Electrolysis

This article is about the chemical process. For the cosmetic hair removal procedure, see Electrology.

Illustration of an electrolysis apparatus used in a school laboratory.

In chemistry and manufacturing, electrolysis (pronounced /iˌlɛkˈtrɒlɨsɪs/, from the Greek ἤλεκτρον [ɛ̌ːlektron] "amber" and λύσις [lýsis] "dissolution") is a method of using a direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially highly important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell.

History

• 1800 – William Nicholson and Johann Ritter decomposed water into hydrogen and oxygen.
• 1807 – Potassium, sodium, barium, calcium and magnesium were discovered by Sir Humphry Davy using electrolysis.
• 1875 – Paul Emile Lecoq de Boisbaudran discovered gallium using electrolysis.
• 1886 – Fluorine was discovered by Henri Moissan using electrolysis.
• 1886 – Hall-Héroult process developed for making aluminium
• 1890 – Castner-Kellner process developed for making sodium hydroxide

Overview

Electrolysis is the passage of a direct electric current through an ionic substance that is either molten or dissolved in a suitable solvent, resulting in chemical reactions at the electrodes and separation of materials.
The main components required to achieve electrolysis are :

• An electrolyte : a substance containing free ions which are the carriers of electric current in the electrolyte. If the ions are not mobile, as in a solid salt then electrolysis cannot occur.
• A direct current (DC) supply : provides the energy necessary to create or discharge the ions in the electrolyte. Electric current is carried by electrons in the external circuit.
• Two electrodes : an electrical conductor which provides the physical interface between the electrical circuit providing the energy and the electrolyte

Electrodes of metal, graphite and semiconductor material are widely used. Choice of suitable electrode depends on chemical reactivity between the electrode and electrolyte and the cost of manufacture.

 Process of electrolysis

The key process of electrolysis is the interchange of atoms and ions by the removal or addition of electrons from the external circuit. The required products of electrolysis are in some different physical state from the electrolyte and can be removed by some physical processes. For example, in the electrolysis of brine to produce hydrogen and chlorine, the products are gaseous. These gaseous products bubble from the electrolyte and are collected.

2 NaCl + 2 H₂O → 2 NaOH + H₂ + Cl₂

A liquid containing mobile ions (electrolyte) is produced by

• Solvation or reaction of an ionic compound with a solvent (such as water) to produce mobile ions
• An ionic compound is melted (fused) by heating

An electrical potential is applied across a pair of electrodes immersed in the electrolyte.
Each electrode attracts ions that are of the opposite charge. Positively charged ions (cations) move towards the electron-providing (negative) cathode, whereas negatively charged ions (anions) move towards the positive anode.
At the electrodes, electrons are absorbed or released by the atoms and ions. Those atoms that gain or lose electrons to become charged ions pass into the electrolyte. Those ions that gain or lose electrons to become uncharged atoms separate from the electrolyte. The formation of uncharged atoms from ions is called discharging.
The energy required to cause the ions to migrate to the electrodes, and the energy to cause the change in ionic state, is provided by the external source of electrical potential.

 Oxidation and reduction at the electrodes

Oxidation of ions or neutral molecules occurs at the anode, and the reduction of ions or neutral molecules occurs at the cathode. For example, it is possible to oxidize ferrous ions to ferric ions at the anode:

Fe2+ aq → Fe3+ aq + e^–

It is also possible to reduce ferricyanide ions to ferrocyanide ions at the cathode:

Fe(CN)3- 6 + e^– → Fe(CN)4- 6

Neutral molecules can also react at either electrode. For example: p-Benzoquinone can be reduced to hydroquinone at the cathode:
+ 2 e^– + 2 H⁺ →
In the last example, H⁺ ions (hydrogen ions) also take part in the reaction, and are provided by an acid in the solution, or the solvent itself (water, methanol etc.). Electrolysis reactions involving H⁺ ions are fairly common in acidic solutions. In alkaline water solutions, reactions involving OH^- (hydroxide ions) are common.
The substances oxidised or reduced can also be the solvent (usually water) or the electrodes. It is possible to have electrolysis involving gases.

 Energy changes during electrolysis

The amount of electrical energy that must be added equals the change in Gibbs free energy of the reaction plus the losses in the system. The losses can (in theory) be arbitrarily close to zero, so the maximum thermodynamic efficiency equals the enthalpy change divided by the free energy change of the reaction. In most cases, the electric input is larger than the enthalpy change of the reaction, so some energy is released in the form of heat. In some cases, for instance, in the electrolysis of steam into hydrogen and oxygen at high temperature, the opposite is true. Heat is absorbed from the surroundings, and the heating value of the produced hydrogen is higher than the electric input.

 Related techniques

The following techniques are related to electrolysis:

• Electrochemical cells, including the hydrogen fuel cell, utilise differences in Standard electrode potential in order to generate an electrical potential from which useful power can be extracted. Although related via the interaction of ions and electrodes, electrolysis and the operation of electrochemical cells are quite distinct. A chemical cell should not be thought of as performing "electrolysis in reverse".

 Faraday's laws of electrolysis

Main article: Faraday's laws of electrolysis

 First law of electrolysis

In 1832, Michael Faraday reported that the quantity of elements separated by passing an electric current through a molten or dissolved salt is proportional to the quantity of electric charge passed through the circuit. This became the basis of the first law of electrolysis:

 Second law of electrolysis

Faraday also discovered that the mass of the resulting separated elements is directly proportional to the atomic masses of the elements when an appropriate integral divisor is applied. This provided strong evidence that discrete particles of matter exist as parts of the atoms of elements.

 Industrial uses

Hall-Heroult process for producing aluminium

• Production of aluminium, lithium, sodium, potassium, magnesium, calcium
• Coulometric techniques can be used to determine the amount of matter transformed during electrolysis by measuring the amount of electricity required to perform the electrolysis
• Production of chlorine and sodium hydroxide
• Production of sodium chlorate and potassium chlorate
• Production of perfluorinated organic compounds such as trifluoroacetic acid
• Production of electrolytic copper as a cathode, from refined copper of lower purity as an anode.

Electrolysis has many other uses:

• Electrometallurgy is the process of reduction of metals from metallic compounds to obtain the pure form of metal using electrolysis. For example, sodium hydroxide in its molten form is separated by electrolysis into sodium and oxygen, both of which have important chemical uses. (Water is produced at the same time.)
• Anodization is an electrolytic process that makes the surface of metals resistant to corrosion. For example, ships are saved from being corroded by oxygen in the water by this process. The process is also used to decorate surfaces.
• A battery works by the reverse process to electrolysis.
• Production of oxygen for spacecraft and nuclear submarines.
• Electroplating is used in layering metals to fortify them. Electroplating is used in many industries for functional or decorative purposes, as in vehicle bodies and nickel coins.
• Production of hydrogen for fuel, using a cheap source of electrical energy.
• Electrolytic Etching of metal surfaces like tools or knives with a permanent mark or logo.

Electrolysis is also used in the cleaning and preservation of old artifacts. Because the process separates the non-metallic particles from the metallic ones, it is very useful for cleaning old coins and even larger objects.

Competing half-reactions in solution electrolysis

Using a cell containing inert platinum electrodes, electrolysis of aqueous solutions of some salts leads to reduction of the cations (e.g., metal deposition with, e.g., zinc salts) and oxidation of the anions (e.g. evolution of bromine with bromides). However, with salts of some metals (e.g. sodium) hydrogen is evolved at the cathode, and for salts containing some anions (e.g. sulfate SO₄²⁻) oxygen is evolved at the anode. In both cases this is due to water being reduced to form hydrogen or oxidised to form oxygen. In principle the voltage required to electrolyse a salt solution can be derived from the standard electrode potential for the reactions at the anode and cathode. The standard electrode potential is directly related to the Gibb's free energy, ΔG, for the reactions at each electrode and refers to an electrode with no current flowing. An extract from the table of standard electrode potentials is shown below.

Half-reaction	E° (V)	Ref.
Na+ + e− Na(s)	−2.71
Zn2+ + 2e− Zn(s)	−0.7618
2H+ + 2e− H2(g)	≡ 0
Br2(aq) + 2e− 2Br−	+1.0873
O2(g) + 4H+ + 4e− 2H2O	+1.23
Cl2(g) + 2e− 2Cl−	+1.36
S2O82– + 2e− 2SO2− 4	+2.07

In terms of electrolysis, this table should be interpreted as follows

• oxidised species (often a cation) nearer the top of the table are more difficult to reduce than oxidised species further down. For example it is more difficult to reduce sodium ion to sodium metal than it is to reduce zinc ion to zinc metal.
• reduced species (often an anion) near the bottom of the table are more difficult to oxidise than reduced species higher up. For example it is more difficult to oxidise sulfate anions than it is to oxidise bromide anions.

Using the Nernst equation the electrode potential can be calculated for a specific concentration of ions, temperature and the number of electrons involved. For pure water (pH 7):

• the electrode potential for the reduction producing hydrogen is −0.41 V
• the electrode potential for the oxidation producing oxygen is +0.82 V.

Comparable figures calculated in a similar way, for 1M zinc bromide, ZnBr₂, are −0.76 V for the reduction to Zn metal and +1.10 V for the oxidation producing bromine. The conclusion from these figures is that hydrogen should be produced at the cathode and oxygen at the anode from the electrolysis of water which is at variance with the experimental observation that zinc metal is deposited and bromine is produced. The explanation is that these calculated potentials only indicate the thermodynamically preferred reaction. In practice many other factors have to be taken into account such as the kinetics of some of the reaction steps involved. These factors together mean that a higher potential is required for the reduction and oxidation of water than predicted, and these are termed overpotentials. Experimentally it is known that overpotentials depend on the design of the cell and the nature of the electrodes.
For the electrolysis of a neutral (pH 7) sodium chloride solution, the reduction of sodium ion is thermodynamically very difficult and water is reduced evolving hydrogen leaving hydroxide ions in solution. At the anode the oxidation of chlorine is observed rather than the oxidation of water since the overpotential for the oxidation of chloride to chlorine is lower than the overpotential for the oxidation of water to oxygen. The hydroxide ions and dissolved chlorine gas react further to form hypochlorous acid. The aqueous solutions resulting from this process is called electrolyzed water and is used as a disinfectant and cleaning agent.

 Electrolysis of water

Main article: Electrolysis of water

One important use of electrolysis of water is to produce hydrogen.

2 H₂O(l) → 2 H₂(g) + O₂(g); E₀ = +1.229 V

Hydrogen can be used as a fuel for powering internal combustion engines by combustion or electric motors via hydrogen fuel cells (see Hydrogen vehicle). This has been suggested as one approach to shift economies of the world from the current state of almost complete dependence upon hydrocarbons for energy (See hydrogen economy.)
The energy efficiency of water electrolysis varies widely. The efficiency is a measure of what fraction of electrical energy used is actually contained within the hydrogen. Some of the electrical energy is converted to heat, an almost useless byproduct. Some reports quote efficiencies between 50% and 70%. This efficiency is based on the Lower Heating Value of Hydrogen. The Lower Heating Value of Hydrogen is total thermal energy released when hydrogen is combusted minus the latent heat of vaporisation of the water. This does not represent the total amount of energy within the hydrogen, hence the efficiency is lower than a more strict definition. Other reports quote the theoretical maximum efficiency of electrolysis as being between 80% and 94%. The theoretical maximum considers the total amount of energy absorbed by both the hydrogen and oxygen. These values refer only to the efficiency of converting electrical energy into hydrogen's chemical energy. The energy lost in generating the electricity is not included. For instance, when considering a power plant that converts the heat of nuclear reactions into hydrogen via electrolysis, the total efficiency is more likely to be between 25% and 40%.^{[citation needed]}
NREL found that a kilogram of hydrogen (roughly equivalent to a gallon of gasoline) could be produced by wind powered electrolysis for between $5.55 in the near term and $2.27 in the long term.
About four percent of hydrogen gas produced worldwide is created by electrolysis, and normally used onsite. Hydrogen is used for the creation of ammonia for fertilizer via the Haber process, and converting heavy petroleum sources to lighter fractions via hydrocracking.

Mixture

For other uses, see Mix (disambiguation).

In chemistry, a mixture is a material system made up by two or more different substances which are mixed together but are not combined chemically. Mixture refers to the physical combination of two or more substances the identities of which are retained and are mixed in the form of alloys, solutions, suspensions, and colloids.
Mixtures are the product of a mechanical blending or mixing of chemical substances like elements and compounds, without chemical bonding or other chemical change, so that each ingredient substance retains its own chemical properties and makeup. Nonetheless, despite there are no chemical changes to its constituents, the physical properties of a mixture, such as its melting point, may differ from those of the components. Some mixtures can be separated into their components by physical (mechanical or thermal) means. Azeotropes can be considered as a kind of mixture which usually pose considerable difficulties regarding the separation processes required to obtain their constituents (physical or chemical processes or, even a blend of them).
Mixtures can be either homogeneous or heterogeneous. An homogeneous mixture is a type of mixture in which the composition is uniform. An heterogeneous mixture is a type of mixture in which the components can easily be identified, as there are two or more phases present. Air is a homogeneous mixture of the gaseous substances nitrogen, oxygen, and smaller amounts of other substances. Salt, sugar, and many other substances dissolve in water to form homogeneous mixtures. A homogeneous mixture in which there is both a solute and solvent present is also a solution. The following table shows the main properties of the three families of mixtures.

	Solution	Colloid	Coarse dispersion
Mixture homogeneity	Homogeneous	Visually homogeneous but microscopically heterogeneous	Heterogeneous
Particle size	< 1 nanometer	between 1 nanometer and 1 micrometer	> 1 micrometer
Physical stability	Yes	Yes	No: needs stabilizing agents
Tyndall effect	No	Yes	Yes
Separates by centrifugation	No	Yes	Yes
Separates by decantation	No	No	Yes

The following table shows examples of the three types of mixtures.

Dissolved or dispersed phase	Continuous medium	Solution	Colloid	Coarse dispersion
Gas	Gas	Gas mixture: air (oxygen and other gases in nitrogen)	None	None
Liquid	Gas	None	Aerosol: fog, mist, vapor, hair sprays	Aerosol
Solid	Gas	None	Solid aerosol: smoke, cloud, air particulates	Solid aerosol: dust
Gas	Liquid	Solution: oxygen in water	Foam: whipped cream, shaving cream	Foam
Liquid	Liquid	Solution: alcoholic beverages	Emulsion: miniemulsion, microemulsion	Emulsion: milk, mayonnaise, hand cream
Solid	Liquid	Solution: sugar in water	Sol: pigmented ink, blood	Suspension: mud (soil, clay or silt particles are suspended in water), chalk powder suspended in water
Gas	Solid	Solution: hydrogen in metals	Solid foam: aerogel, styrofoam, pumice	Foam: dry sponge
Liquid	Solid	Solution: amalgam (mercury in gold), hexane in paraffin wax	Gel: agar, gelatin, silicagel, opal	Wet sponge
Solid	Solid	Solution: alloys, plasticizers in plastics	Solid sol: cranberry glass	Gravel, granite

==Physics and Chemistry== Lol :) A heterogeneous mixture is a mixture of two or more compounds. Examples are: mixtures of sand and water or sand and iron filings, a conglomerate rock, water and oil, a salad, trail mix, and concrete (not cement).
Making a distinction between homogeneous and heterogeneous mixtures is a matter of the scale of sampling. On a coarse enough scale, any mixture can be said to be homogeneous, if you'll allow the entire article to count as a "sample" of it. On a fine enough scale, any mixture can be said to be heterogeneous, because a sample could be as small as a single molecule. In practical terms, if the property of interest of the mixture is the same regardless of which sample of it is taken for the examination used, the mixture is homogeneous.
Gy's sampling theory quantitatively defines the heterogeneity of a particle as:

where h_i, c_i, c_batch, m_i, and m_aver are respectively: the heterogeneity of the ith particle of the population, the mass concentration of the property of interest in the ith particle of the population, the mass concentration of the property of interest in the population, the mass of the ith particle in the population, and the average mass of a particle in the population.
During sampling of heterogeneous mixtures of particles, the variance of the sampling error is generally non-zero.

Pierre Gy derived, from the Poisson sampling model, the following formula for the variance of the sampling error in the mass concentration in a sample:

in which V is the variance of the sampling error, N is the number of particles in the population (before the sample was taken), q _i is the probability of including the ith particle of the population in the sample (i.e. the first-order inclusion probability of the ith particle), m _i is the mass of the ith particle of the population and a _i is the mass concentration of the property of interest in the ith particle of the population.
It must be noted that the above equation for the variance of the sampling error is an approximation based on a linearization of the mass concentration in a sample.
In the theory of Gy, correct sampling is defined as a sampling scenario in which all particles have the same probability of being included in the sample. This implies that q _i no longer depends on i, and can therefore be replaced by the symbol q. Gy's equation for the variance of the sampling error becomes:

where a_batch is that concentration of the property of interest in the population from which the sample is to be drawn and M_batch is the mass of the population from which the sample is to be drawn.

Monera

Monera is a superseded kingdom that contains unicellular organisms without a nucleus (i.e., a prokaryotic cell organization), such as bacteria.

The taxon Monera was first proposed as a phylum by Ernst Haeckel in 1866; subsequently, the taxon was raised to the rank of kingdom in 1925 by Édouard Chatton, gaining common acceptance, and the last commonly accepted mega-classication with the taxon Monera was the five-kingdom classification system established by Robert Whittaker in 1969. Under the three-domain system of taxonomy, which was established in 1990 and reflects the evolutionary history of life as currently understood, the organisms found in kingdom Monera have been divided into two domains, Archaea and Bacteria (with Eukarya as the third domain). Furthermore the taxon Monera is paraphyletic. The term "moneran" is the informal name of members of this group and is still sometimes used (as is the term "prokaryote") to denote a member of either domain.

Despite most bacteria being classified under Monera, the bacterial phylum Cyanobacteria (the blue-green algae) was not classified under Monera, but under Plantae given the ability of its members to photosynthesise.

History

Haekel's classification

Tree of Life in Generelle Morphologie der Organismen (1866)

Traditionally the natural world was classified as animal, vegetable, or mineral as in Systema Naturae. After the discovery of microscopy, attempts were made to fit microscopic organisms into either the plant or animal kingdoms. In 1676, Antonie van Leeuwenhoek discovered bacteria and called them "animacules," assigning them to the class Vermes of the Animalia.Due to the limited tools — the sole references for this group were shape, behaviour, and habitat — the description of genera and their classification was extremely limited, which was accentuated by the perceived lack of importance of the group.

Ten years after The Origin of Species by Darwin, in 1866 Ernst Haeckel, a supporter of evolution, proposed a three-kingdom system which added the Protista as a new kingdom that contained most microscopic organisms. One of his eight major divisions of Protista was composed of the monerans (called Moneres in German) and defines them as completely structureless and homogeneous organisms, consisting only of a piece of plasma. Haeckel's Monera included not only bacterial groups of early discovery but also several small eukaryotic organisms; in fact the genus Vibrio is the only bacterial genus explicitly assigned to the phylum, while others are mentioned indirectly, which has led Copeland to speculate that Haeckel considered all bacteria to belong to the genus Vibrio, ignoring other bacterial genera. One notable exception were the members of the modern phylum Cyanobacteria, such as Nostoc, which were placed in the phylum Archephyta of Algae (vide infra: Blue-green algae).

The Neolatin noun Monera and the German noun Moneren/Moneres are derived from the ancient Greek noun moneres (μονήρης) which Haeckel states to mean "simple", however it actually means "single, solitary". Haeckel also describes the protist genus Monas in the two pages about Monera in his 1866 book. The informal name of a member of the Monera was initially moneron, but later moneran was used.

Due to its lack of features, the phylum was not fully subdivided, but the genera therein were divided into two groups:

• die Gymnomoneren (no envelope [sic.]): Gymnomonera
• Protogenes — such as Protogenes primordialis, an unidentified amaeba (eukaryote) and not a bacterium
• Protamaeba — an incorrectly described/fabricated species
• Vibrio — a genus of comma-shaped bacteria first described in 1854
• Bacterium — a genus of rod-shaped bacteria first described in 1828. Haeckel does not explicitly assign this genus to the Monera.
• Bacillus — a genus of spore-forming rod-shaped bacteria first described in 1835 Haeckel does not explicitly assign this genus to the Monera.
• Spirochaeta — thin spiral-shaped bacteria first described in 1835  Haeckel does not explicitly assign this genus to the Monera.
• Spirillum — spiral-shaped bacteria first described in 1832 Haeckel does not explicitly assign this genus to the Monera.
• etc.: Haeckel does provide a comprehensive list.
• die Lepomoneren (with envelope): Lepomonera
• Protomonas — identified to a synonym of Monas, a flagellated protozoan, and not a bacterium. The name was reused in 1984 for an unrelated genus of bacteria.
• Vampyrella — now classed as a eukaryote and not a bacterium.

 Subsequent classifications

Like Protista, the Monera classification was not fully followed at first and several different ranks were used and located with animals, plants, protists or fungi. Furthermore, Häkel's classification lacked specificity and was not exhaustive —it in fact covers only a few pages—, consequently a lot of confusion arose even to the point that the Monera did not contain bacterial genera and others according to Huxley. The most popular scheme was created in 1859 by C. Von Nägeli who classified non-phototrophic Bacteria as the class Schizomycetes.

The class Schizomycetes was then emended by Walter Migula (along with the coinage of the genus Pseudomonas in 1894) and others. This term was in dominant use even in 1916 as reported by Robert Earle Buchanan, as it had priority over other terms such as Monera. However, starting with Ferdinand Cohn in 1872 the term bacteria (or in German der Bacterien) became prominently used to informally describe this group of species without a nucleus: Bacterium was in fact a genus created in 1828 by Christian Gottfried Ehrenberg Additionally, Cohn divided the bacteria according to shape namely:

• Spherobacteria for the cocci
• Microbacteria for the short, non-filamentous rods
• Desmobacteria for the longer, filamentous rods and Spirobacteria for the spiral forms.

Successively, Cohn created the Schizophyta of Plants which contained the non-photrophic bacteria in the family Schizomycetes and the phototrophic bacteria (blue green algae/Cyanobacteria) in the Schizophyceae This union of blue green algae and Bacteria was much later followed by Haeckel, who classified the two families in a revised phylum Monera in the Protista.

Rise to prominence

The term Monora, became well established in the 20s and 30s when to rightfully increase the importance of the difference between species with a nucleus and without, In 1925 Édouard Chatton divided all living organisms into two empires Prokaryotes and Eukaryotes: the Kingdom Monera being the sole member of the Prokaryotes empire.

The anthropic importance of the crown group of animals, plants and fungi was hard to depose consequently several other megaclassification schemes ignored on the empire rank, but maintained the kingdom Monera consisting of bacteria, such Copeland in 1938 and Whittaker in 1969. The latter classification system was widely followed and in which Robert Whittaker proposed a five kingdom system for classification of living organisms. Whittaker's system placed most single celled organisms into either the prokaryotic Monera or the eukaryotic Protista. The other three kingdoms in his system were the eukaryotic Fungi, Animalia, and Plantae. Whittaker, however, did not believe that all his kingdoms were monophyletic. Whittaker subdiveded the kingdom into two branches containing several phyla:

• Myxomonera branch
• Cyanophyta, now called Cyanobacteria
• Myxobacteria
• Mastigomonera branch
• Eubacteriae
• Actinomycota
• Spirochaetae

Alternative ommonly followed subdivision systems were based on Gram stains. This culminated in the Gibbons and Murray classification of 1978:

• Gracilicutes (gram negative)
• Photobacteria (photosynthetic): class Oxyphotobacteriae (water as electron acceptor, includes the order Cyanobacteriales=blue green algae, now phylum Cyanobacteria) and class Anoxyphotobacteriae (anaerobic phototrophs, orders: Rhodospirillales and Chlorobiales
• Scotobacteria (non-photosynthetic, now the Proteobacteria and other gram negative nonphotosynthetic phyla)
• Firmacutes [sic] (gram positive, subsequently corrected to Firmicutes)
• several orders such as Bacillales and Actinomycetales (now in the phylum Actinobacteria)
• Mollicutes (gram variable, e.g. Mycoplasma)
• Mendocutes (uneven gram stain, "methanogenic bacteria" now known as the Archaea)

Three-domain system

In 1977, a PNAS paper by Carl Woese and George Fox demonstrated that the archaea (initially called archaebacteria) are not significantly closer in relationship to the bacteria than they are to eukaryotes. The paper received front-page coverage in The New York Times , and great controversy initially. The conclusions have since become accepted, leading to replacement of the kingdom Monera with the two kingdoms Bacteria and Archaea. However, Thomas Cavalier-Smith has never accepted the importance of the division between these two groups, and has published classifications in which the archaebacteria are part of a subkingdom of the Kingdom Bacteria.

 Blue-green algae

Although it was generally accepted that one could distinguish prokaryotes from eukaryotes on the basis of the presence of a nucleus, mitosis versus binary fission as a way of reproducing, size, and other traits, the monophyly of the kingdom Monera (or for that matter, whether classification should be according to phylogeny) was controversial for many decades. Although distinguishing between prokaryotes from eukaryotes as a fundamental distinction is often credited to a 1937 paper by Édouard Chatton (little noted until 1962), he did not emphasize this distinction more than other biologists of his era. Roger Stanier and C. B. van Niel believed that the bacteria (a term which at the time did not include blue-green algae) and the blue-green algae had a single origin, a conviction which culminated in Stanier writing in a letter in 1970, "I think it is now quite evident that the blue-green algae are not distinguishable from bacteria by any fundamental feature of their cellular organization". Other researchers, such as E. G. Pringsheim writing in 1949, suspected separate origins for bacteria and blue-green algae. In 1974, the influential Bergey's Manual published a new edition coining the term cyanobacteria to refer to what had been called blue-green algae, marking the acceptance of this group within the Monera.

 Summary

Linnaeus 1735 2 kingdoms	Haeckel 1866 3 kingdoms	Chatton 1925 2 empires	Copeland 1938 4 kingdoms	Whittaker 1969 5 kingdoms	Woese et al. 1977 6 kingdoms	Woese et al. 1990 3 domains	Cavalier-Smith 2004 6 kingdoms
(not treated)	Protista	Prokaryota	Monera	Monera	Eubacteria	Bacteria	Bacteria
					Archaebacteria	Archaea
		Eukaryota	Protoctista	Protista	Protista	Eukarya	Protozoa
							Chromista
Vegetabilia	Plantae		Plantae	Plantae	Plantae		Plantae
			Protoctista	Fungi	Fungi		Fungi
Animalia	Animalia		Animalia	Animalia	Animalia		Animalia