Periodic Table > Potassium
 

Terminology


Allotropes
Some elements exist in several different structural forms, these are called allotropes.


For more information on Murray Robertson’s image see Uses and properties facts below.

 

Fact box terminology


Group
Elements appear in columns or ‘groups’ in the periodic table. Members of a group typically have similar properties and electron configurations in their outer shell.


Period
Elements are laid out into rows or ‘periods’ so that similar chemical behaviour is observed in columns.


Block
Elements are organised into blocks by the orbital type in which the outer electrons are found. These blocks are named for the characteristic spectra they produce: sharp, principal, diffuse, and fundamental.


Atomic Number
The number of protons in the nucleus.


Atomic Radius/non -bonded (Å)
based on Van der Waals forces (where several isotopes exist, a value is presented for the most prevalent isotope). These values were calculated using a multitude of methods including crystallographic data, gas kinetic collision cross sections, critical densities, liquid state properties, for more details please refer to the CRC Handbook of Chemistry and Physics.


Electron Configuration
The arrangements of electrons above the last (closed shell) noble gas.


Isotopes
Elements are defined by the number of protons in its centre (nucleus), whilst the number of neutrons present can vary. The variations in the number of neutrons will create elements of different mass which are known as isotopes.


Melting Point (oC)
The temperature at which the solid-liquid phase change occurs.


Melting Point (K)
The temperature at which the solid-liquid phase change occurs.


Melting Point (oF)
The temperature at which the solid-liquid phase change occurs.


Boiling Point (oC)
The temperature at which the liquid-gas phase change occurs.


Boiling Point (K)
The temperature at which the liquid-gas phase change occurs.


Boiling Point (oF)
The temperature at which the liquid-gas phase change occurs.


Sublimation
Elements that do not possess a liquid phase at atmospheric pressure (1 atm) are described as going through a sublimation process.


Density (kgm-3)
Density is the weight of a substance that would fill 1 m3 (at 298 K unless otherwise stated).


Relative Atomic Mass
The mass of an atom relative to that of Carbon-12. This is approximately the sum of the number of protons and neutrons in the nucleus. Where more than one isotope exists the value given is the abundance weighted average.


Key Isotopes (% abundance)
An element must by definition have a fixed number of protons in its nucleus, and as such has a fixed atomic number, however variants of an element can exist with differing numbers of neutrons, and hence a different atomic masses (e.g. 12C has 6 protons and 6 neutrons and 13C has 6 protons and 7 neutrons).


CAS number
The Chemical Abstracts Service registry number is a unique identifier of a particular chemical, designed to prevent confusion arising from different languages and naming systems (where several isotopes exist, a value is presented for the most prevalent isotope).

Fact box

 
Group Melting point 63.5 oC, 146.3 oF, 336.65 K 
Period Boiling point 759 oC, 1398.2 oF, 1032.15 K 
Block Density (kg m-3) 862 
Atomic number 19  Relative atomic mass 39.098  
State at room temperature Solid  Key isotopes 39
Electron configuration [Ar] 4s1  CAS number 7440-09-7 
ChemSpider ID 4575326 ChemSpider is a free chemical structure database
 

Uses and properties terminology


Image Explanation

Murray Robertson is the artist behind the images which make up Visual Elements. This is where the artist explains his interpretation of the element and the science behind the picture.


Natural Abundance

Where this element is most commonly found in nature.


Biological Roles

The elements role within the body of humans, animals and plants. Also functionality in medical advancements both today and years ago.


Appearance

The description of the element in its natural form.

Uses and properties

 
Image explanation

The image features the alchemical symbol for Potash from which it was first isolated.

Appearance
A soft, silvery metal that tarnishes within minutes.
Uses

The greatest demand for potassium compounds is in fertilisers. Many other potassium salts are of great importance, including the nitrate, carbonate, chloride, bromide, cyanide and sulfate. In water, it reacts rapidly and often explosively to release hydrogen, which burns with a lilac flame.

Biological role

Potassium, in the form of potassium cations, is essential to life, being a constituent of all cells. Animals tend to bathe their cells with fluids, such as blood, rich in sodium ions, but in plants there is no such fluid, making them rich in potassium but poor in sodium. Herbivores often lack sodium and need to find salt-licks. It also means that soils from which harvests are taken every year need to have their potassium replenished, hence the need for potassium-based fertilisers. The average human consumes up to 7 grams of potassium a day, and has a store of some 140 grams in the cells of the human body. Normal diets contain enough potassium, but some foods such as instant coffee, sardines, nuts, raisins, potatoes and chocolate have more than average. One of its natural isotopes is radioactive, and although this radioactivity is mild, it may be one natural cause of genetic mutation in man.

Natural abundance

Potassium is the seventh most abundant metal in the Earth’s crust making up 2.4% by mass. There are deposits of billions of tonnes of potassium chloride throughout the world and mining extracts about 50 million tonnes a year, mainly for use in fertilisers. Most minerals (eg those found in igneous rocks) containing potassium are sparingly soluble and the metal is difficult to obtain from them. Certain minerals, however, such as sylvite, sylvinite and carnallite, are found in deposits formed by evaporation of old seas or lakes, and potassium salts can be easily recovered from these. Potassium salts are also found in the ocean but in smaller amounts compared with sodium.

 
Atomic data terminology

Atomic radius/non -bonded (Å)
Based on Van der Waals forces (where several isotopes exist, a value is presented for the most prevalent isotope). These values were calculated using a multitude of methods including crystallographic data, gas kinetic collision cross sections, critical densities, liquid state properties,for more details please refer to the CRC Handbook of Chemistry and Physics.


Electron affinity (kJ mol-1)
The energy released when an additional electron is attached to the neutral atom and a negative ion is formed (where several isotopes exist, a value is presented for the most prevalent isotope). *


Electronegativity (Pauling scale)
The degree to which an atom attracts electrons towards itself, expressed on a relative scale as a function bond dissociation energies, Ed in eV. χA - χB =(eV)-1/2sqrt(Ed(AB)-[Ed(AA)+Ed(BB)]/2), with χH set as 2.2 (where several isotopes exist, a value is presented for the most prevalent isotope).


1st Ionisation energy (kJ mol-1)
The minimum energy required to remove an electron from a neutral atom in its ground state (where several isotopes exist, a value is presented for the most prevalent isotope).


Covalent radius (Å)
The size of the atom within a covalent bond, given for typical oxidation number and coordination (where several isotopes exist, a value is presented for the most prevalent isotope). ***

Atomic data

 
Atomic radius, non-bonded (Å) 2.750 Covalent radius (Å) 2
Electron affinity (kJ mol-1) 48.367 Electronegativity
(Pauling scale)
0.820
Ionisation energies
(kJ mol-1)
 
1st
418.810
2nd
3051.829
3rd
4419.604
4th
5876.917
5th
7975.471
6th
9590.635
7th
11342.807
8th
14943.637
 
Bond enthalpies terminology

Covalent Bonds
The strengths of several common covalent bonds.

Bond enthalpies

 
Covalent bonds
K–K  49  kJ mol -1  
 

Mining/Sourcing Information

Data for this section of the data page has been provided by the British Geological Survey. To review the full report please click here or please look at their website here.


Key for numbers generated


Governance indicators

1 (low) = 0 to 2

2 (medium-low) = 3 to 4

3 (medium) = 5 to 6

4 (medium-high) = 7 to 8

5 (high) = 9


Reserve base distribution

1 (low) = 0 to 30 %

2 (medium-low) = 30 to 45 %

3 (medium) = 45 to 60 %

4 (medium-high) = 60 to 75 %

5 (high) = 75 %

(Where data are unavailable an arbitrary score of 2 was allocated. For example, Be, As, Na, S, In, Cl, Ca and Ge are allocated a score of 2 since reserve base information is unavailable. Reserve base data are also unavailable for coal; however, reserve data for 2008 are available from the Energy Information Administration (EIA).)


Production Concentration

1 (low) = 0 to 30 %

2 (medium-low) = 30 to 45 %

3 (medium) = 45 to 60 %

4 (medium-high) = 60 to 75 %

5 (high) = 75 %


Crustal Abundance

1 (low) = 100 to 1000 ppm

2 (medium-low) =10 to 100 ppm

3 (medium) = 1 to 10 ppm

4 (medium-high) = 0.1 to 1 ppm

5 (high) = 0.1 ppm

(Where data are unavailable an arbitrary score of 2 was allocated. For example, He is allocated a score of 2 since crustal abundance data is unavailable.)


Explanations for terminology


Crustal Abundance (ppm)

The abundance of an element in the Earth's crust in parts-per-million (ppm) i.e. The number of atoms of this element per 1 million atoms of crust.


Sourced

The country with the largest reserve base.


Reserve Base Distribution

This is a measure of the spread of future supplies, recording the percentage of a known resource likely to be available in the intermediate future (reserve base) located in the top three countries.


Production Concentrations

This reports the percentage of an element produced in the top three countries. The higher the value, the larger risk there is to supply.


Total Governance Factor

The World Bank produces a global percentile rank of political stability. The scoring system is given below, and the values for all three production countries were summed.


Relative Supply Risk Index

The Crustal Abundance, Reserve Base Distribution, Production Concentration and Governance Factor scores are summed and then divided by 2, to provide an overall Relative Supply Risk Index.

Supply risk

 
Scarcity factor 4.5
Country with largest reserve base Canada
Crustal abundance (ppm) 22774
Leading producer Canada
Reserve base distribution (%) 61.10
Production concentration (%) 20.90
Total governance factor(production) 6
Top 3 countries (mined)
  • 1) Canada
  • 2) Russia
  • 3) Belarus
Top 3 countries (production)
  • 1) Canada
  • 2) Russia
  • 3) Belarus
 

Oxidation states and isotopes


Key for Isotopes


Half Life
  y years
  d days
  h hours
  m minutes
  s seconds
Mode of decay
  α alpha particle emission
  β negative beta (electron) emission
  β+ positron emission
  EC orbital electron capture
  sf spontaneous fission
  ββ double beta emission
  ECEC double orbital electron capture

Terminology


Common Oxidation states
The oxidation state of an atom is a measure of the degree of oxidation of an atom. It is defined as being the charge that an atom would have if all bonds were ionic. Free atoms have an oxidation state of 0, and the sum of oxidation numbers within a substance must equal the overall charge.


Important Oxidation states
The most common oxidation states of an element in its compounds.


Isotopes
Elements are defined by the number of protons in its centre (nucleus), whilst the number of neutrons present can vary. The variations in the number of neutrons will create elements of different mass which are known as isotopes.

Oxidation states and isotopes

 
Common oxidation states 1
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  39K 38.964 93.258
  40K 39.964 0.012 1.248 x 109 β- 
        β+ 
  41K 40.962 6.73
 

Pressure and temperature - advanced terminology


Molar Heat Capacity (J mol-1 K-1)

Molar heat capacity is the energy required to heat a mole of a substance by 1 K.


Young's modulus (GPa)

Young's modulus is a measure of the stiffness of a substance, that is, it provides a measure of how difficult it is to extend a material, with a value given by the ratio of tensile strength to tensile strain.


Shear modulus (GPa)

The shear modulus of a material is a measure of how difficult it is to deform a material, and is given by the ratio of the shear stress to the shear strain.


Bulk modulus (GPa)

The bulk modulus is a measure of how difficult to compress a substance. Given by the ratio of the pressure on a body to the fractional decrease in volume.


Vapour Pressure (Pa)

Vapour pressure is the measure of the propensity of a substance to evaporate. It is defined as the equilibrium pressure exerted by the gas produced above a substance in a closed system.

Pressure and temperature data – advanced

 
Molar heat capacity
(J mol-1 K-1)
29.6 Young's modulus (GPa) Unknown
Shear modulus (GPa) Unknown Bulk modulus (GPa) 3.1
Vapour pressure  
Temperature (K)
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Pressure (Pa)
1.88
x 10-2
96.9 - - - - - - - - -
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History

Potassium salts in the form of saltpetre (potassium nitrate, KNO3), alum (potassium aluminium sulfate, KAl(SO4)2), and potash (potassium carbonate, K2CO3) have been known for centuries. They were used in gunpowder, dyeing, and soap making. They were scraped from the walls of latrines, manufactured from clay and sulfuric acid, and collected as wood ash respectively. Reducing them to the element defeated the early chemists and potassium was classed as an ‘earth’ by Antoine Lavoisier. Then in 1807, Humphry Davy exposed moist potash to an electric current and observed the formation of metallic globules of a new metal, potassium. He noted that when they were dropped into water they skimmed around on the surface, burning with a lavender-coloured flame.

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Podcasts

Listen to Potassium Podcast
Transcript :

Chemistry in Its Element - Potassium


  (Promo)

 

You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry

(End promo)

 

Chris Smith

Hello, this week the story of the first alkaline metal ever isolated, why its an alkaline metal at all and why its symbol begins with the letter K.   Here's Peter Wothers. 

 

Peter Wothers

Potassium -   the only element named after a cooking utensil.   It was named in 1807 by Humphry Davy after the compound from which he isolated the metal, potash, or potassium hydroxide.

An extract from the 1730s by the Dutch chemist [Herman] Boerhaave describes how potash got its name:

             "Potas or Pot-ashes is brought yearly by the Merchant's Ships in great abundance from Coerland [now part of Latvia and Lithuania], Russia, and Poland.   It is prepared there from the Wood of green Fir, Pine, Oak, and the like, of which they make large piles in proper Trenches, and burn them till they are reduced to Ashes... These ashes are then dissolved in boiling Water, and when the Liquor at top, which contains the Salt, is depurated, i.e. freed from impurities, by standing quiet, it is poured off clear.   This, then, is immediately put into large copper Pots, and is there boiled for the space of three days, by which means they procure the Salt they call Potas, (which signifies Pot-Ashes) on account of its being thus made in Pots.

 

Even earlier in the 16th Century, Conrad Gesner tells us that "Of the hearbe called Kali, doe certayne prepare a Salt"

He describes this plant, Kali whose Latin name is Salsola kali but is more commonly known as Saltwort:

Kali is of two Cubites of heygth, hauing no prickles or thornes, & is sometymes very red, saltye in taste, with a certayne vngratefull smell, found & gathered in saltie places: out of which, the Salt of Alkali maye be purchased"

 

His method of production of this Salt of Alkali is pretty similar to that described by Boerhaave with both processes actually yielding an impure mixture of what we would now call potassium   and sodium carbonate; the wood ash method yielding more potassium carbonate, potash, the salty herbs giving more sodium carbonate, soda.   However, it is from the herb kali, that we owe the word that describes both -   al-kali or alkali; the 'al' prefix simply being Arabic definite article 'the'.

 

The crude potash can be made more caustic or 'pure' by treating a solution of it with lime water, calcium hydroxide.   The potassium carbonate and calcium hydroxide solutions react with a bit of chemical partner-swapping: insoluble calcium carbonate or chalk precipitates out, leaving a solution of potassium hydroxide.   It was from this pure hydroxide that Davy first isolated the metal potassium.   To do this he [used] the relatively new force of electricity.

 

Almost twenty years earlier, the Italian physician Luigi Galvani noticed that when he touched the central nerve of a dead frog with a metal knife the muscles in the frog's leg violently contracted.   After further experiments, he found that combinations of different metals gave even stronger effects.   His fellow countryman, Alessandro Volta, recognised the electrical origin of these observations, and went on to develop the so-called Voltaic pile.   This [device consisted] of alternating plates of two different metals, such as copper and zinc, sandwiched with cloth soaked in a solution of brine.

 

Such a Voltaic pile had been used in spectacular fashion at the turn of the century by English chemists William Nicholson and Anthony Carlisle to decompose - or electrolyse - water into its constituent elements hydrogen and oxygen.   Humphry's younger brother John Davy stated that this experiment "immediately impressed powerfully the mind of my brother".

In his notebook in August 1800 Humphry records "I cannot close this notice without feeling grateful to Volta,   Nicholson, and Carlisle, whose experience has placed such a wonderful and important instrument of analysis in my power".

 

It was to caustic potash that Humphry focussed his attention.   It had been suspected for a while that this substance was not elemental, but its exact nature unknown.   Lavoisier's "Elements of Chemistry" from which Humphry taught himself chemistry as a young lad made it quite clear that both potash and soda were compounds and likely to be decomposed into their true constituent elements in the future.   Davy himself suspected potash to be composed of phosphorus or sulphur united to nitrogen.

 

After unsuccessfully trying to electrolyse aqueous solutions of potash, during which he only succeeded in breaking apart the water, he reasoned that he needed to do away with the water and try to electrolyse molten caustic potash.   This he did on the sixth of October, 1807 using the large Voltaic pile he had built at the Royal Institute in London.   His younger cousin, Edmund Davy, was assisting Humphry at the time and he relates how when Humphry first saw "the minute globules of potassium burst through the crust of potash, and take fire as they entered the atmosphere, he could not contain his joy - he actually danced about the room in ecstatic delight;   some little time was required for him to compose himself sufficiently to continue the experiment"

 

Davy had every right to be delighted with this amazing new metal: it looked just like other bright, shiny metals but its density was less than that of water. This meant the metal would float on water --at least, it would do if it didn't explode as soon as it came into contact with the water.   Potassium is so reactive , it will even react and burn a hole through ice!   This was the first alkali metal to be isolated, but Davy went on to isolate sodium, calcium, magnesium and barium.

 

Whilst Davy named his new metal potassium after the potash [from whence it came], Berzelius, the Swedish chemist who invented the international system of chemical symbols now used by chemists the world over, preferred the name kalium for the metal, better reflecting its true origins, he thought.   Hence it is due a small salty herb that we now end up with the symbol K for the element pot-ash-ium, potassium.

Chris Smith

Cambridge Chemist Peter Wothers.   Next time beautiful but deadly is the name of the game.

 

Bea Perks

 

Arsenic gets its name from a Persian word for the yellow pigment now known as orpiment. For keen lexicographers apparently the Persian word in question Zarnikh was subsequently borrowed by the Greeks for their word arsenikon which means masculine or potent. On the pigment front, Napoleon's wallpaper just before his death is reported to have incorporated a so called Scheele's green which exuded an arsenic vapour when it got damp.

 

Chris Smith

So potent or not, licking the wallpaper in Napoleon's apartments is definitely off the menu.   That's Bea Perks who will be with us next time to tell us the deadly tale of arsenic, I hope you can join us.   I'm Chris Smith, thank you for listening and goodbye.

 

(Promo)

 

Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by thenakedscientists dot com. There's more information and other episodes of Chemistry in its element on our website at chemistryworld dot org forward slash elements. 

 

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Resources

Description :
A teaching resource on The Alkali Metals supported by video clips from the Royal Institution Christmas Lectures® 2012.
Description :
Many elements react with oxygen on heating. These reactions and the properties of their products illustrate the periodic nature of the elements.
Description :
A series of short, fun videos exploring the chemistry of the alkali metals, taken from a lecture by Dr. Peter Wothers at the University of Cambridge.
Description :
A series of short, fun videos exploring the chemistry of the alkali metals, taken from a lecture by Dr. Peter Wothers at the University of Cambridge.
Description :
This demonstration experiment can be used to show the flame colours given by alkali metal, alkaline earth metal, and other metal salts.
Description :
Filter paper soaked in potassium iodide solution which also contains starch and phenolphthalein is placed on an aluminium sheet which forms one electrode of an electric circuit. The other electrode i...
 

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References

 
Images:  Visual Elements © Murray Robertson 2011
Mining and Sourcing data:  British Geological Survey – natural environment research council.
Text:  John Emsley Nature’s Building Blocks: An A-Z Guide to the Elements, Oxford University Press, 2nd Edition, 2011.
Additional information for platinum, gold, neodymium and dysprosium obtained from Material Value Consultancy Ltd www.matvalue.com
Data: CRC Handbook of Chemistry and Physics, CRC Press, 92nd Edition, 2011.
G. W. C. Kaye and T. H. Laby Tables of Physical and Chemical Constants, Longman, 16th Edition, 1995.
Members of the RSC can access these books through our library.