Alkali metals: what they are, characteristics

You alkali metals are the metallic elements of group 1 of the Periodic table. Hydrogen, despite being in this group, is not considered an alkali metal, as it is not a metallic element. All alkali metals have a single valence electron, located in the s subshell. Alkali metals are reactive, have low density and melting point, in addition to a low first ionization energy. In compounds, they are always in the M+ form, that is, with a charge equal to +1.

Read too: Non-metals — the main aspects of non-metallic chemical elements

Topics of this article

• 1 - Summary of alkali metals
• 2 - What are the alkali metals?
• 3 - Properties of alkali metals
• 4 - Characteristics of alkali metals
• 5 - Applications of alkaline metals
• 6 - Differences between alkaline and alkaline earth metals
• 7 - Solved exercises on alkaline metals

• They are the chemical elements from group 1 of the Periodic Table, except for hydrogen.
• They have a single valence electron, located in the s subshell.
• They are of low density and have a low melting point.
• In them the first ionization energy is very low, but the second ionization energy is very high. Therefore, they are reactive and present with a +1 charge in compounds.
• Those with the greatest number of everyday applications are lithium, sodium and potassium. Cesium and rubidium have more laboratory applications, while francium, very rare and synthetic, has no practical applications.

You metals alkaline correspond to group 1 elements of the Periodic Table, except for the hydrogen:

• lithium, symbol Li, Z = 3;
• sodium, symbol Na, Z = 11;
• potassium, symbol K, Z = 19;
• rubidium, symbol Rb, Z = 37;
• cesium, symbol Cs, Z = 55;
• francium, Fr symbol, Z = 87.

Do not stop now... There's more after the publicity ;)

All alkali metals have only one electron of valence, located in a sublevel s. are also the elements of greater atomic radius of their respective periods and therefore present a first ionization energy low compared to the other elements.

As a consequence, are quite reactive and present themselves as ions M⁺ in their compounds and in solution. The second ionization energy is very high, which prevents the formation of M ions.²⁺ of alkali metals. Check out, in the table below, some of these mentioned properties.

Metal	electronic configuration	metallic radius (Å)	Ionization energy (kJ.mol^-1)	electronegativity
1ª	2ª
read	[He] 2s^1	1,52	520,1	7296	0,98
At	[Ne] 3s^1	1,86	495,7	4563	0,93
K	[Air] 4s^1	2,27	418,7	3069	0,82
Rb	[Kr] 5s^1	2,48	402,9	2640	0,82
cs	[Xe] 6s^1	2,65	375,6	2260	0,79

Important: Francium is a synthetic element, the second rarest on the planet, obtained by nuclear processes and difficult to isolate. Therefore, a good part of its properties are estimated, and its instability (its most stable isotope has half life of 22 minutes) makes it difficult to collect experimental data.

Pieces of potassium metal placed in mineral oil so as not to react.

A low electronegativity of alkali metals makes it clear that their compounds will be essentially ionic, with no possibility of forming covalent compounds; it also justifies the behavior of the metallic character, which grows towards cesium.

The alkali metals have grayish coloring, with the exception of cesium, which has a yellowish color. They are also characterized by low melting points, softness (some can be cut with a knife), low density (Lithium, for example, is the least dense metal in the entire Periodic Table) and very exothermic reaction with water. The following table provides more information about alkali metals.

Metal	Melting point (°C)	Boiling point (°C)	Density (g.cm^-3, 20°C)
read	180,5	1326	0,534
At	97,8	883	0,968
K	63,7	756	0,856
Rb	39,0	688	1,532
cs	28,5	690	1,90

Alkaline metals also stand out for their good electrical conductivity, and present a different flame coloring when being vaporized: crimson for lithium; yellow for sodium; violet for potassium; reddish violet for rubidium; and blue for cesium.

Scheme demonstrating the differences between alkali metals in the flame test.

Of the alkali metals, rubidium and cesium have fewer commercial and industrial applications, becoming more restricted to academic applications. Very low stability francium has no practical and known applications so far. Lithium, sodium and potassium have compounds with considerable commercial value.

• Lithium: taking advantage of its low density, it is applied in metallic alloys with magnesium It is aluminum, which can be used in aerospace craft components. Added to the low density, the high oxidation potential of lithium (+3.02 V) makes it used in the manufacture of batteries (lithium ion), widely used in electronics such as cell phones, tablets, notebooks, smartwatches, among others. others. Finally, lithium carbonate is used as a drug to treat bipolar disorder (manic depression), although large amounts of this metal can damage the central nervous system.
• Sodium: Of the sodium compounds, the one that stands out the most and has the greatest production is the sodium chloride (NaCl), not only because it is the basic constituent of table salt, the most used seasoning in the world, but because it can be used to make sodium hydroxide (NaOH) and the gas chlorine (Cl₂). Although it is not part of our Brazilian reality, NaCl is also widely used to clean roads frozen in winter, as it is capable of lowering the freezing point of water and thus unclogging pathways taken by snow. However, for environmental reasons, it has been replaced by acetate calcium and magnesium. Sodium and metallic alloys of sodium and potassium are used as nuclear reactors, mainly due to the low melting point, low viscosity and high thermal capacity.

• Potassium: in the form of potassium chloride, plays an important role in the fertilizers, as it is part of the macronutrients of the plant. Sodium and potassium, by the way, are among the approximately 25 essential elements for animal life. In our organism, the concentrations of Na⁺ and K⁺, inside and outside the cell fluids, is different, creating a potential difference responsible for the transmission of nerve impulses.

Truck throwing sodium chloride on the track to melt the ice.

Although similar in some important ways, alkali metals have some differences from alkaline earth metals.

In the first place, you alkaline earth metals include the elements of group 2 of the Periodic Table. They are less reactive than the alkali metals and are somewhat brittle. Yet, have two electrons in valence shell, unlike alkali metals, which only have one. Therefore, alkaline earths only have a charge equal to +2 in their compounds.

Know also: Noble metals — metals less reactive than hydrogen

question 1

(Ufscar) In the Periodic Classification, column 1 refers to alkaline elements and column 17 refers to halogens. Alkali metals like lithium, sodium and cesium react with halogen gases like Cl₂. The products of the reactions of lithium, sodium and cesium metals with Cl gas₂, are ionic solids whose formulas are, respectively,

A) LiCl₂, NaCl, CsCl.

B) LiCl, NaCl₂, CsCl.

C) LiCl₂, NaCl₂, CsCl₂.

D) LiCl₃, NaCl₃, CsCl₃.

E) LiCl, NaCl, CsCl.

Resolution:

Alternative E

Alkali metals, when reacting with chlorine gas, make the following reaction:

2 M + Cl₂ → 2 MCl

In the product, alkali metals must have an oxidation number equal to +1.

question 2

(Uespi) In the periodic classification of elements, we find the alkali metals. The meaning of alkali is “plant ashes”, in which we can find mainly sodium and potassium. In relation to this family, mark the correct alternative.

A) It is called alkali metals because of the ease with which they donate protons.

B) Their elements have similar properties, mainly because they have a valence electron.

C) They are highly electronegative, due to their ability to donate electrons.

D) They carry out ionic bonds, being electron acceptors.

E) They are less reactive elements.

Resolution:

Alternative B

Because sodium and potassium are in the same group, it's natural for them to have similar properties.

Choice A is incorrect because alkali metals do not break down protons with ease, but electrons (more specifically the valence ones).

Choice C is incorrect as these elements are not highly electronegative.

Option D is incorrect because, despite performing ionic bonds, these metals tend to donate electrons.

Choice E is incorrect because alkali metals are very reactive.

By Stefano Araujo Novais
Chemistry teacher

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Learn more about francium, its characteristics, properties, production, applications and history.

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Have you ever heard of the chemical element potassium? Click here and learn about its characteristics, properties, applications, production and history, in addition to its biological role.

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