2_1 s-Block Elements: Electron Configurations and Where They Occur

The s-block is the two columns on the far left of the periodic table: group 1 (IA) and group 2 (IIA). They get the name because the last electron added in each element goes into an s orbital of the outermost shell. Group 1, apart from hydrogen, is the alkali metals: lithium, sodium, potassium, rubidium, caesium and francium. Group 2 is the alkaline earth metals: beryllium, magnesium, calcium, strontium, barium and radium. These include the most electropositive and reactive metals of all, and almost every property they have traces back to their simple outer electron arrangement.

Alkali Metals: One Outer Electron

Every alkali metal has a noble gas core plus a single electron in the next s orbital, written

ns^1

. That lone electron is shielded from the nucleus by all the inner shells, so it is held weakly and the first ionization energy (the energy to remove one electron from each atom in a mole of gaseous atoms) is very low. Losing it leaves a stable noble gas configuration, so these metals form

\mathrm{M^+}

ions, show only the +1 oxidation state and make ionic compounds. The name comes from an Arabic word for plant ashes, which are rich in sodium and potassium compounds, and the metals all react with water to give strongly alkaline solutions.

Alkali metals: configuration and selected properties

Property	Li	Na	K	Rb	Cs
Configuration	[He]2s¹	[Ne]3s¹	[Ar]4s¹	[Kr]5s¹	[Xe]6s¹
1st ionization energy (kJ/mol)	520	496	419	403	376
Electronegativity	1.0	0.9	0.8	0.8	0.7
Melting point (°C)	181	98	63	39	28
Density (g/cm³)	0.53	0.97	0.86	1.53	1.93

Going down the group, each element has one more occupied shell than the one above, so atoms and ions get larger and ionization energy falls. Metallic bonding weakens too, because one electron per atom is shared over a larger, more diffuse lattice. That is why the melting point drops so steeply that caesium melts on a warm day. Density generally rises, but potassium is slightly less dense than sodium because its atoms gain more in volume than in mass.

Alkaline Earth Metals: Two Outer Electrons

Group 2 atoms have the configuration

ns^2

. Both outer electrons are lost to give

\mathrm{M^{2+}}

ions, so the usual oxidation state is +2. Removing two electrons costs more energy than removing one, but the extra energy is more than repaid by the stronger lattice and hydration energies of a doubly charged ion. Compared with the alkali metal in the same period, each group 2 atom has one more proton pulling on the same shell, so it is smaller, harder, denser and less reactive.

Alkaline earth metals: configuration and selected properties

Property	Be	Mg	Ca	Sr	Ba
Configuration	[He]2s²	[Ne]3s²	[Ar]4s²	[Kr]5s²	[Xe]6s²
1st ionization energy (kJ/mol)	899	738	590	549	503
Electronegativity	1.5	1.2	1.0	1.0	0.9
Melting point (°C)	1287	650	842	777	727
Radius of M²⁺ ion (pm)	31	65	99	113	135

The same downward trends appear: larger atoms, lower ionization energy and greater reactivity. Melting points are much higher than in group 1 because each atom contributes two electrons to the metallic bonding. They do not fall smoothly, because the metals do not all share the same crystal structure.

Why They Are Never Found Free

Metals that lose electrons this readily react with oxygen, water and carbon dioxide almost as soon as they are exposed. Over geological time, all of them have ended up as compounds: chlorides, carbonates, sulphates, silicates and phosphates. Sodium and potassium are each a few percent of the Earth's crust, mostly locked in aluminosilicate rocks, while dissolved sodium and magnesium salts make sea water salty. Rubidium and caesium are rare and turn up alongside potassium minerals. Francium is so radioactive and short-lived that only traces exist, and radium is also rare and radioactive.

Some important s-block minerals

Metal	Mineral	Formula
Lithium	Spodumene	LiAl(SiO₃)₂
Sodium	Rock salt (halite)	NaCl
Sodium	Chile saltpetre	NaNO₃
Sodium	Trona	Na₂CO₃·NaHCO₃·2H₂O
Potassium	Sylvite	KCl
Potassium / magnesium	Carnallite	KCl·MgCl₂·6H₂O
Beryllium	Beryl	Be₃Al₂(SiO₃)₆
Magnesium	Magnesite / dolomite	MgCO₃ / MgCO₃·CaCO₃
Calcium	Calcite (limestone) / gypsum	CaCO₃ / CaSO₄·2H₂O
Calcium	Fluorite / phosphorite	CaF₂ / Ca₃(PO₄)₂
Strontium / barium	Strontianite / barite	SrCO₃ / BaSO₄

Calcium carbonate is a good example of how these minerals move around. It is almost insoluble in pure water, but rainwater carrying dissolved carbon dioxide slowly converts it into soluble calcium hydrogencarbonate. This is how limestone caves form and why water from chalky ground is hard. When the water evaporates or loses

\mathrm{CO_2}

, the reaction runs backwards and deposits calcium carbonate again as stalactites or kettle scale.

[Ca+2].[O-]C([O-])=O.O=C=O.O>>[Ca+2].OC(=O)[O-].OC(=O)[O-]

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s-Block Elements in Living Things

Several s-block ions are essential to life. A magnesium ion sits at the centre of every chlorophyll molecule. Calcium, mostly as calcium phosphate and calcium carbonate, forms bones, teeth, shells and egg shells. Sodium and potassium ions carry nerve impulses across cell membranes, because their different concentrations inside and outside a cell set up an electrical potential.

Tips for remembering

• One for group 1, two for group 2: the group number is the number of outer s electrons, and also the charge of the ion the metal forms (M⁺ or M²⁺).
• Too reactive to be free: an element that gives up its outer electrons this easily has already done so in nature, so s-block metals are always found as compounds.
• Bigger down the group, easier to ionize: each step down adds a shell, so the outer electron sits further out and the ionization energy falls.