IMAT Chemistry · Periodic Table

Alkali Metals, Alkaline Earth Metals, Halogens and Noble Gases

Each main group has a typical chemistry set by its valence electrons: group 1 and 2 metals lose them, halogens gain one, noble gases keep a full shell.

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Why this matters

Group questions ask you to recognise a group or element from its chemistry: group 2 from its reaction with water and its oxide in 2019, iodine from its colours and its chloride in 2017, the properties of group 1 in 2018, and in 2011 which pair of elements reacts most vigorously.

Concept 1 of 4: Group 1: the alkali metals

Every alkali metal has one electron alone in a new shell, held weakly. Giving it away is easy, so these metals react fast with water, oxygen and halogens, always forming 1+ ions. The further down the group, the further out that electron sits and the faster the reaction.

Definition

One valence electron ( $ns^1$ ); always the +1 oxidation state, as $\mathrm{M^+}$ ions.
Soft metals with low densities (lithium, sodium and potassium float on water) and low melting points that fall down the group.
React with cold water to give hydrogen and an alkaline hydroxide: $\mathrm{2M + 2H_2O \rightarrow 2MOH + H_2}$ . The reaction is more vigorous down the group.
Strong reducing agents: they reduce halogens to halide ions, e.g. $\mathrm{2Na + I_2 \rightarrow 2NaI}$ .
With hydrogen they form ionic hydrides such as $\mathrm{NaH}$ , in which hydrogen is −1.
Metallic bonding: the valence electrons are delocalised in the solid and in the liquid, so the metals conduct in both. They are stored under oil because they react with air and water.

Metal	Melting point (°C, approx.)	Reaction with cold water	Flame colour
Lithium	181	Fizzes steadily	Crimson red
Sodium	98	Melts into a ball and darts across the surface	Yellow-orange
Potassium	63	Melts and the hydrogen catches fire	Lilac
Rubidium	39	Violent	Red-violet
Caesium	28	Explosive	Blue-violet

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In a metal hydride, hydrogen is negative

Hydrogen is +1 in water and acids, but in ionic hydrides of groups 1 and 2 it is the

\mathrm{H^-}

ion, oxidation state −1. The metal is always positive.

Concept 2 of 4: Group 2: the alkaline earth metals

Group 2 metals have two valence electrons to lose instead of one, and a slightly stronger nuclear pull, so they are less reactive than their group 1 neighbours. They still react with water, faster further down the group. Their oxides are ionic solids that behave as bases.

Definition

Two valence electrons ( $ns^2$ ); the +2 oxidation state, as $\mathrm{M^{2+}}$ ions.
Oxides have the formula MO. They are ionic solids with high melting points, which do not conduct when solid, and are basic: they react with acids to give a salt and water.
With water: $\mathrm{M + 2H_2O \rightarrow M(OH)_2 + H_2}$ , more vigorous down the group. Magnesium reacts only slowly with cold water but quickly with steam: $\mathrm{Mg + H_2O \rightarrow MgO + H_2}$ .
Down the group, hydroxides become more soluble and sulfates less soluble.
Less reactive than the group 1 metal in the same period.

Metal	Reaction with water	Hydroxide	Note
Beryllium	No reaction, even with steam	$\mathrm{Be(OH)_2}$ , amphoteric	Its oxide is amphoteric, unlike the rest
Magnesium	Very slow with cold water; fast with steam, giving MgO	$\mathrm{Mg(OH)_2}$ , barely soluble	Burns with a bright white flame
Calcium	Steady fizzing with cold water	$\mathrm{Ca(OH)_2}$ , slightly soluble (limewater)	Limewater turns milky with $\mathrm{CO_2}$
Barium	Vigorous with cold water	$\mathrm{Ba(OH)_2}$ , quite soluble	$\mathrm{BaSO_4}$ is insoluble (used in X-ray imaging)

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The oxide formula gives the group away

A metal oxide

\mathrm{M_2O}

points to group 1, MO to group 2 and

\mathrm{M_2O_3}

to group 13. A metal that reacts with water and forms an ionic, basic oxide MO belongs to group 2.

Concept 3 of 4: Group 17: the halogens

A halogen atom is one electron short of a full shell, so it grabs one from a metal or shares one in a covalent bond. The smaller the atom, the harder it grabs: fluorine is the most reactive. Going down the group the molecules get bigger, so the forces between them grow and the elements go from gas to liquid to solid.

Definition

Seven valence electrons ( $ns^2\,np^5$ ); non-metals that exist as diatomic molecules $\mathrm{X_2}$ .
With metals they form halide ions $\mathrm{X^-}$ (oxidation state −1). Fluorine is only ever −1; chlorine, bromine and iodine also take positive states when bonded to oxygen or to a more electronegative halogen (iodine is +1 in ICl).
Down the group: melting and boiling points rise and colours darken; reactivity, oxidising power, electronegativity and first ionisation energy fall.
Displacement: a halogen oxidises the halide ions of any halogen below it. $\mathrm{Cl_2 + 2KBr \rightarrow 2KCl + Br_2}$ .
Iodine dissolves poorly in water but well in potassium iodide solution, giving a brown solution, and turns starch blue-black.

Halogen	State at room temperature	Colour	Oxidising power
Fluorine, $\mathrm{F_2}$	Gas	Pale yellow	Strongest
Chlorine, $\mathrm{Cl_2}$	Gas	Yellow-green	Strong
Bromine, $\mathrm{Br_2}$	Liquid	Red-brown	Moderate
Iodine, $\mathrm{I_2}$	Solid	Shiny grey-black solid; violet vapour when heated	Weakest of the four

Practice this conceptself-check · 3 quick reps

Halogen reactivity falls down the group

Fluorine is the most reactive halogen and iodine the least of the four common ones, the opposite of group 1. Electronegativity and first ionisation energy also fall down the group, while melting and boiling points rise.

Concept 4 of 4: Group 18: the noble gases

A noble gas atom already has a full outer shell, which is a very low-energy, stable arrangement. It has no electron to spare and no room for one more, so it hardly reacts at all and its atoms do not even pair up into molecules.

Definition

Full outer shells: helium $1s^2$ (2 electrons); the others $ns^2\,np^6$ (8 electrons).
Monatomic gases: they exist as single atoms.
Very unreactive, because each has the highest first ionisation energy in its period and gains no energy by accepting an electron.
Boiling points rise down the group (stronger London forces between larger atoms); helium has the lowest boiling point of any substance.
Not completely inert: xenon forms compounds with fluorine and oxygen, such as $\mathrm{XeF_4}$ .

Gas	Outer-shell electrons	Typical use
Helium	2	Balloons (light and non-flammable); cooling the magnets in MRI scanners
Neon	8	Advertising signs that glow red
Argon	8	Inert atmosphere for welding; filling light bulbs
Xenon	8	Bright lamps; forms compounds such as $\mathrm{XeF_4}$

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Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (4)

Group 1: the alkali metals5 rows

Metal	Melting point (°C, approx.)	Reaction with cold water	Flame colour
Lithium	181	Fizzes steadily	Crimson red
Sodium	98	Melts into a ball and darts across the surface	Yellow-orange
Potassium	63	Melts and the hydrogen catches fire	Lilac
Rubidium	39	Violent	Red-violet
Caesium	28	Explosive	Blue-violet

Group 2: the alkaline earth metals4 rows

Metal	Reaction with water	Hydroxide	Note
Beryllium	No reaction, even with steam	$\mathrm{Be(OH)_2}$ , amphoteric	Its oxide is amphoteric, unlike the rest
Magnesium	Very slow with cold water; fast with steam, giving MgO	$\mathrm{Mg(OH)_2}$ , barely soluble	Burns with a bright white flame
Calcium	Steady fizzing with cold water	$\mathrm{Ca(OH)_2}$ , slightly soluble (limewater)	Limewater turns milky with $\mathrm{CO_2}$
Barium	Vigorous with cold water	$\mathrm{Ba(OH)_2}$ , quite soluble	$\mathrm{BaSO_4}$ is insoluble (used in X-ray imaging)

Group 17: the halogens4 rows

Halogen	State at room temperature	Colour	Oxidising power
Fluorine, $\mathrm{F_2}$	Gas	Pale yellow	Strongest
Chlorine, $\mathrm{Cl_2}$	Gas	Yellow-green	Strong
Bromine, $\mathrm{Br_2}$	Liquid	Red-brown	Moderate
Iodine, $\mathrm{I_2}$	Solid	Shiny grey-black solid; violet vapour when heated	Weakest of the four

Group 18: the noble gases4 rows

Gas	Outer-shell electrons	Typical use
Helium	2	Balloons (light and non-flammable); cooling the magnets in MRI scanners
Neon	8	Advertising signs that glow red
Argon	8	Inert atmosphere for welding; filling light bulbs
Xenon	8	Bright lamps; forms compounds such as $\mathrm{XeF_4}$

Watch out for (3)

In a metal hydride, hydrogen is negative
→ Group 1: the alkali metals
The oxide formula gives the group away
→ Group 2: the alkaline earth metals
Halogen reactivity falls down the group
→ Group 17: the halogens

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