First ionisation energy decreases down groups 1 and 2

Ionisation energy is a measure of the strength of electrostatic attraction between the outer electrons and the nucleus. The first ionisation energy of group 1 and group 2 elements decreases as you move down each group.

Ionisation energy decreases down groups 1 and 2 because:

• Nuclear charge - Increases down the group as more protons are added, increasing attraction for electrons.
• Atomic radius - Increases down the group as more electron shells are added, moving electrons away from nucleus.
• Electron shielding - Increases down group as more inner electron shells reduce nuclear attraction. The atomic radius and shielding effects down groups are greater than the nuclear charge effect, leading to an overall decrease in ionisation energies as you move down groups 1 and 2.

Reactivity increases down groups 1 and 2

The reactivity of group 1 and group 2 elements increases as you move down each group. This trend in reactivity directly correlates with the decrease in ionisation energy of the elements. As it becomes easier to remove the outer electron(s) from atoms further down the group, these elements become more reactive.

Group 1 elements are generally more reactive than group 2 elements in the same period because:

• Group 1 elements have only one electron to lose, while group 2 elements have two.
• The first ionisation energy of group 1 elements is lower than that of group 2 elements in the same period.

Reaction of group 1 and 2 elements with oxygen

When group 1 and 2 elements react with oxygen, they form solid white oxides. However, the nature of these oxides differs between the two groups.

Group 1 elements react vigorously with oxygen, following the general equation:

4M_(s) + O_2(g) ➔ 2M₂O_(s)

The products are oxides containing M⁺ and O^2- ions. For example, lithium reacts vigorously when heated in oxygen:

4Li_(s) + O_2(g) ➔ 2Li₂O_(s)

Group 2 elements also react with oxygen, but form different oxides:

2M_(s) + O_2(g) ➔ 2MO_(s)

In this case, the products contain M²⁺ and O^2- ions. For example, magnesium burns with a characteristic bright white flame:

2Mg_(s) + O_2(g) ➔ 2MgO_(s)

Reaction of group 1 and 2 elements with chlorine

Both group 1 and 2 elements react with chlorine gas to form solid white chlorides, but the resulting compounds have different formulas.

For group 1 elements, the general equation is:

2M_(s) + Cl_2(g) ➔ 2MCl_(s)

The products are chlorides containing M⁺ and Cl^- ions. The reactivity increases down the group, but this trend is less pronounced than with oxygen or water. For example, potassium reacts with chlorine gas when heated:

2K_(s) + Cl_2(g) ➔ 2KCl_(s)

For group 2 elements, the general equation is:

M_(s) + Cl_2(g) ➔ MCl_2(s)

Here, the products contain M²⁺ and Cl^- ions. Again, reactivity increases down the group. For example, magnesium burns vigorously when placed in chlorine gas:

Mg_(s) + Cl_2(g) ➔ MgCl_2(s)

Reaction of group 1 and 2 elements with water

The reactions of group 1 and 2 elements with water demonstrate clear trends in reactivity.

Group 1 elements react vigorously with water, producing hydrogen gas and metal hydroxides:

2M_(s) + 2H₂O_(l) ➔ 2MOH_(aq) + H_2(g)

The reactivity increases dramatically down the group. Lithium fizzes and floats on water, sodium may produce a flame, and potassium reacts so vigorously that the hydrogen produced usually ignites immediately. For example:

2K_(s) + 2H₂O_(l) ➔ 2KOH_(aq) + H_2(g)

Group 2 elements follow a similar pattern, but with some key differences:

M_(s) + 2H₂O_(l) ➔ 2M(OH)_2(aq) + H_2(g)

Beryllium doesn't react with water at all, while magnesium reacts very slowly. Calcium reacts more noticeably, producing effervescence, and the reactivity increases further with strontium and barium. For example:

Ca_(s) + 2H₂O_(l) ➔ 2Ca(OH)_2(aq) + H_2(g)

Interestingly, while magnesium reacts slowly with water, it reacts rapidly with steam to produce magnesium oxide and hydrogen:

Mg_(s) + H₂O_(g) ➔ MgO_(s) + H_2(g)