2.4.2 Reactivity - Groups 1 & 2

Group 1 metals

• The reactivity of the group 1 metals increases as you go down the group
• When a group 1 element reacts its atoms only need to lose electron, as there is only 1 electron in the outer shell
  • When this happens, 1+ ions are formed

• The next shell down automatically becomes the outermost shell and since it is already full, a group 1 ion obtains noble gas configuration
• As you go down group 1, the number of shells of electrons increases by 1
  • This means that the outermost electron gets further away from the nucleus, so there are weaker forces of attraction between the outermost electron and the nucleus
  • Less energy is required to overcome the force of attraction as it gets weaker, so the outer electron is lost more easily
  • So, the alkali metals get more reactive as you descend the group

Group 2 metals

• The reactivity of the Group 2 metals also increases down the group for the same reasons 
  • The outermost electron gets further away from the nucleus, so there are weaker forces of attraction between the outermost electron and the nucleus
  • Less energy is required to overcome the force of attraction as it gets weaker, so the outer electron is lost more easily
• This can be observed when the Group 2 metals react with water as well
  • Magnesium reacts extremely slowly with cold water
  • Calcium reacts fairly vigorously with cold water in an exothermic reaction

Group 1 metals 

Reaction with oxygen

• The alkali metals react with oxygen in the air forming metal oxides, which is why the alkali metals tarnish when exposed to the air
• The metal oxide produced is a dull coating which covers the surface of the metal
• The metals tarnish more rapidly as you go down the group

Summary of the Reactions of the First Three Alkali Metals with Oxygen

Reaction with chlorine 

• The reactions of the Group 1 elementrs with chlorine are similar in appearance to the reactions of the Group 1 metals with oxygen
• Sodium, for example, burns with an intense orange flame in chlorine in exactly the same way that it does in pure oxygen
• The other metals also behave the same way in both gases
  • In each case, a white solid is formed, the simple chloride

2M (s) + Cl₂ (g) → 2MCl (s) 

Reactions with water

• The reactions of the alkali metals with water get more vigorous as you descend the group

Summary of the Reactions of the First Three Alkali Metals with Water

Group 2 metals

Reactions with water and oxygen

• The reaction of Group 2 metals with oxygen follows the following general equation:

2M (s) + O₂ (g) → 2MO (s)

• Where M is any metal in Group 2
• Remember than Sr and Ba also form a peroxide, MO₂
• The reaction of all metals with water follows the following general equation:

M (s) + 2H₂O (l) → M(OH)₂ (s) + H₂ (g)

• Except for, Be which does not react with water

Group 2 Metals Reacting with Water and Oxygen - Equations

• Magnesium reacts extremely slowly with cold water:

Mg (s) + 2H₂O (l) → Mg(OH)₂ (aq) + H₂ (g)

• The solution formed is weakly alkaline (pH 9-10) as magnesium hydroxide is only slightly soluble
• However, when magnesium is heated in steam, it reacts vigorously with steam to make magnesium oxide and hydrogen gas:

Mg (s) + H₂O (g) → MgO (s) + H₂ (g)

Reactions with chlorine

• Group 2 metals react with chlorine gas to give the metal chloride 
  • For example

Mg (s) + Cl₂ (g) → MgCl₂ (s)