Group I and Group II

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Lithium, sodium and potassium all belong to Group 1.

This is because they all have 1 electron in their outer shell which is why they react in similar ways.

Properties:

Soft metals that can be cut with a knife.

Low density - can float on water.

Low melting points in comparison with other metals.

They react violently (in some cases) with water to form alkaline solutions - hence the name, alkali metals.

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Reactivity increases as you descend the group.

Potassium is more reactive than lithium, since although they both need to lose one electron to have full outer shells, potassium's outer electron is furthest from the positive attractions of the nucleus. Therefore, it is easier for potassium to lose its outer electron than it is for lithium.

Other trends:

Melting point and boiling point decreases down the group.

Magnesium, Calcium and Strontium all belong to Group 2.

All Group 2 elements have two outer electrons, therefore they wish to lose two when bonding to create compounds. Losing two electrons allows them to have full outer shells, and achieve stability.

Properties:

Silvery metals.

Higher melting and boiling points than Group I elements.

Less reactive than Group I elements. This is because it is more difficult to lose two electrons compared to losing just one electron.

React with water to form alkaline solutions. Reactivity increases down the group. This is because the smaller the atom the closer the outer electrons are to the nucleus. Therefore there is a greater attraction between the nucleus and electrons in magnesium than there is in calcium.

Melting points and boiling points decrease down the group due to weaker forces of attraction between atoms.

Hardness increases as you descend down the group.

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