Explain how the atomic structure of metals and non-metals relates to their…

The idea

An element's position in the periodic table is determined by its atomic structure, particularly its atomic number and electron arrangement. Metals occupy the left and centre of the periodic table because their atoms have relatively few outer electrons and lose them easily to form positive ions. Non-metals sit on the right and top, with atoms that have more outer electrons and tend to gain or share electrons. For Groups 1 to 7, the group number equals the number of outer electrons. Group 0 elements, however, have full outer shells (2 for helium, 8 for the rest). The period number equals the number of occupied electron shells.

For example: Sodium (Group 1, Period 3) has one outer electron and three shells; chlorine (Group 7, Period 3) has seven outer electrons and three shells.

What earns the marks

• For elements in Groups 1 to 7, the group number equals the number of electrons in the outer shell, which controls chemical behaviour.
• Group 0 noble gases have full outer shells (two electrons for helium, eight for the others), which explains their position on the far right and lack of reactivity.
• The period number equals the number of occupied electron shells in the atom.
• Metals are found on the left and in the centre because they have few outer electrons and lose them to form positive ions.
• Non-metals are found on the right and towards the top because they have more outer electrons and gain or share electrons to achieve a full shell.

Common mistakes and how to put them right

• Stating that the group number always equals the number of outer electrons.

  Put it right: This is true for Groups 1 to 7, but Group 0 elements have full outer shells (8 electrons, except helium which has 2).

• Confusing group number with period number.

  Put it right: Group number relates to outer electrons, while period number gives the number of occupied shells.

• Thinking atomic number equals the number of outer electrons.

  Put it right: Atomic number is the number of protons, which equals the total electrons in a neutral atom, not just the outer ones.

Exam tips

• When explaining reactivity, connect the number of outer electrons to the ease of losing or gaining electrons to achieve a full shell.
• Remember to explicitly mention helium's two outer electrons if discussing Group 0, as it is an exception to the octet rule.