Elements show periodic relationships in their chemical and physical…

The idea

The periodic table arranges elements by increasing atomic number so that elements with similar properties recur at regular intervals. Physical properties such as melting point, boiling point, density and state at room temperature change gradually across a period and repeat in a pattern down a group. Chemical properties also repeat: Group 1 metals react vigorously with water, forming an alkaline metal hydroxide and hydrogen, and reactivity increases down the group; Group 7 halogens react with metals to form salts and reactivity decreases down the group; Group 0 noble gases are unreactive because they have full outer shells.

For example: Lithium, sodium and potassium all fizz in water and form alkaline solutions, showing a periodic relationship.

What earns the marks

• States that elements are arranged in order of increasing atomic number, so similar properties recur periodically.
• Describes a physical trend, such as melting point decreasing down Group 1 or boiling point increasing down Group 7.
• Describes a chemical trend, such as Group 1 metals reacting with water to form an alkaline hydroxide and hydrogen.
• Explains that Group 0 elements are unreactive because they have full outer electron shells.
• Compares reactivity trends, for example Group 1 reactivity increases down the group while Group 7 reactivity decreases.
• Uses a named example, such as sodium reacting with water or chlorine displacing bromine from potassium bromide solution.

Common mistakes and how to put them right

• Treating the periodic table as ordered by relative atomic mass rather than atomic number.

  Put it right by stating that atomic number determines position and repeating properties.

• Assuming all Group 1 reactions are equally vigorous.

  Put it right by noting reactivity increases down the group, so potassium reacts more vigorously than lithium.

• Saying noble gases react readily because they are in Group 0.

  Put it right by explaining their full outer shells make them unreactive.

Exam tips

• Quote the group number and a specific trend, then support it with one named element and observation.
• When asked to compare, give both directions, such as reactivity increases down Group 1 but decreases down Group 7.
• Use data from a table or graph to justify a trend rather than relying on memory alone.