The elements in the periodic table are arranged in order of atomic…

The idea

The periodic table orders elements by increasing atomic (proton) number, which is the number of protons in the nucleus and equals the number of electrons in a neutral atom. Reading left to right across a period, one proton and one electron are added each time, so nuclear charge and outer-shell occupancy change gradually. Reading down a group, atoms gain filled inner shells but keep the same outer-shell electron count, so chemical behaviour repeats. For example, sodium (atomic number 11) sits in Group 1 with lithium (3) and potassium (19) because each has one outer electron, while chlorine (17) sits in Group 7 with fluorine (9) because each has seven. Columns are called groups; rows are periods.

What earns the marks

• State that elements are placed in order of increasing atomic (proton) number, not relative atomic mass.
• Identify that the atomic number equals the number of protons and, in a neutral atom, the number of electrons.
• Explain that vertical columns are called groups and that elements within a group show similar chemical properties.
• Link similar properties to the same number of outer-shell electrons rather than to equal total electron numbers.
• Use a named example, such as lithium, sodium and potassium in Group 1, to show the pattern.
• Recognise that horizontal rows are periods and that across a period the outer-shell electron count increases by one per element.

Common mistakes and how to put them right

• Saying elements are arranged in order of relative atomic mass.

  Put it right by stating the order is increasing atomic (proton) number, which resolves anomalies such as argon before potassium.

• Confusing groups with periods.

  Put it right by defining groups as vertical columns with similar properties and periods as horizontal rows.

• Claiming all atoms in a group have identical electron arrangements.

  Put it right by saying they share the same number of outer-shell electrons while inner shells differ.

Exam tips

• When asked why elements are ordered as they are, name atomic (proton) number explicitly and define it as proton count.
• Support any statement about similar properties with a group example and the shared outer-electron count.
• Check whether a question asks about position, order or properties, and answer each part separately to avoid losing credit.