Atoms, elements and compounds: There are about 100 different elements. Elements are shown in the periodic table.

The idea

An element is a pure substance made of only one type of atom. There are about 100 different elements. Each element has a unique symbol, such as H, O or Na, and is placed in the periodic table in order of increasing atomic number. The table groups elements with similar properties in vertical columns called groups, and shows metals on the left and non-metals on the right. For example, all Group 1 elements react vigorously with water, while Group 7 elements form coloured vapours. Knowing an element's position helps predict its reactivity and physical properties. Compounds are formed from elements by chemical reactions.

What earns the marks

• State that an element is a pure substance containing only one type of atom.
• Recall that there are about 100 different elements, each with a unique atomic number.
• Identify the periodic table as the arrangement of elements in order of increasing atomic number.
• Describe how groups organise elements with similar properties in vertical columns, while periods are horizontal rows.
• Use a given element symbol or position to predict its metal/non-metal character or reactivity trend.

Common mistakes and how to put them right

• Confusing elements with compounds.

  Put it right by checking the chemical formula: O₂ is an element, H₂O is a compound.

• Assuming elements in the same period have similar properties.

  Put it right by linking similar reactivity to group number.

Exam tips

• When asked to define an element, include both 'pure substance' and 'only one type of atom' to secure the mark.
• Use the periodic table provided in the exam to locate an element by symbol or atomic number rather than relying on memory.