VCE · Topic explainer

How to read the periodic table of elements

The periodic table arranges elements by atomic number, the number of protons in each atom. Rows are periods and columns are groups. Main-group elements in the same group often react in similar ways. Use the data-book legend to distinguish atomic number from relative atomic mass and to locate electronegativity values.

Worked example

Find chlorine in group 17 and period 3. Its atomic number is 17, so a neutral chlorine atom has 17 protons and 17 electrons. A chloride ion has one additional electron and the same 17 protons.

The relative atomic mass is about 35.5 because the value averages the naturally occurring isotopes by abundance. Use the relative atomic mass for a molar mass calculation, and the atomic number when you need to count protons.

Atomic number identifies the element. Period is a row. Group is a column.. Relative atomic mass averages isotopes.

Official exam practice

2025 VCE Chemistry data book, Section 16

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