Elements in the periodic table are arranged in periods and groups. Explain what the period number and the group number of an element tell you about the structure of its atoms.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The period number tells you the number of electron shells (energy levels) in the atom.
The group number tells you the number of electrons in the outer shell (outermost energy level).
For example, sodium is in period 3, so it has 3 electron shells; it is also in group 1, so it has 1 electron in its outer shell.
Elements in the same group have similar chemical properties because they have the same number of outer‑shell electrons.
The group number tells you the number of electrons in the outer shell (outermost energy level).
For example, sodium is in period 3, so it has 3 electron shells; it is also in group 1, so it has 1 electron in its outer shell.
Elements in the same group have similar chemical properties because they have the same number of outer‑shell electrons.
Examiner tips
- Use the exact terms "electron shells" and "outer‑shell electrons". Give a clear example linking period to shells and group to outer electrons. Show the reasoning that similar outer‑shell electrons give similar properties.
Common mistakes
- Confusing period with group or vice‑versa. Saying period tells about valence electrons instead of shells. Omitting the example or the link to chemical properties.
Mark scheme (4 marks)
- The period number tells you the number of electron shells (energy levels) in the atom
- The group number tells you the number of electrons in the outer shell (outermost energy level)
- Correct application: a named example linking period to number of shells OR group to outer electrons (e.g. sodium is in period 3 so it has 3 shells; sodium is in group 1 so it has 1 electron in its outer shell)
- Elements in the same group have similar chemical properties because they have the same number of outer shell electrons
Key terms in this question
Related
- All Eduqas GCSE Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Atomic structure and the periodic table questions
- Explain how the atomic number and mass number of an atom can be used to determin…
- Describe how scientists' understanding of atomic structure changed with the deve…
- Explain why atoms have no overall electrical charge, even though they contain ch…
- Isotopes of an element have the same chemical properties but different physical …