Magnesium (Mg) is in Period 3 and Group 2 of the periodic table. Silicon (Si) is also in Period 3 but in Group 4. Explain how the positions of magnesium and silicon in the periodic table reflect the structure of their atoms, and describe one trend you would expect to observe as you move across Period 3 from left to right.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Scientists use the periodic table to predict the properties and structures of elements. Elements in the same period have the same number of electron shells, whilst elements in the same group share similar chemical properties.
Model answer (5 marks)
Magnesium is in Group 2, so its valence shell contains 2 electrons – the outermost shell is the 3rd energy level, giving Mg a configuration of 1s²2s²2p⁶3s².
Silicon is in Group 4, so its valence shell contains 4 electrons – the outermost shell is also the 3rd energy level, giving Si a configuration of 1s²2s²2p⁶3s²3p².
Both elements are in Period 3, therefore each has three electron shells (n = 1, 2, 3). The different numbers of valence electrons explain why Mg is an alkaline‑earth metal and Si is a metalloid.
Across Period 3, the number of valence electrons increases from 1 to 8, so the metallic character decreases and the reactivity of the metals falls while the reactivity of the non‑metals rises.
Silicon is in Group 4, so its valence shell contains 4 electrons – the outermost shell is also the 3rd energy level, giving Si a configuration of 1s²2s²2p⁶3s²3p².
Both elements are in Period 3, therefore each has three electron shells (n = 1, 2, 3). The different numbers of valence electrons explain why Mg is an alkaline‑earth metal and Si is a metalloid.
Across Period 3, the number of valence electrons increases from 1 to 8, so the metallic character decreases and the reactivity of the metals falls while the reactivity of the non‑metals rises.
Examiner tips
- State the group number to justify valence electrons. Mention the period to give the number of shells. Explain how valence electrons relate to chemical behaviour. Give a clear trend (e.g. decreasing metallic character).
Common mistakes
- Confusing the number of valence electrons with the total electron count. Forgetting that both elements share the same principal quantum number. Describing the trend as ‘reactivity increases’ for all elements rather than specifying metals vs non‑metals.
Mark scheme (5 marks)
- Magnesium has 2 electrons in its outer shell / highest energy level because it is in Group 2
- Silicon has 4 electrons in its outer shell / highest energy level because it is in Group 4
- Both magnesium and silicon are in Period 3, so both have 3 electron shells / 3 energy levels
- Elements in the same group have similar chemical properties / react in similar ways
- Across Period 3 from left to right, the number of outer electrons increases (from 1 to 8) / reactivity of metals decreases OR reactivity of non-metals increases / metallic character decreases
Key terms in this question
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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