Crossing a period
Period 3 runs from sodium (Na) to argon (Ar): Na, Mg, Al, Si, P, S, Cl, Ar. All have three occupied electron shells, but the number of valence electrons increases from 1 to 8 across the period, and the proton number rises steadily.
Trends across the period
- Atomic size decreases across the period: the growing nuclear charge pulls the same-shell electrons in more tightly.
- Electronegativity increases: atoms attract bonding electrons more strongly from left to right.
- Character changes from metallic (Na, Mg, Al) through metalloid (Si) to non-metallic (P, S, Cl), ending with the noble gas Ar.
Key idea
Across Period 3, atomic radius decreases and non-metallic character increases because the nuclear charge increases while the number of shells stays the same.
Oxides
The nature of the oxides changes across the period: metal oxides (Na2O, MgO) are basic, aluminium oxide (Al2O3) is amphoteric (reacts with both acids and alkalis), and non-metal oxides (SO2, SO3, P4O10) are acidic.
Worked example
Which of Na and Cl has the larger atom? Both are in Period 3, but Cl has more protons (17 vs 11). The greater nuclear charge pulls the electrons in more tightly, so Na has the larger atom.
Remember
- Same number of shells; valence electrons 1 → 8.
- Atomic size decreases; electronegativity increases.
- Oxides: basic → amphoteric → acidic.