Chapter 9 · Electrons in Atoms and the Periodic Table
9.9Periodic Trends: Atomic Size, Ionization Energy, and Metallic Character
12 min · two checks
Predict
Which is larger, a potassium atom or a bromine atom in the same period?
The idea
- Predict atomic size, ionization energy, and metallic character from position.
- Explain the trends with effective nuclear charge and shell number.
Atomic radius increases down a group because a new principal shell is occupied, farther from the nucleus. It decreases across a period because protons are added while the new electrons enter the same shell and shield each other poorly. The nucleus gets a stronger grip. First ionization energy — the energy to remove the outermost electron — runs roughly opposite to size: high at the top right, low at the bottom left. Noble gases are peaks. Small dips exist (Be to B, N to O) because of subshell and pairing details; the overall trend is the one to master first.
Metallic character tracks how easily an element loses electrons: high at the bottom left, low at the top right. That is why Group 1 gets more reactive downward, and why fluorine is a more aggressive nonmetal than iodine. Cations are smaller than their atoms; anions are larger. You can reason that without a chart: fewer electron–electron repulsions shrink a cation, and extra repulsions swell an anion.
Keep these
- Size: down and to the left. Ionization energy: up and to the right.
- Metallic character follows easy loss of electrons: down and to the left.
- Cations shrink relative to the atom. Anions grow.
Worked path
Rank S, Cl, and Se by increasing atomic radius, and say which has the largest first ionization energy.
Observe
S and Cl are in period 3. Se is below S.