2.3 · Periodic TrendsPractice · 元素周期律

Ca is [Ar]4s²; its last electrons go into 4s, so it is in the s block (group 2).

Across a period the number of shells stays the same while the nuclear charge grows, pulling the electrons in, so atoms shrink. Covalent radii: Na 166 pm, Mg 141 pm, Si 111 pm, Cl 102 pm. Na, at the left, is largest.

Down a group, each atom has another shell, so the outer electron is farther from the nucleus and easier to remove; the first ionization energy falls. Data (kJ/mol): Li 520, Na 496, K 419, Rb 403. Li is largest.

Electronegativity rises across a period and falls down a group, so F, at the upper right, is the highest of all elements. Values: O 3.44, F 3.98, Cl 3.16, N 3.04.

In water, an alkali metal loses its single outer electron. Down the group that electron comes off more easily (first ionization energies, kJ/mol: Li 520, Na 496, K 419, Rb 403), so reactivity increases. Rb reacts most violently of these.

Noble gases have full outer shells (1s² for He, ns²np⁶ for the rest). They neither lose electrons easily (they have the highest ionization energy in each period) nor gain them, so they rarely react. Being a gas has nothing to do with it.

The proton count is unchanged but there is one more electron, so electron–electron repulsion grows and the cloud spreads out: anions are larger than their atoms (Cl atom 102 pm, $\ce{Cl-}$ 181 pm). Cations are the opposite: they lose electrons, often a whole shell, and shrink.

Chemical reactions mostly involve the outer (valence) electrons. Elements in a group share the same valence configuration (every alkali metal is ns¹), so they behave alike. Elements with the same number of shells are in the same period, not group.

Mg is 1s²2s²2p⁶3s², with 10 inner electrons (n = 1 and 2). Effective nuclear charge ≈ 12 − 10 = 2. The inner electrons screen most of the nuclear charge, so the 3s electrons feel only about +2.

Melting / boiling points: $\ce{F2}$ −219.6 / −188.1 °C; $\ce{Cl2}$ −101.5 / −34 °C; $\ce{Br2}$ −7.2 / 58.8 °C; $\ce{I2}$ 113.7 / 184.3 °C. Room temperature (about 25 °C) falls between bromine’s melting and boiling points, so $\ce{Br2}$ is a liquid; $\ce{F2}$ and $\ce{Cl2}$ are gases and $\ce{I2}$ is a solid. Down the group, larger molecules have stronger dispersion forces and higher melting and boiling points.