Chemistry · Ch 8 — Elements of Group 1 and 2
Trends in atomic and physical properties of elements of group 1 and group 2
Trends in atomic and physical properties of elements of group 1 and group 2
Both groups share a broad physical character: the alkali metals are silvery-white and soft, and because of their comparatively large atomic size they have low densities — they are, as a group, the most electropositive elements in the periodic table. The alkaline earth metals are likewise generally silvery-white, lustrous and soft, but noticeably harder than the alkali metals, and while they too are strongly electropositive, they are somewhat less electropositive than the alkali metals of the corresponding period. Table 8.4 tabulates atomic radius, ionic radius, density, ionization enthalpy, electronegativity, melting point, crustal abundance and standard reduction potential for the group 1 elements (excluding hydrogen), and Table 8.5 gives the same set of properties for group 2. Reading down either table reveals consistent periodic trends: atomic radius, ionic radius and density all increase going down the group (each successive element has one more occupied shell, and more mass, even as the added shell is loosely bound); ionization enthalpy and electronegativity both decrease going down the group (the outer electron is progressively farther from, and more shielded from, the nucleus, so it is easier to remove and the atom holds onto shared electron density less strongly); and both groups, throughout, show high negative standard reduction potentials, meaning their metals are strong reducing agents. The unipositive ions of every group 1 element, and the divalent ions of every group 2 element, have an inert-gas electronic configuration with no unpaired electrons — so their co …
Table 8.4 — symbol | atomic radius (pm) | ionic radius (pm) | density (g/cm³) | ionization enthalpy (kJ/mol) | electronegativity | melting point (K) | abundance in lithosphere | standard reduction potential E° (V):
Li: 152, 76, 0.54, 520, 1.0, 454, 18 ppm, −3.04.
Na: 186, 102, 0.97, 496, 0.9, 371, 2.27%, −2.714.
K: 227, 138, 0.86, 419, 0.8, 336, 1.84%, −2.925.
Rb: 248, 152, 1.53, 403, 0.8, 312, 78.12 ppm, −2.930.
Cs: 265, 167, 1.90, 376, 0.7, 302, 2.6 ppm, −2.927. …
Table 8.5 — symbol | atomic radius (pm) | ionic radius (pm) | density (g/cm³) | 1st ionization enthalpy (kJ/mol) | 2nd ionization enthalpy (kJ/mol) | electronegativity | melting point (K) | abundance in lithosphere | standard reduction potential E° (V):
Be: 111, 31, 1.84, 899, 1757, 1.5, 1560, 2 ppm, −1.97.
Mg: 160, 72, 1.74, 737, 1450, 1.2, 924, 2.76%, −2.36.
Ca: 197, 100, 1.55, 590, 1145, 1.0, 1124, 4.6%, −2.84.
Sr: 215, 118, 2.63, 549, 1064, 1.0, 1062, 384 ppm, −2.89.
Ba: 222, 135, 3.59, 503, 965, 0.9, 1002, 390 ppm, −2.92. …
Worked out. Worked problem: explain the observed values 496 kJ/mol and 737 kJ/mol for the first ionization enthalpies of Na and Mg respectively. Solution: Na is [Ne]3s¹ and Mg is [Ne]3s². Removing one electron from Na gives Na⁺, which is isoelectronic with Ne and therefore very stable — so Na's first ionization enthalpy is low. Removing one electron from Mg gives Mg⁺ with configuration [Ne]3s¹, which does not correspond to any noble gas and is comparatively less stable, so more energy is required to form it — hence Mg's first ionizati …
Worked out. Worked problem: the atomic radii of Na, K and Mg are 186, 227 and 160 pm respectively — explain the differences. Solution: Na and K both belong to group 1; K has one more occupied shell than Na, so K's atomic radius is larger. Na and Mg belong to the same period and so have the same number of shells, but Mg's nuclear charge is larger than Na's, which pulls Mg's valence electrons in more tightly — so Mg's atomic radius is …
Worked out. Worked problem: a magnesium strip only slowly tarnishes in air, but metallic calcium is readily attacked by air — explain. Solution: Mg (period 2) and Ca (period 3) both belong to group 2. Metallic character — the tendency to lose valence electrons — increases down a group, so Ca has greater metallic character, a greater tendency to lose its valence electrons, and a lower ionization enthalpy than Mg. As a result Mg reacts only slowly with air, forming a thin protective oxide film (tarnishing), while Ca reacts readily at room temperature with both th …