What is Period, group, block?

Example 2.2 (Reading a position)

Selenium, $[\ce{Ar}]\,3\text{d}^{10} 4\text{s}^2 4\text{p}^4$, is in period 4 (valence shell $n = 4$), in the p block, and in group 16 (two s electrons, ten d and four p: $2 + 10 + 4$); its valence configuration $n\text{s}^2 n\text{p}^4$ is that of oxygen and sulfur above it.

Example 2.24 (Lithium in a battery)

Lithium has the lowest electronegativity of period 2 and loses its 2s electron easily, while it is the lightest metal: it carries the most transferable charge per gram of any element, the reason it powers rechargeable batteries. Its first ionisation energy, $5.39\,\mathrm{eV}$, is larger than that of sodium (5.14) or potassium (4.34), yet in water it is the most reducing alkali metal, a paradox resolved by the strong hydration of the small $\ce{Li+}$ ion (Chapter 13).