The properties of atoms are linked to how well they hold on to or attract electrons. These properties depend on the outermost orbitals that contain electrons.
The Two Master Trends
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Moving down a group weakens the electron–nucleus attraction. As you move down a group, the outermost orbitals get larger. Going from H to Li to Na, the outermost orbitals are 1s, 2s, then 3s. The farther an electron is from the nucleus, the lower its attraction to the nucleus.
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Moving left-to-right across a period strengthens the attraction. Going from B to C to N, each step to the right adds one proton to the nucleus and one electron to the outermost orbitals. Attraction to the added proton outweighs repulsion from the added electron, so the net attraction is stronger.
Big picture: Atoms in the top right of the periodic table hold their electrons most tightly.
Atoms in the lower left hold their electrons least tightly.
How well atoms hold electrons
Cs (least tightly)
He (most tightly)