Drop a small piece of lithium into water and it fizzes gently. Drop in sodium and it melts into a ball and zips across the surface. Potassium reacts so fast that the hydrogen gas it releases catches fire with a lilac flame. These three metals sit in the same column of the periodic table, they all react in a similar way, and the reaction gets more violent going down the column. Why? Mendeleev found the pattern, but he could not explain it. The explanation is inside the atom, in the arrangement of its electrons.
The electrons in an atom's outermost shell are called valence electrons, and that shell is the valence shell. They are the electrons that take part in chemical reactions, so they decide how an element behaves. Two patterns connect the table to the atom. First, the period number equals the number of occupied electron shells: sodium is in period 3 and has 3 shells (2, 8, 1). Second, the group tells you the number of valence electrons: groups 1 and 2 have 1 and 2, and groups 13 to 18 have 3 to 8 (take away 10 from the group number). Helium is the exception in group 18: it has only 2 electrons, but that fills its only shell.
| Group | Valence electrons | What the atom tends to do | Typical ion charge |
|---|---|---|---|
| 1 (alkali metals) | 1 | Lose 1 electron | +1 (Na⁺, K⁺) |
| 2 (alkaline earth metals) | 2 | Lose 2 electrons | +2 (Mg²⁺, Ca²⁺) |
| 13 | 3 | Aluminium loses 3 electrons | +3 (Al³⁺) |
| 14 | 4 | Usually shares electrons instead | Rarely forms simple ions |
| 15 | 5 | Gain 3 electrons | −3 (N³⁻, P³⁻) |
| 16 | 6 | Gain 2 electrons | −2 (O²⁻, S²⁻) |
| 17 (halogens) | 7 | Gain 1 electron | −1 (F⁻, Cl⁻) |
| 18 (noble gases) | 8 (helium 2) | Outer shell already full | Does not form ions |
Patterns in reactivity
Metals react by losing electrons. Going down group 1, each atom has one more shell, so the single valence electron is farther from the nucleus and more inner electrons sit between it and the positive protons. The nucleus holds that outer electron less tightly, so it is easier to lose. That is why reactivity increases down the alkali metals: lithium, then sodium, then potassium, then rubidium and cesium. The same pattern holds for the alkaline earth metals.
Non-metals react by gaining electrons. A halogen needs to pull in one more electron. In a small atom like fluorine, the valence shell is close to the nucleus, so the nucleus attracts an incoming electron strongly. Going down group 17, the atoms get bigger and the pull gets weaker. So reactivity decreases down the halogens: fluorine is the most reactive, then chlorine, bromine and iodine.
- React by losing valence electrons
- Reactivity increases going down the group
- Most reactive: bottom-left of the table
- Example: potassium is more reactive than sodium
- React by gaining electrons
- Reactivity increases going up the group
- Most reactive: fluorine, top right (not counting noble gases)
- Example: chlorine is more reactive than iodine
Patterns in atom size
Down a group, atoms get bigger, because each period adds another shell of electrons. Across a period from left to right, atoms get smaller. That surprises most people, since the atoms are gaining particles. But across a period the electrons go into the same shell while the number of protons rises, so the more positive nucleus pulls all the electrons in closer. Sodium (11 protons) is a larger atom than chlorine (17 protons), even though both have 3 shells.