Periodic Table Trends

The Periodic Table is arranged by increasing atomic (proton) number. Rows are called periods; columns are called groups. Elements in the same group have similar chemical properties because they have the same number of outer (valence) electrons.

Across a Period (left → right)

• Metallic character decreases; non-metallic character increases.
• Atoms get slightly smaller as the nucleus pulls electrons in more strongly.
• Metals on the left tend to lose electrons (form positive ions like $\text{Na}^+$); non-metals on the right tend to gain or share electrons (form negative ions like $\text{Cl}^-$).

Down a Group (top → bottom)

• Atoms get larger (more electron shells). Think of adding more coats—each coat makes the atom bigger.
• Group 1 (alkali metals): reactivity increases down the group; melting point decreases; density generally increases.
• Group 17 (halogens): reactivity decreases down the group; density increases.

Group Number and Ion Charge

• Group 1 forms $+1$ ions (e.g., $\text{Na}^+$).
• Group 2 forms $+2$ ions (e.g., $\text{Mg}^{2+}$).
• Group 16 forms $-2$ ions (e.g., $\text{O}^{2-}$).
• Group 17 forms $-1$ ions (e.g., $\text{Cl}^-$).

Why Elements in a Group Are Similar

They have the same number of outer electrons, so they react in similar ways. For example, Li, Na and K each have 1 outer electron and react with water in similar reactions, but at different speeds.

Group Facts to Know

• Group 1 alkali metals: soft, very reactive; reactivity increases Li → Na → K.
• Group 17 halogens: at room temperature: chlorine is a green gas, bromine is a red-brown liquid, iodine is a dark grey solid (purple vapor when warmed). Reactivity: Cl > Br > I.

Common Misconceptions

• Period number tells you the number of electron shells, not the charge of the ion.
• Not all metals are highly reactive; position matters (e.g., Group 1 vs. transition metals).
• Halogens exist as diatomic molecules (e.g., $\text{Cl}_2$, $\text{Br}_2$, $\text{I}_2$).