The Periodic Trends

Across a period (left to right), the number of protons in the nucleus INCREASES while electrons go into the SAME shell. The increasing nuclear charge pulls the existing electrons in more strongly → atomic radius DECREASES. So sodium (Na) is larger than chlorine (Cl) even though they're in the same period.

Down a group, each element has ONE MORE filled electron shell than the one above. Extra shells make the atom much bigger → atomic radius INCREASES. Lithium is small; caesium is huge.

These patterns matter because:

• Bigger atoms have outer electrons FURTHER from the nucleus → easier to lose → more reactive (for metals).
• Smaller atoms attract electrons MORE strongly → more reactive (for non-metals).

Group 1 — alkali metals. All have ONE outer electron. They react by LOSING this electron. The further from the nucleus it is, the easier to lose → reactivity INCREASES down the group.

• Lithium: vigorous fizz with water.
• Sodium: rapid reaction with cold water; floats and dissolves.
• Potassium: ignites the hydrogen produced (lilac flame).
• Rubidium / caesium: explosive reaction.

Group 7 — halogens. All have SEVEN outer electrons. They react by GAINING one. Closer to the nucleus = stronger pull on the incoming electron = MORE reactive. So fluorine (closest) is most reactive; iodine (furthest down) is least reactive.

• Fluorine: extremely reactive, attacks glass.
• Chlorine: gas, kills bacteria.
• Bromine: liquid at room temp, less reactive than chlorine.
• Iodine: solid, even less reactive.

A neat experiment: chlorine water added to potassium bromide solution → chlorine 'displaces' bromine (Cl₂ + 2KBr → 2KCl + Br₂) because chlorine is more reactive. This is a displacement reaction.

Group 0/18 — noble gases. All have FULL outer shells → very low tendency to react. He, Ne, Ar, Kr stay essentially inert. (Only the heavier ones — Xe, Rn — form a few compounds in extreme conditions.)

Going LEFT to RIGHT across a period, elements gradually change from metallic to non-metallic. Period 3 illustrates this beautifully:

Element	Type	Notes
Na	metal	shiny, reactive, conducts
Mg	metal	shiny, less reactive than Na
Al	metal	shiny, lighter
Si	metalloid	semi-conducting properties
P	non-metal	red and white forms exist
S	non-metal	yellow solid
Cl	non-metal	green gas
Ar	noble gas	unreactive

The 'zigzag' on the right of the Periodic Table separates metals (left) from non-metals (right). Elements ON the zigzag (B, Si, Ge, As, Sb, Te) are metalloids — they have intermediate properties used in semiconductors.

Going DOWN a group, metallic character generally INCREASES. Lead is more metallic than carbon despite both being Group 14.

This explains why elements like germanium and silicon, sitting right between metals and non-metals on the table, are vital in modern electronics.