Periodic Table: Periods 1–3

Carbon (C)

Oxygen (O)

Sodium (Na)

Chlorine (Cl)

That is why they react in similar ways.

More protons pull electrons closer; more shells push them farther out.

Their outer electron shell is already full.

The number of protons in the nucleus.

Na

Halogens

H, He, Li, Be, B, C, N, O, F, Ne

Na, Mg, Al, Si, P, S, Cl, Ar

Mg

Silicon

The period number equals the number of occupied electron shells (main energy levels).

The group number minus 10: group 13 has 3, 14 has 4, 15 has 5, 16 has 6, 17 has 7.

1s² 2s² 2p⁶ 3s¹, or [Ne] 3s¹

1s² 2s² 2p⁶ 3s² 3p⁵, or [Ne] 3s² 3p⁵

Soft, very reactive metals with one valence electron (Li, Na, K); react with water to form hydroxides and hydrogen gas.

Reactive metals with two valence electrons (Be, Mg, Ca), less reactive than group 1.

Noble gases (He, Ne, Ar)

No. It is a nonmetal gas; it sits in group 1 only because it has one valence electron.

Li, Be, Na, Mg and Al.

Boron and silicon.

H₂, N₂, O₂, F₂ and Cl₂.

Sodium (metal) and chlorine (nonmetal).

Small dips at B and Al (a new p subshell starts) and at O and S (the first paired p electron is easier to remove).

Noble gases are usually left out because they rarely form bonds.

It decreases across a period (left to right) and increases down a group.

Na⁺, Mg²⁺, Al³⁺, O²⁻, F⁻, Cl⁻

It loses its entire outer (3s) shell, and the remaining electrons are pulled in by the same 11 protons.

The extra electron adds repulsion between electrons with no extra proton, so the electron cloud spreads out.

Atoms of the same element with different numbers of neutrons, e.g. carbon-12 and carbon-14.

12 (mass number 23 − atomic number 11).

It is a weighted average of its isotopes: about 76% chlorine-35 and 24% chlorine-37.

Oxygen (about 46% by mass), followed by silicon.