Period 2: lithium to neon

Filling the second shell: 2s, then 2p

The second shell has two subshells. The 2s subshell takes two electrons and fills first, in lithium ([He] 2s¹) and beryllium ([He] 2s²). The three 2p orbitals take six more, filling from boron ([He] 2s² 2p¹) to neon ([He] 2s² 2p⁶).

Underneath, every atom in the row keeps the same 1s² pair, the helium core, so only the outer shell changes from box to box. Neon’s complete set of eight is so stable that it forms no known stable compounds, and many other elements gain, lose or share electrons to copy it.

Left to right across period 2

Each step adds a proton, so the nucleus grips the same shell more tightly. Electronegativity climbs from 0.98 for lithium through 2.55 for carbon to 3.98 for fluorine, the highest of all elements, and first ionization energy rises from 5.392 eV for lithium to 21.565 eV for neon.

Two small dips reveal the subshells. Boron (8.298 eV) is easier to ionize than beryllium (9.323 eV) because its outer electron sits in the higher-energy 2p subshell, and oxygen (13.618 eV) is easier than nitrogen (14.534 eV) because its fourth 2p electron has to share an orbital.

Lithium and beryllium are metals, boron is a metalloid, and carbon through neon are nonmetals. Carbon melts at 3823 K, the highest in the row, while the last four elements are gases at room temperature.

Where you meet period 2

Nitrogen and oxygen make up about 99 percent of the air you breathe, and carbon forms the backbone of every molecule in living things. Lithium, the least dense metal at 0.534 g/cm³, powers rechargeable batteries. Fluoride in toothpaste helps protect teeth, boron compounds go into heat-resistant glass, and neon glows red-orange in advertising signs.