Chemistry · Rung 2 of 24

The periodic table

The table is not a list, it is a map, and position is what matters. Columns are groups and share valence electrons: group 1 has one, which is why it burns in water; group 17 has seven and needs one more; group 18 has a full set and reacts with nothing. Rows are periods and say how many shells there are. Going down a group the atom gets bigger, because there is one more shell; going right across a period it gets smaller, because the nucleus pulls harder on the same electrons. Electronegativity does the opposite: it rises upward and to the right, and the champion is fluorine at 3.98. Those two trends predict half of chemistry without memorising anything.

Where this sits

Comes after
What everything is made of
Opens up
Bonds

Exercises from this rung

Generated by the app, from easiest to hardest

  1. What do elements in the same column share?

    • The number of protons
    • The atomic mass
    • The number of shells
    • The electrons in the outer shell

    Why A column is a group, and what defines a group is how many electrons it has in its outer shell. Since those electrons do all the chemistry, elements in the same group behave alike: lithium, sodium and potassium all react with water, each more violently than the last. The number of shells is given by the row, not the column.

    easier

  2. Why do noble gases barely react with anything?

    • Because their outer shell is already full
    • Because their nucleus is unstable
    • Because they are very heavy
    • Because they have no electrons

    Why An atom reacts in order to fill its outer shell. Noble gases already have it filled: helium with two electrons, the rest with eight. They have no need to give, take or share anything, so they keep to themselves — hence the name — and are used wherever no reaction is wanted: argon in welding, helium in balloons.

    in between

  3. Sodium and potassium both react violently with water. Why?

    • Because both have a single electron in the outer shell
    • Because both are in the same period
    • Because both are metals
    • Because both are light

    Why They are in the same group, group 1, and have one electron too many to leave the shell below complete. Letting it go is so easy that they do it with water, releasing hydrogen and enough heat to ignite it. And since the atom is bigger the further down you go, the electron sits further out and leaves more readily: potassium reacts more violently than sodium, and caesium explodes.

    harder