Atoms, elements, and why the periodic table is shaped like that
Mendeleev left gaps for elements nobody had found, and said what they would be like. He was right.
Everything is made of about a hundred kinds of atom. An atom is mostly empty space with a tiny dense nucleus of protons and neutrons, surrounded by electrons.
The scale is hard to hold on to. If a nucleus were a marble on a football pitch, the nearest electron would be somewhere near the stands. Solid matter is almost entirely nothing, and feels solid because of electrical repulsion between electrons rather than because anything is touching.
What makes an element itself
- Protons - positive, in the nucleus. The count IS the element. 6 protons is carbon, always.
- Neutrons - neutral, in the nucleus. Changing the count gives an isotope - same element, different mass.
- Electrons - negative, around the outside. Changing the count gives an ion - charged, same element.
So carbon-12 and carbon-14 are both carbon. They behave identically in chemistry because chemistry is electrons, and they differ only in nuclear stability - which is what makes carbon-14 useful for dating.
Why the table has that shape
Electrons occupy shells, and each shell holds a limited number: 2 in the first, then 8, then 8 again for the elements most people meet.
The table is arranged so that a row ends when a shell fills. That is the entire explanation for its odd staircase outline - the first row has 2 elements because the first shell holds 2, the next rows have 8.
It follows that everything in a column has the same number of outer electrons. And since chemistry is almost entirely about outer electrons, a column is a family with similar behaviour.
Sodium explodes in water and argon sits in light bulbs doing nothing, and the difference is one electron in the outer shell. Position on the table predicts behaviour, which is what makes it a theory rather than a catalogue.
Mendeleev's gaps
In 1869 Mendeleev arranged the known elements by mass and noticed properties repeating. To keep the pattern intact he left empty spaces, and predicted the properties of elements nobody had ever seen.
He described one he called eka-silicon: mass about 72, grey metal, density around 5.5. In 1886 germanium was discovered - mass 72.6, grey metal, density 5.35.
That is the difference between a list and a theory. A list records what you have; a theory tells you what you will find. It is also a clean example of the falsifiability idea - those predictions could have been wrong, which is exactly why getting them right was convincing.
He had it slightly wrong and it still worked. Mendeleev ordered by atomic mass; the correct ordering is by proton number. They almost always agree, and where they disagree he swapped the elements anyway because the chemistry demanded it. He was right for a reason he could not have known - protons were not discovered for another forty years.
Bonding, in one idea
Atoms react to reach a full outer shell, and there are two ways to do it.
- Ionic - one atom gives electrons to another. Both become charged ions and stick together by electrical attraction. Sodium hands its one electron to chlorine, and you get table salt.
- Covalent - atoms share electrons so both count them as theirs. Two hydrogens and an oxygen share, and you get water.
Nearly all of school chemistry is those two ideas applied repeatedly. Which one happens depends on how strongly each atom pulls on electrons, and that depends on where it sits on the table.