Atoms, elements and the periodic table
Chemistry · Key Stage 3 · The School
Inside an atom
A nucleus of protons (positive) and neutrons (neutral), with electrons (negative) in shells around it. Almost all the mass is in the nucleus; almost all the volume is empty space. The number of PROTONS decides which element it is — change that and you have changed the element itself.
Element, compound, mixture
An element contains one kind of atom (copper, oxygen). A compound has different atoms chemically BONDED in fixed proportions (water is always H₂O). A mixture is substances together but not bonded (air, salty water) — which is why a mixture can be separated physically and a compound cannot.
Reading the table
Groups are the columns: same number of outer electrons, so similar reactions. Group 1 metals are violently reactive; group 0 noble gases barely react at all because their outer shell is full. Periods are the rows: each one fills the next shell. The table is not a list, it is a map of behaviour.
Copper wire: one type of atom only, so an ELEMENT. Water: hydrogen and oxygen chemically bonded in a fixed ratio of two hydrogens to one oxygen, with properties like neither — a COMPOUND. Air: nitrogen, oxygen, argon and carbon dioxide, in no fixed ratio, each keeping its own properties and separable by physical means — a MIXTURE. The three tests are: how many kinds of atom, fixed ratio or not, and can it be separated physically.
Mendeleev arranged the table in 1869, sixty years before anyone knew an atom had electrons. He ordered by mass and by how elements behaved — and where the pattern demanded an element nobody had found, he left a GAP and predicted what it would be like. Gallium and germanium were discovered later and matched closely. A pattern that predicts things not yet seen is doing more than sorting what you already have.