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.