Inside the atom and through reactions. The nuclear atom and its electron shells explain the periodic table; atoms rearrange and are never lost, which is why mass is conserved and equations balance; and reactions happen when particles collide hard enough.
Protons, neutrons and electrons; atomic and mass numbers (AC9S9U06, Year 9).
An atom has a tiny, dense nucleus of protons and neutrons, with electrons in shells around it. The number of protons (the atomic number) decides the element; protons plus neutrons give the mass number.
Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons. They react in the same way but have different masses, and some are unstable: they are radioactive.
An unstable nucleus decays by giving out alpha particles (2 protons and 2 neutrons), beta particles (fast electrons made in the nucleus) or gamma rays (energy). Alpha and beta decay change the element; gamma does not. Alpha is stopped by paper, beta by aluminium, and gamma is only reduced by lead.
Each unstable nucleus decays at random, but a large sample halves in a fixed time: its half-life. After one half-life half is left, after two a quarter, after three an eighth. Half-lives range from minutes to billions of years, which decides what each isotope is used for.
An unstable nucleus often decays into another unstable one, and a decay chain ends only when a stable nucleus is reached. Each alpha decay lowers the atomic number by 2 and the mass number by 4; each beta decay raises the atomic number by 1 and leaves the mass number alone. Uranium-238 reaches stable lead-206 after eight alpha and six beta decays.
Reactions rearrange atoms; none are made or destroyed; balanced equations show it (AC9S9U07, Year 9).
In a chemical reaction the reactants are used up and new substances, the products, are made. A word equation names them: reactants → products. It must include every product (burning a fuel makes water as well as carbon dioxide), but it shows no amounts or atoms.
In a chemical reaction atoms are rearranged into new particles, never made or destroyed. Counting atoms before and after shows why mass is conserved and why equations must balance.
A balanced equation has the same number of each kind of atom on both sides, because atoms are only rearranged. You balance it by changing the coefficients in front of formulas, never the formulas themselves.
In a chemical reaction atoms are rearranged, not made or destroyed, so the total mass of the reactants equals the total mass of the products. In a closed system the mass reading stays the same. In an open system it can seem to change because a gas escapes (mass falls) or a gas from the air joins in (mass rises); counting the gas shows the mass is still conserved.
Electron shells, and why elements in a group behave alike (AC9S10U06, Year 10).
Electrons fill shells around the nucleus in order: 2 in the first, 8 in the second, 8 in the third (for the first 20 elements). The electrons in the outer shell decide how an element behaves.
Elements in the same group have the same number of outer-shell electrons, so they react in similar ways. Group 1 metals lose their one outer electron easily; group 17 non-metals gain one easily; group 18 has full shells and hardly reacts.
Across a period the outer electrons go up from 1 to 8; then a new shell starts and the count goes back to 1, so a new period begins and the properties repeat. The period number is the number of shells. Across a period, elements change from metals to non-metals.
The periodic table is not a list to look up but a tool for prediction. An element's group gives its number of outer electrons, and so how reactive it is and what it bonds with; its period gives its number of shells. From the position alone you can predict whether an element is a metal, how it reacts and what its atom looks like.
Particles must collide with enough energy to react: collision theory (AC9S10U07, Year 10).
Many reactions follow one of three patterns. Synthesis: two substances join into one (A + B → AB). Decomposition: one substance breaks into simpler ones (AB → A + B). Displacement: a more reactive element takes the place of a less reactive one (A + BC → AC + B). Spotting the pattern lets you predict the products.
Particles react only when they collide with enough energy (the activation energy). Collision theory explains why temperature, concentration, surface area and catalysts change the rate of a reaction.
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