Atoms and isotopes

Everything around you is made of atoms, and every atom is made of three kinds of particle. Protons have a positive charge, electrons a negative charge of the same size, and neutrons no charge at all. Protons and neutrons sit together in a tiny, heavy middle called the nucleus; the electrons spread out around it and take up nearly all the room.

The number of protons decides which element an atom is. That count is the atomic number, Z. Every atom with 6 protons is carbon, every atom with 8 is oxygen, and every atom with 79 is gold. A neutral atom has as many electrons as protons, so the charges cancel.

Neutrons can vary. Atoms of the same element with different numbers of neutrons are isotopes. The mass number, A, is protons plus neutrons, and it's written after the name or top left of the symbol: carbon-12 has 6 protons and 6 neutrons, carbon-14 has 6 protons and 8 neutrons. They react the same way, because chemistry is about electrons; carbon-14 just happens to be radioactive.

An atom that has gained or lost electrons is an ion. Lose electrons and it's positive (a cation): a sodium atom that loses one electron is Na⁺, with 11 protons and 10 electrons. Gain electrons and it's negative (an anion): chlorine that gains one is Cl⁻.

The atomic mass on a periodic table is an average over the isotopes found in nature. About 76% of chlorine atoms are chlorine-35 and 24% are chlorine-37, so the average is about 35.45, not a whole number. Masses of atoms are measured in atomic mass units (u), where a carbon-12 atom is exactly 12 u; a proton or neutron is close to 1 u, and an electron is about 1/1836 of that.