Atomic structure
Atomic structure tests subatomic particles, atomic and mass numbers, isotopes, relative atomic mass and electron arrangement.
Subatomic particles
Atoms contain protons, neutrons and electrons. A proton has relative charge +1 and relative mass 1; a neutron has charge 0 and relative mass 1; an electron has charge −1 and negligible relative mass. The atomic number Z is the number of protons and identifies the element. The mass number A is protons + neutrons. A neutral atom has equal numbers of protons and electrons; an ion forms when electrons are gained or lost, never protons.
Isotopes and relative atomic mass
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have identical chemical properties because the electron arrangement is the same, but slightly different physical properties such as mass and density. Relative atomic mass is the weighted mean mass of the isotopes compared with 1/12 of a carbon-12 atom.
Electron arrangement
Electrons occupy shells in the order 2, 8, 8. The group number equals the number of outer-shell electrons; the period number equals the number of occupied shells. Sodium (Z = 11) is 2,8,1 and forms Na+ (2,8); chlorine (Z = 17) is 2,8,7 and forms Cl− (2,8,8).
Worked example
Chlorine is 75% chlorine-35 and 25% chlorine-37 by abundance. Relative atomic mass = (35 × 75 + 37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 3550 ÷ 100 = 35.5. The calculation uses percentage abundances as weights, so the answer lies between 35 and 37, closer to 35.
Traps
Mass number is not the number of neutrons. Isotopes have the same atomic number, not the same mass number. Electron configuration 2,8,8 applies to the first 20 elements; transition metals fill shells differently. When an ion forms, only electrons change, so the atomic number stays the same.