Chemistry - atoms, molecules, ions, and the periodic table
What the exam tests
Atomic structure, ions, molecules, bonding and the periodic table. Questions ask you to use proton and electron numbers to explain charges and formulas.
Core rules
- Atomic number = protons = electrons in a neutral atom. Mass number = protons + neutrons. Isotopes have the same protons but different neutrons.
- Electrons occupy shells (2, 8, 8...). Atoms react to gain, lose or share electrons to achieve a stable outer shell.
- Metal atoms lose electrons to form positive ions; non-metal atoms gain electrons to form negative ions.
- Ionic compounds are held by electrostatic attraction, have high melting points, and conduct when molten or dissolved.
- Covalent molecules share electrons and usually have low melting points.
- Groups have similar outer-shell electrons; periods show repeating trends. Group 1 is alkali metals, Group 17 halogens, Group 18 noble gases.
Worked example
Sodium (atomic number 11) has electron arrangement 2,8,1 and loses one electron to become Na⁺, arrangement 2,8. Chlorine (17) is 2,8,7 and gains one electron to become Cl⁻, 2,8,8. The ions combine in a 1:1 ratio, so the formula is NaCl.
Traps
- The number of neutrons is mass number minus atomic number, not the atomic number.
- Ion charge comes from electron gain or loss, not from changing protons.
- A formula such as MgCl₂ balances charges (Mg²⁺ and two Cl⁻), not atom counts alone.