Atomic structure, the Periodic Table and chemical bonding
Atomic structure, the Periodic Table and chemical bonding tests subatomic particles, electron configuration, periodic trends and why substances have particular structures and properties.
Atomic structure
- Proton: +1, mass 1; neutron: 0, mass 1; electron: -1, mass negligible.
- Atomic number Z = protons; mass number A = protons + neutrons. Isotopes have same Z but different A.
- Electrons occupy shells 2,8,8. Group number equals outer electrons; period equals number of shells.
- Metals lose electrons to form positive ions; non-metals gain electrons to form negative ions.
Bonding
- Ionic: transfer of electrons, electrostatic attraction between oppositely charged ions in a giant lattice. High melting point; conducts when molten or dissolved, not solid.
- Covalent: shared electron pairs. Simple molecular substances have weak intermolecular forces, low melting points, do not conduct. Giant covalent substances such as diamond and silicon dioxide have high melting points.
- Metallic: positive ions in a sea of delocalised electrons; conducts, malleable.
Worked example
Sodium (Z=11) has configuration 2,8,1 and loses one electron to form Na+. Chlorine (Z=17) has 2,8,7 and gains one electron to form Cl-. The ions have full outer shells and attract electrostatically in a 1:1 lattice, NaCl. The formula is NaCl because the charges balance.
Traps
Confusing mass number with relative atomic mass; saying isotopes have different chemical properties; writing NaCl as molecules; forgetting 'delocalised' for metallic electrons; incorrect ion charges.