CHEM10007 · Fundamentals Of Chemistry
Atoms, the Periodic Table & Electronic Structure
The opening fortnight builds chemistry's vocabulary from nothing: what matter is, how atoms are structured, and why the Periodic Table is arranged the way it is. You move from protons, neutrons, electrons and isotopes through the Bohr and quantum-mechanical models to writing electron configurations and explaining periodic trends. These ideas underpin every later topic — bonding, reactivity and acidity all trace back to electron arrangement and effective nuclear charge.
What this chapter covers
- 01Matter: elements, compounds and mixtures; empirical vs molecular formulae
- 02The Periodic Table: groups (alkali metals, alkaline earths, halogens, noble gases), periods, metals/non-metals/metalloids
- 03Atomic structure: atomic number Z, mass number A = p + n, isotopes
- 04Relative atomic mass as an abundance-weighted average: Ar = Σ(isotope mass × fractional abundance)
- 05Bohr model: fixed energy levels, line spectra, ΔE = hν = hc/λ, and its limitations
- 06Quantum-mechanical model: shells (n) → sub-shells (s, p, d, f) → orbitals; 2 electrons per orbital
- 07Electron configuration: Aufbau order, Pauli exclusion, Hund's rule; s/p/d blocks; valence electrons
- 08Periodic trends: atomic radius and ionisation energy driven by effective nuclear charge (Zeff) and shielding
Isotopes, ion symbol and electron configuration
- 1 mark — protons and neutronsZ(Mg) = 12, so the nucleus has 12 protons; neutrons = A − Z = 26 − 12 = 14.
- 1 mark — electron count for the ionLosing 2 electrons from a neutral atom (12 e−) gives a 2+ ion with 12 − 2 = 10 electrons.
- 1 mark — correct symbolFull ion symbol: 26Mg2+ (mass number top-left, charge top-right).
- 1 mark — configuration10 electrons fill 1s22s22p6 = [Ne]; Mg2+ is isoelectronic with neon.
Key terms
- Isotope
- Atoms of the same element (same atomic number Z) that differ in neutron number, and therefore in mass number A. Chemically near-identical, physically distinguishable by mass.
- Relative atomic mass (Ar)
- The abundance-weighted average mass of an element's naturally occurring isotopes, Ar = Σ(isotope mass × fractional abundance).
- Orbital
- A region of space described by the quantum-mechanical model that holds up to 2 electrons. Sub-shells contain 1 (s), 3 (p) or 5 (d) orbitals.
- Effective nuclear charge (Zeff)
- The net positive pull felt by a valence electron after inner electrons shield it from the nucleus. Rising Zeff across a period shrinks atoms and raises ionisation energy.
- Ionisation energy
- The energy needed to remove an electron from a gaseous atom. It increases across a period and decreases down a group.
Atoms, the Periodic Table & Electronic Structure FAQ
What are the main limitations of the Bohr model?
It cannot account for electron–electron repulsion in multi-electron atoms, cannot predict the spectra of atoms larger than hydrogen or magnetic behaviour, and (classically) a circulating electron should radiate energy and spiral into the nucleus. The quantum-mechanical model replaces fixed orbits with probability orbitals.
How do I write an electron configuration quickly?
Follow the Aufbau filling order (1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p …), place at most 2 electrons per orbital (Pauli), and fill degenerate orbitals singly before pairing (Hund). Use a noble-gas core, e.g. [Ar]4s2, to save time to the end of Period 4.
Why does atomic radius decrease across a period?
Electrons are added to the same shell while protons (and Zeff) increase, so the nucleus pulls the electron cloud in more tightly. Down a group, new shells are added, so radius increases.
Exam move
Drill the Z/A/isotope bookkeeping until it is automatic, then practise configurations to the end of Period 4 with noble-gas cores. Tie every periodic trend back to one cause — effective nuclear charge versus shielding — so you can reason out a trend you have not memorised. Keep the Bohr-model limitations as a ready-made written-answer paragraph, since they recur as a Section B explanation.
