Structure of the Atom and the Periodic Table
Core concepts
The atom is a nucleus of protons and neutrons surrounded by electrons occupying quantised energy levels. Atomic number (Z) fixes the element; mass number (A) fixes the isotope; isotopes differ only in neutron count, so they share chemistry but differ in mass and in nuclear stability. Relative atomic mass is the weighted mean over natural abundances, not the mass of any single atom.
Electron configuration follows the Aufbau order, Hund's rule and the Pauli exclusion principle. Write configurations in shells for the first 20 elements and in subshell notation beyond that; the 4s orbital fills before 3d, but 3d electrons are lost first when transition metals ionise.
Periodic trends
Across a period, nuclear charge rises while shielding stays roughly constant, so atomic radius falls, ionisation energy and electronegativity rise, and metallic character falls. Down a group, extra shells increase shielding and distance, so radius rises and ionisation energy falls. These trends explain reactivity: Group 1 metals lose an electron ever more easily down the group; Group 17 non-metals gain electrons less readily down the group.
Traps
- Confusing mass number with relative atomic mass.
- Forgetting that ionisation energy is always endothermic and measured in the gaseous state.
- Assuming a small jump in successive ionisation energies always means a noble gas configuration; check where the jump occurs.
- Writing 3d before 4s in electron configurations.
Rule of thumb
For any trend question, state the competing factors: nuclear charge, shielding, and distance of the outer electron. Trend = attraction of the outer electron to the nucleus. More attraction means higher ionisation energy and smaller radius.
Worked example
Magnesium (Z = 12) has configuration 1s² 2s² 2p⁶ 3s². The third ionisation energy is far larger than the second because the third electron is removed from the stable neon core (2p⁶), closer to the nucleus and less shielded.
Exam technique
Always quote data when explaining a trend, and link structure to property explicitly.