Periodic Classification
Periodic Classification
This domain tests the modern periodic table: periodicity, groups and periods, blocks, valency, and trends in atomic size, metallic character, ionisation energy and electronegativity.
Core rules
- Modern periodic law: properties of elements are periodic functions of their atomic numbers.
- The table has 7 periods (horizontal rows) and 18 groups (vertical columns).
- For main-group elements, group number is related to valence electrons: Group 1 has 1, Group 2 has 2, Group 13 has 3, Group 14 has 4, Group 15 has 5, Group 16 has 6, Group 17 has 7, Group 18 has 8 (stable octet).
- Period number equals the number of occupied electron shells.
- Across a period, nuclear charge increases and atomic radius decreases; ionisation energy and electronegativity generally increase; metallic character decreases.
- Down a group, shells increase, atomic radius increases, ionisation energy and electronegativity decrease, metallic character increases.
- Elements are classified as metals, non-metals and metalloids; transition elements occupy Groups 3–12.
Worked example
An element has atomic number 17. Its electron configuration is 2, 8, 7. It is in Period 3 because three shells are occupied. Its group is 17 because it has seven valence electrons, so it is a halogen with valency 1. It gains one electron to form a −1 ion and reacts with sodium to form an ionic chloride, NaCl.
Traps
Do not place elements by atomic mass when atomic number is given. Group number is not always the number of valence electrons for transition elements. Noble gases are Group 18, not Group 8. Atomic radius decreases across a period but increases down a group.