Periodic Classification of Elements
What the exam tests
Periodic Classification of Elements tests Mendeleev's periodic law and its limitations, the modern periodic law, position of elements from atomic number, and trends in properties. BSEB often asks for the group, period and valency of an element from its electronic configuration.
Core rules
- Mendeleev arranged elements by atomic mass and left gaps for undiscovered elements. His law: properties are a periodic function of atomic mass.
- Modern periodic law: properties are a periodic function of atomic number. Atomic number is the number of protons and determines the position.
- Periods are horizontal rows (1–7); groups are vertical columns (1–18).
- The number of shells gives the period; the number of valence electrons gives the group for main-group elements.
- Across a period, atomic radius decreases and metallic character decreases.
- Down a group, atomic radius increases, metallic character increases and ionisation energy decreases.
- Group 1: alkali metals; group 2: alkaline earth metals; group 17: halogens; group 18: noble gases with stable octets (He has 2).
- Isotopes occupied the same place in the modern table because they have the same atomic number.
Worked example
An element has atomic number 17. Write its electronic configuration and predict its group, period, valency and nature. Configuration: 2, 8, 7. There are three shells, so period 3. Seven valence electrons place it in group 17. It needs one electron to complete the octet, so valency is 1. It is a non-metal and forms Cl−.
Traps that cost marks
- Using atomic mass instead of atomic number to place an element in the modern table.
- Saying all groups have 8 valence electrons; helium has 2.
- Reversing the trend: metallic character increases down a group, not across a period.
- Forgetting that Mendeleev's table could not explain isotopes and placed some elements out of order.