03Class 11 Chemistry

Classification of Elements and Periodicity in Properties short notes

Short notes on periodic trends, with the exceptions examiners like to ask about. Learn why each trend happens, and you won't need to memorise it.

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The periodic table

The modern periodic law says properties are periodic functions of atomic number.
There are 18 groups and 7 periods.
s-block: groups 1 and 2. p-block: groups 13 to 18. d-block: groups 3 to 12. f-block: lanthanoids and actinoids.
Elements with Z above 100 get systematic names, such as Unq for 104 at first, before official names were given.

Atomic radius falls across a period, and rises down a group.
Cations are smaller than their parent atoms, and anions are larger.
In isoelectronic species, a higher nuclear charge means a smaller size.
Ionisation enthalpy generally rises across a period, and falls down a group.
Electronegativity rises across a period, and falls down a group. Fluorine is the most electronegative.
Metallic character falls across a period, and rises down a group.
Oxides turn from basic on the left to acidic on the right.

Ionisation enthalpy: Be is greater than B, and N is greater than O, because of stable filled and half-filled subshells.
Electron gain enthalpy: Cl is more negative than F, because F is so small that its electrons repel the new one.
Noble gases have positive electron gain enthalpies.
Diagonal relationships include Li with Mg, and Be with Al.

Saying fluorine has the most negative electron gain enthalpy. That's chlorine.
Forgetting that N has a higher ionisation enthalpy than O.
Confusing electronegativity, a tendency within a bond, with electron gain enthalpy, a measured energy.

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