Periodicity of Properties
Periodicity (with reference to elements) refers to recurrence of same property after a fixed interval when the elements are arranged in increasing order of their atomic numbers.
When we move across a period or in a group a periodic trends are observed in atomic sizes, ionization enthalpies, electron gain enthalpies, electronegativity and valence.
The size of an atom is measured by its radius. The atomic radii decrease while going from left to right in a period and increase with atomic number in a group.
Ionization enthalpies generally increase across a period and decrease down a group. Electronegativity in general also shows a similar trend.
Electron gain enthalpies, in general, become more negative across a period and less negative down a group. There is some periodicity in valence, for example, among representative elements; the valence is either equal to the number of electrons in the outermost orbitals or eight minus this number.
Diagonal relationship exists between certain pairs of diagonally placed elements in the second and third periods of the periodic table. Pairs that show the diagonal relationship are lithium and magnesium, beryllium and aluminium and boron and silicon.
Metallic character decreases on moving from left to right in a period. Tendency of an element to gain the electrons increases; thus, the non-metallic character increases on moving from left to right.
Chemical reactivity is highest at the two extremes of a period and is lowest in the centre. The reactivity on the left extreme of a period is because of the ease of electron loss (or low ionization enthalpy). Highly reactive elements do not occur in free state in nature; they usually occur in the combined form. Oxides formed of the elements on the left are basic and of the elements on the right are acidic in nature. Oxides of elements in the centre are amphoteric or neutral.
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