Trends in Modern Periodic Table
In a group – Elements have the same number of valence electrons.
Down the group – number of shells increases.
In a period – Elements have the same number of shells.
Along the period – valence shell electrons increase by one unit.
Variation of Valency
Valency of an element can be calculated from the electronic configuration in two ways –
Valency = number of valence electrons (if they are 1, 2, 3 or 4).
All the elements of a group have the same number of valence electrons. Therefore, they all have the same valency.
Variation of Atomic Size
Atomic size or radii: It is defined as the distance from the centre of the nucleus to the valence shell of the atom.
Along the period – Atomic radius decreases because effective nuclear charge increases by one unit and it pulls valence electrons or the electron cloud closer to the nucleus.
Down the group – Atomic radius increases because new shells are added, hence, the distance between the nucleus and valence electrons or the electron cloud increases.
Variation of Metallic Properties
Along the period – Metallic character decreases because the tendency to lose valence electrons decreases due to increasing nuclear charge.
Down the group – As the distance between the nucleus and outermost electron increases, nuclear pull decreases. This increases the tendency of an atom to lose valence electron/s, hence metallic character increases.
Variation of Nonmetallic Properties
Along the period – Non-metallic character increases as the tendency to gain electrons in the valence shell increases due to increasing nuclear charge.
Down the group – As the distance between the nucleus and valence shell increases, nuclear pull decreases. This decreases the tendency of an atom to gain an electron its valence shell, hence non-metallic character decreases.
Variation of Electronegativity
Along the period – Electronegativity increases as the tendency to gain electrons in the valence shell increases due to increasing nuclear charge.
Down the group – As the distance between the nucleus and valence shell increases, nuclear pull decreases. This decreases the tendency of an atom to gain an electron, hence electronegativity decreases.