Which element has the largest size in the second period?

(i) The classification of elements is required for a convenient and systematic study of the behaviour and nature of substances.

(ii) Dobereiner, Newlands and Mendeleev proposed the classification of elements on the basis of their chemical properties and atomic masses (atomic weight).

2. Mendeleev's periodic table:

Mendeleev classified the elements into groups and periods. He proposed the periodic law that the properties of elements are periodic functions of their atomic masses.

3. Modern periodic table:

(i) Modern periodic law is based on the atomic numbers and electronic configurations of elements. This law states that properties of elements are periodic functions of their atomic numbers.

(ii) Modern periodic table is divided into $7$ horizontal rows called periods and $18$  vertical columns called groups.

(iii) On the basis of similarities and dissimilarities in the electronic configurations all the known elements have been classified into blocks. These are called s-block elements, $\mathrm{p}$-block elements, $\mathrm{d}$-block elements and $\mathrm{f}$-block elements.

(iv) $\mathrm{s}$-and $\mathrm{p}$-block elements are called representative elements.

(v) $\mathrm{s}$-block elements with $1$ valence electron are called alkali metals. These are placed in group $1$ of the periodic table; $\mathrm{s}$-block elements with $2$ valence electrons are called alkaline earth metals. These are placed in group $2$.

(vi) Elements of group $13$, group $14$, group $15$, group $16$, group $17$ and group $18$ collectively constitute p-block elements.

(vii) $\mathrm{d}$-block elements are called transition elements. They are positioned between $\mathrm{s}$-and $\mathrm{p}$-block elements of the periodic table.

(viii) Lanthanides and actinides are placed in two separate rows a9t the bottom of the periodic table. This is done to save space and to have the elements with similar chemical properties at a single place.

4. Trends in modern periodic table:

(i) In a period, the number of valence electrons increases on moving from left to right.

(ii) In a group the number of valence electrons does not change on moving from top to bottom.

(iii) In a period, the valency of elements varies from $1$ to $4$, then decreases from $4$ to $1$ and becomes zero for noble gases.

(iv) Each element of a group has the same valency which is a characteristic of that group.

(v) In a group the atomic radii increase with increasing atomic number from top to bottom.

(vi) In a period, the atomic radii decrease with increasing atomic number from left to right. But the atomic radius of a noble gas is largest in the period.

(vii) The radius of a cation is smaller than the radius of the atom from which it is formed. This is due to increase in effective nuclear charge. The radius of an anion is greater than the radius of the atom from which it is formed. This is due to decrease in the effective nuclear charge.

(viii) Ionisation energy is the energy needed for the removal of an electron from a gaseous isolated atom to form a gaseous cation.

(ix) Ionisation energy increases from left to right in a period. For alkali metals it has the lowest value in a period and for a noble gas it is the largest in the period.

(x) In a group, the Ionisation energy decreases with increasing atomic number from top to bottom.

(xi) The elements at the extreme left of the periodic table are metals.

(xii) The elements at the extreme right of the periodic table are nonmetals.

(xiii) The elements at the border line between metals and nonmetals are called metalloids or semimetals (e.g. $\mathrm{Si}, \mathrm{Ge}, \mathrm{As}$).

(xiv) In a period, the metallic character decreases from left to right as the atomic number increases. That is, nonmetallic character increases across a period as the atomic number increases.

(xv) In a group, the metallic character increases and nonmetallic character decreases from top to bottom as the atomic number increases.

(xvi) In a group, the reactivity of metals increases with increasing atomic number from top to bottom.

(xvii) In a group, the reactivity of nonmetal decreases from top to bottom.