Question

Elements in a group generally possess similar properties, but elements along a period have different properties. How do you explain this statement?

Answer 1

Elements in the periodic table are placed in different groups and periods according to their properties. Elements belonging to the same group have the same properties while elements belonging to different groups have different properties.

Complete step by step answer:
Let us first discuss the classification of elements in the modern periodic table. Modern periodic table is most accepted due to many advantages over Mendeleev’s periodic table. Mode parodic classification was given by Moseley in 1912. According to him physical and chemical properties of elements are a periodic function of their atomic number.
Modern periodic table relates the position of an element in the table to its electronic configuration more clearly.
The elements in the same group show similar properties due to similar outer electronic configuration.
Let us find electronic configuration${1^{st}}$group element.
$Li \to 3 \to 1{s^2}2{s^1} \to \left[ {He} \right]2{s^1}$
$Na \to 11 \to 1{s^2}2{s^2}2{p^2}3{s^2} \to \left[ {Ne} \right]3{s^1}$
$K \to 19 \to 1{s^2}2{s^2}2{p^2}3{s^2}3{p^6}4{s^1} \to \left[ {Ar} \right]4{s^1}$
$Rb \to 37 \to \left[ {Kr} \right]5{s^1}$
$Cs \to 55 \to \left[ {Xe} \right]6{s^1}$
Valence electrons in above elements is 1.
Therefore, most chemical properties are similar because chemical properties of elements depend on the number of valence electrons in its outermost orbit. They are commonly called alkali metals.
Second group element$\left[ {n{s^2}n{p^1}} \right]$ are called alkali earth metals and ${17^{th}}$group element$\left[ {n{s^2}n{p^5}} \right]$ are called halogens.
If we move along the same period electron added to the same energy level, it changes the atomic size of the atom of element and thus also properties of elements.
Let us discuss atomic size of second period elements

| Na| Mg| Al| Si| P| S| Cl| Ar
---|---|---|---|---|---|---|---|---
EC$\to$| 2,8,1| 2,8,2| 2,8,3| 2,8,4| 2,8,5| 2,8,6| 2,8,7| 2,8,8
Atomic radius $\to$| 186| 160| 143| 117| 110| 104| 99| 191

As we move from left to right in periodic table atomic size decreases due to entry of electron in the same energy level, that increases attraction between electron and proton in nucleus therefore atomic radius decreases and also atomic size decreases.

Note:
Elements belonging to the same group have the same properties but with the same period they show gradation in properties.