Question

(a) Explain why
(i) All the elements of a group have similar chemical properties.
(ii) All the elements of a period have different chemical properties.
(b) The atomic radii of three elements X, Y and Z of a period of the periodic table are 186 pm; 104 pm and 143 pm respectively. Giving a reason, arrange these elements in the increasing order of atomic numbers in the period.

Answer 1

(a)(i) Elements in the same group have the same number of valence electrons in their outermost shells. Since the number of valence electrons determines the chemical properties of an element and how it reacts with other elements, elements in the same group have similar properties. For e.g. Sodium and Potassium are placed in the same group and have similar chemical properties.

(a)(ii) Elements in a period have the same number of shells, however this does not affect their chemical properties. However, all of them have different number of valence electrons in the valence shell. Therefore, they have different chemical properties and react differently with other elements. For e.g, Sodium and Argon are placed in the same period, but sodium is a very reactive metal whereas argon is inert and does not react.

(b)(i) The atomic radii of elements in a period decreases from left to right. This is due to the increasing number of protons and subsequent increase in the effective nuclear charge, pulling the electron shells closer to the nucleus.

Therefore, X, Y, Z would be arranged in the following order based on their atomic numbers:

X(186pm), Z(143pm), Y(104pm)

Or X < Z < Y

Answer 2

Explanation:

• (a) (i) In a group, all the elements have similar chemical properties. This is due to the reason that in the outermost shell, they have the same number of valence electrons. Their atoms have same no. of electrons in the energy level that is highly occupied.
• (ii) In a period, all the elements have various chemical properties. This is due to the fact that in their atoms, they have different number of valence electrons.
• (b) Order of elements in atomic number: $Y>Z>X$. This is due to the fact that in a period, the atomic number increases from left to right. And, there will be a decrease in the size of the atoms.