NCERT Solutions For Class 9 Science Chapter 4 Structure of the Atom

8

Isotopes of an element have different number of neutrons.

Atomic nucleus 

2,8,1

(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons. (F)

(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral. (F)

(c) The mass of an electron is about 1/2000 times that of proton. (T)

(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine. (T)

We know that

Mass number = Number of protons + Number of neutrons

Mass number of X = 6 + 6 = 12

Mass number of Y = 6 + 8 = 14

X and Y have the same atomic number but different mass numbers therefore; X and Y are the Isotopes.

Z represents the atomic number of an element. Atomic number 3 is of the element Lithium.

Electronic configuration of Lithium is 2,1. In order to complete its duplet it will lose its valence electron therefore, its valency is 1.

Atomic number of Sodium (Na) is 11.

Electronic configuration of Sodium (Na) is 2,8,1.

In order to complete its octet it will lose its valence electron and form Sodium ion (Na⁺).

Electronic configuration of sodium ion (Na⁺) is 2,8.

In the electronic configuration of sodium ion both K and L shell are completely filled. 

According to J.J. Thomson’s model of an atom, an atom consists of a positively charged sphere and the electrons are embedded in it. However, later it was found that the positively charged particles are present at the centre of the atom (nucleus) and electrons revolve around the nucleus.

Valency is the number of electrons gained, lost or shared by an atom so as to make the octet of the electrons in the outermost shell.

For example,

1. Atomic number of Silicon is 14.

Electronic configuration of Silicon is 2,8,4.

Since the number of valence electrons is 4 therefore, valency of Silicon is 4.

2. Atomic number of Oxygen is 8.

Electronic configuration of Oxygen is 2,6.

Since the number of valence electrons is more than 4 therefore, valency of Oxygen is 8 – 6 i.e., 2.

The limitations of Rutherford’s model of the atom are:

• According to the electromagnetic theory, if a charged particle moves around another charged particle, it accelerates and continuously loses energy in the form of the radiant energy. Loss of energy would slow down the speed of the electron and eventually, the electron would fall into the nucleus. However, such a collapse did not occur and Rutherford's model was unable to explain the reason behind it.
• It does not say anything about the distribution of electrons around the nucleus and the energy of electrons.

(i) Atomic number:

• It represents the total number of protons present in the nucleus of an atom.
• It is denoted by Z.
• All the atoms of an element have the same atomic number.
• The number of electrons and protons in an atom are equal.

For example,

Atomic number of Boron is 5

Atomic number of Aluminium is 13

Atomic number of Argon is 18

(ii) Mass number:

• It represents the total number of protons and neutrons present in the nucleus of an atom.
• It is denoted by A.
• The mass of an atom is due to the protons and neutrons present in the nucleus of an atom.
• Protons and neutrons are collectively called the nucleons.

For example,

Mass number of Boron is 11

Mass number of Aluminium is 27

Mass number of Argon is 40

(iii) Isotopes:

• These are the atoms of the same element that have the same atomic number but different mass number.
• Isotopes of a particular element have the same chemical properties but different physical properties.

For example,

Carbon has three isotopes ¹²C, ¹³C, ¹⁴C

Boron has two isotopes ¹⁰B, ¹¹B

(iv) Isobars:

• These are the atoms that have the same mass numbers but different atomic numbers.
• Isobars have different chemical as well as physical properties.

For example,

Ar⁴⁰, ₁₉K⁴⁰, ₂₀Ca⁴⁰

C¹⁴, ₇N¹⁴

Uses of Isotopes

1. An isotope of uranium is used as fuel in the nuclear reactor.
2. An isotope of cobalt is used in the treatment of cancer.

The rules for distributing the electrons in various shells are given below.

1.The maximum number of electrons present in a shell (n) is given by the formula 2n², where ‘n’ is an integer.

For example,

Maximum number of electrons in K shell is 2(1)² = 2 electrons

Maximum number of electrons in L shell is 2(2)² = 8 electrons

2.The maximum number of electrons in the outermost orbit of an atom is 8.

For example,

Total number of electrons 13

Expected distribution is 2,11 but actual distribution is 2,8,3

3.Electrons are not accommodated in a given shell, unless the inner shells are filled, i.e., shells are filled in a step-wise manner.

For example,

In order to accommodate 13 electrons in K, L and M shell, the correct distribution is 2,8,3 and not 2,7,4.

1. Electrons revolve around the nucleus in certain discrete orbits (energy levels) without losing any energy.
2. An electron present in a particular orbit possesses a definite amount of energy.
3. An electron does not radiate energy (lose energy) even though it accelerates the motion around the nucleus.