Structure of the Atom

Class 9 – NCERT Science Solutions

Intext Questions 1

Question 1

What are the canal rays?

Answer

The canal rays are positively charged radiations present in a gas discharge. They helped in the discovery of another sub-atomic particle that was positively charged called the proton.

Question 2

If an atom contains one electron and one proton, will it carry any charge or not?

Answer

As the atom contains one electron, so it is negatively charged whereas the presence of proton makes the atom positively charged particle. The net charge becomes zero as both particles neutralize the charge present on each other.

Intext Questions 2

Question 1

On the basis of Thompson’s model of an atom, explain how the atom is neutral as a whole.

Answer

As per Thompson’s model of an atom,

(i) An atom consists of a positively charged sphere and the electrons are embedded in it.

(ii) Electrons (negatively charged) and protons (positively charged) are equal in magnitude. So, the atom as a whole, is electrically neutral.

Question 2

On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?

Answer

As per Rutherford’s model of an atom, the positively charged protons are present in the nucleus of an atom.

Question 3

Draw a sketch of Bohr’s model of an atom with three shells.

Answer

Below diagram shows Bohr’s model of an atom with three shells:

Question 4

What do you think would be the observation if the ∝–particle scattering experiment is carried out using a foil of a metal other than gold?

Answer

If the ∝–particle scattering experiment is carried out using a foil of a metal (as thin as gold) other than gold, then also the observation would be same because the structure of an atom, when considered individually is same.

However, other metals are not as malleable as gold so it is difficult to turn those metals into such thin sheets. If thick foil is used then it will result in the change of scattering angles which might affect the predictions of the experiment.

Intext Questions 3

Question 1

Name the three subatomic particles of an atom.

Answer

The three subatomic particles of an atom are:

1. Protons – Positively charged
2. Electrons – Negatively charged
3. Neutrons – No charge (neutral)

Question 2

Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

Answer

Given,

Atomic mass of helium atom = 4u,

2 protons in helium nucleus

We know,

Atomic mass = number of protons + number of neutrons

4 = 2 + number of neutrons

So, number of neutrons = 4 – 2 = 2

Hence, Helium has 2 neutrons.

Intext Questions 4

Question 1

Write the distribution of electrons in carbon and sodium atoms.

Answer

(i) A carbon atom has 6 electrons.

In the first orbit or K-shell = 2 electrons;

In the second orbit or L-shell = 4 electrons;

We can also express the electron distribution in a carbon atom as 2, 4.

(ii) A sodium atom has 11 electrons.

In the first orbit or K shell = 2 electrons;

In the second orbit or L-shell = 8 electrons;

In the third orbit or M-shell = 1 electron

We can express the electron distribution in a sodium atom as 2, 8, and 1.

Question 2

If K and L shells of an atom are full, then what would be the total number of electrons in the atom?

Answer

K shell can hold 2 electrons and L shell can hold 8 electrons according to 2n² rule . When both the shells are full, the total number of electrons present in the atom = 2 + 8 = 10 electrons.

Intext Questions 5

Question 1

How will you find the valency of chlorine, sulphur and magnesium?

Answer

(i) Valency of Chlorine :

The electronic configuration of chlorine = 2, 8, 7. It has 7 (more than 4) electrons in its outermost shell. So, the valency of chlorine = 8 – the number of electrons in the outermost shell = 8 – 7 = 1.

Hence, the valency of Cl is 1.

(ii) Valency of Sulphur:

The electronic configuration of Sulphur = 2, 8, 6. It has 6 (more than 4) electrons in its outermost shell. So, the valency of S = 8 – the number of electrons in the outermost shell = 8 – 6 = 2.

Hence, the valency of S is 2.

(iii) Valency of Magnesium:

The electronic configuration of Magnesium = 2, 8, 2. It has 2 (less than 4) electrons in its outermost shell. So, the valency of magnesium = Number of electrons in its outermost shell = 2

Hence, the valency of Mg is 2.

Intext Questions 6

Question 1

If the number of electrons in an atom is 8 and the number of protons is also 8, then

(i) What is the atomic number of the atom? and

(ii) What is the charge on the atom?

Answer

Given,

Number of electrons = 8

Number of protons = 8

(i) As the atomic number of an atom is the number of electrons in that atom; hence, atomic number = 8.

(ii) As the number of electrons and protons are equal in number. Hence, both the charges (positive and negative) neutralize each other and the atom does not possess any charge.

Question 2

With the help of the given table, find out the mass number of oxygen and sulphur atom.

Composition of Atoms of the First Eighteen Elements with Electron Distribution in Various Shells.

Answer

(a) Mass number of oxygen = ?

Given,

Number of protons = 8

Number of neutrons = 8

Mass number = Number of protons + number of neutrons = 8 + 8 = 16

∴ Mass number of oxygen = 16

(b) Mass number of Sulphur = ?

Number of protons = 16

Number of neutrons = 16

Mass number = Number of protons + number of neutrons = 16 + 16 = 32

∴ Mass number of sulphur = 32

Intext Questions 7

Question 1

For the symbol H, D and T, tabulate three subatomic particles found in each of them.

Answer

Element	Symbol	Mass no.	Atomic no.	No. of electrons	No. of protons	No. of neutrons
Hydrogen	H	1	1	1	1	0
Deuterium	D	2	1	1	1	1
Tritium	T	3	1	1	1	2

Question 2

Write the electronic configuration of any one pair of isotopes and isobars.

Answer

(a) Isotopes — Carbon has two isotopes i.e., ₆C¹² and ₆C¹⁴. They have same electronic configuration as they have same atomic numbers = 6. Hence, their electronic configuration is 2, 4.

(b) Isobars — Calcium [₂₀Ca⁴⁰] atomic number 20 and argon [₁₈Ar⁴⁰] atomic number 18 are isobars as both have same mass number i.e., 40 and different atomic numbers.

Electronic configuration of ₂₀Ca⁴⁰ is 2, 8, 8, 2.

Electronic configuration of ₁₈Ar⁴⁰ is 2, 8, 8.

Exercises

Question 1

Compare the properties of electrons, protons and neutrons.

Answer

Property	Electrons	Protons	Neutrons
Charge	Negatively charged	Positively charged	No charge
Location	Located outside the nucleus	Located within the nucleus	Located within the nucleus
Weight	Mass is negligible	1 a.m.u	1 a.m.u
Affinity	Attracted towards positively charged	Attracted towards negatively charged	Not attracted towards any charged particle

Question 2

What are the limitations of J.J. Thomson’s model of the atom?

Answer

Limitations of J.J. Thomson’s model of the atom were:

1. It failed to explain how the positive charge holds on the electrons inside the atom. It also failed to explain the stability of an atom.
2. The concept of the nucleus was not present in his model.
3. It failed to explain the results of Rutherford’s ∝–particle scattering experiment.

Question 3

What are the limitations of Rutherford’s model of the atom?

Answer

1. The revolution of the electron in a circular orbit is not expected to be stable.
2. Any particle in circular orbit would undergo acceleration, and hence, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus.
3. Hence, atoms must be highly unstable and matter would not exist in its known form. We know that atoms are quite stable.

Question 4

Describe Bohr’s model of the atom.

Answer

Neil Bohr put forward the following postulates about the model of an atom:

1. Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.
2. While revolving in discrete orbits the electrons do not radiate energy.

Question 5

Compare all the proposed models of an atom given in this chapter.

Answer

Thomson’s Model	Rutherford’s Model	Bohr’s model
It was proposed by J.J. Thomson.	It was proposed by Ernst Rutherford.	It was proposed by Neil Bohr.
An atom consists of a positively charged sphere and the electrons are embedded in it.	There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.	Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.
The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.	There is very large empty space between nucleus and electrons.	While revolving in discrete orbits the electrons do not radiate energy.
It could not explain the result of Rutherford’s ∝–particle scattering experiment.	It could not explain the stability of atom because revolving electron will lose energy and will ultimately fall into the nucleus.	This model has overcome the limitations of the Rutherford model by defining energy levels for electrons.

Question 6

Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Answer

The following rules are followed for writing the number of electrons in different energy levels or shells for the first eighteen elements:

1. The maximum number of electrons in each shell or orbit is determined by the formula 2n², where n is the shell number.
   K shell, n=1, no. of electrons = 2 x 1² = 2
   L shell, n=2, no. of electrons = 2 x 2² = 8
   M shell, n=3, no. of electrons = 2 x 3² = 18
2. Electrons are not accommodated in a given shell, unless the inner shells are filled. Shells are filled in step-wise manner.
3. The atom’s outermost shell needs 2 electrons for stability (duplet rule) if it has only one shell. If it has multiple shells, it requires 8 electrons (octet rule) in the outermost shell for stability.

Question 7

Define valency by taking examples of silicon and oxygen.

Answer

The definite combining capacity of the atoms of each element, wherein electrons are lost, gained or shared to make the octet of electrons present in the outermost shell, is defined as valency.

(i) The electronic configuration of silicon [14] is 2, 8, 4. It has 4 electrons in its outermost shell. So, the valency of silicon = 8 – the number of electrons in the outermost shell = 8 – 4 = 4.

Hence, valency of silicon = 4.

(ii) The electronic configuration of oxygen [8] is 2, 6. It has 6 (more than 4) electrons in its outermost shell. So, the valency of oxygen = 8 – the number of electrons in the outermost shell = 8 − 6 = 2.

Hence, valency of oxygen = 2.

Question 8

Explain with examples

(i) Atomic number,

(ii) Mass number,

(iii) Isotopes and

(iv) Isobars.

Give any two uses of isotopes.

Answer

(i) Atomic number — The number of protons present in the nucleus of an atom is defined as the atomic number and is denoted by Z.

Example: Atomic number of hydrogen (Z) = 1 as only one proton is present in the nucleus of hydrogen atom.

(ii) Mass number — The total number of protons and neutrons present in the nucleus of an atom is known as the mass number and is denoted by A.

Example: Mass number of Carbon (A) = 12 as it has 6 protons and 6 neutrons.

(iii) Isotopes — The atoms of the same element having the same atomic number but different mass number are known as isotopes.

Example: Carbon has two isotopes ₆C¹² and ₆C¹⁴

(iv) Isobars — Atoms of different elements with different atomic numbers, which have same mass number are known as isobars.

Example: Calcium [₂₀Ca⁴⁰] atomic number 20 and argon [₁₈Ar⁴⁰] atomic number 18 are isobars as both have same mass number i.e., 40 and different atomic numbers.

Isotopes have special properties which makes them useful in various fields like:

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

Question 9

Na⁺ has completely filled K and L shells. Explain.

Answer

The atomic number of sodium is 11. It has 11 electrons in its orbitals. So, its electronic configuration is K-2, L-8, M-1. The maximum electrons in K, L, M shells according to 2n² rule are 2, 8 and 18 respectively.

In case of Na⁺ (10), one electron from the M shell is lost, and it obtains a positive charge. The new electronic configuration is K-1 and L-8. Hence, it has completely filled shells.

Question 10

If the bromine atom is available in the form of, say, two isotopes ₃₅⁷⁹Br (49.7%) and ₃₅⁸¹Br (50.3%), calculate the average atomic mass of bromine atom.

Answer

The atomic mass of an element is the mass of one atom of that element.

Average atomic mass takes into account the isotopic abundance.

Isotope of bromine with atomic mass 79 u = 49.7%

Therefore, Contribution of ₃₅⁷⁹Br to atomic mass = 79×49.710010079×49.7​ = 39.263 u

Isotope of bromine with atomic mass 81 u = 50.3%

Contribution of ₃₅⁸¹Br to the atomic mass of bromine = 81×50.310010081×50.3​ = 40.743u

Hence, the average atomic mass of the bromine atom = 39.263 + 40.743 = 80.006 u

Question 11

The average atomic mass of a sample of element X is 16.2 u. What are the percentages of isotopes ₈¹⁶X and ₈¹⁸X in the sample?

Answer

Let the percentage of ₈¹⁶X be a and that of ₈¹⁸X be 100 – a.

16.2u = 16a10010016a​ + 18(100-a)10018100(100-a)​

⇒ 1620 = 16a + 1800 – 18a

⇒ 1620 = 1800 – 2a

⇒ 2a = 1800 – 1620

⇒ a = 1800−1620221800−1620​

⇒ a = 18022180​

⇒ a = 90

∴ Percentage of ₈¹⁶X = 90%

and

Percentage of ₈¹⁸X = 100 – a = 100 – 90 = 10%

Hence, the percentage of the isotope ₈¹⁶X in the sample is 90% and that of ₈¹⁸X is 10%

Question 12

If Z=3, what would be the valency of the element? Also, name the element.

Answer

Given,

Atomic number, Z = 3

The electronic configuration of the element = 2,1. Hence, its valency = 1

The element with atomic number 3 is Lithium.

Question 13

Composition of the nuclei of two atomic species, X and Y, are given as under

	X	Y
Protons =	6	6
Neutrons =	6	8

Give the mass numbers of X and Y. What is the relation between the two species?

Answer

Mass number = No. of protons + No. of neutrons

Mass number of X = 6 + 6 = 12

Mass number of Y = 6 + 8 = 14

Number of protons is same in both so atomic number is same however as the number of neutrons is different so mass number is different. Hence, these two elements are isotopes.

Question 14

For the following statements, write T for True and F for False.

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

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

(c) The mass of an electron is about 1200020001​ times that of a proton.

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

Answer

(a) False.

Reason — Thomson proposed that:

1. An atom consists of a positively charged sphere and the electrons are embedded in it.
2. The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

(b) False.

Reason — Neutron is not formed by an electron and a proton combining together. It is present in the nucleus of an atom alongside protons.

(c) True.

Reason — The mass of an electron is about 1200020001​ times that of a proton.

(d) False.

Reason — Tincture iodine is an antiseptic that usually contains 2-7% elemental I-127 dissolved in a mixture of ethanol and water. Normal iodine is used in its preparation, not its isotope. ​

Question 15

Rutherford’s alpha–particle scattering experiment was responsible for the discovery of

1. Atomic nucleus
2. Electron
3. Proton
4. Neutron

Answer

Atomic nucleus

Reason — Rutherford’s alpha–particle scattering experiment was responsible for the discovery of atomic nucleus.

Question 16

Isotopes of an element have

1. The same physical properties
2. Different chemical properties
3. Different number of neutrons
4. Different atomic numbers

Answer

Different number of neutrons

Reason — Isotopes of an element have same atomic number (i.e., number of protons) and different mass number (i.e., sum of protons + neutrons).

Question 17

Number of valence electrons in Cl^– ion are

1. 16
2. 8
3. 17
4. 18

Answer

8

Reason — The electronic distribution of Cl is 2, 8, 7. There are 7 valence electrons in chlorine. It gains one electron and forms Cl^–. Hence, its valency is 8.

Question 18

Which one of the following is a correct electronic configuration of Sodium?

1. 2, 8
2. 8, 2, 1
3. 2, 1, 8
4. 2, 8, 1

Answer

2, 8, 1

Reason — Sodium has atomic number 11 and it’s electronic configuration is 2,8,1.

Question 19

Complete the following table.

Atomic Number	Mass Number	Number of Neutrons	Number of Protons	Number of Electrons	Name of the Atomic Species
9		10
16	32				Sulphur
	24		12
	2		1
	1	0	1	0

Answer

Atomic Number	Mass Number	Number of Neutrons	Number of Protons	Number of Electrons	Name of the Atomic Species
9	19	10	9	9	Fluorine
16	32	16	16	16	Sulphur
12	24	12	12	12	Magnesium
1	2	1	1	1	Deuterium
1	1	0	1	0	Protium