CBSE - Structure of the Atom

Canal rays are positively charged radiations that can pass through perforated cathode plate. These rays consist of positively charged particles known as protons.

An electron is a negatively charged particle, whereas a proton is a positively charged particle. The magnitude of their charges is equal. Therefore, an atom containing one electron and one proton will not carry any charge. Thus, it will be a neutral atom.

As per Thomson’s model of the atom, an atom consists both negative and positive charges which are equal in number and magnitude. So, they balance each other as a result of which atom as a whole is electrically neutral.

On the basis of Rutherford's model of an atom, protons are present in the nucleus of an atom.

Bohr's model of an atom with three shells

If α-particle scattering experiment is carried out using a foil of any metal as thin as gold foil used by Rutherford, there would be no change in observations. But since other metals are not so malleable so, such a thin foil is difficult to obtain. If we use a thick foil, then more α-particles would bounce back and no idea about the location of positive mass in the atom would be available with such a certainty. 

The three sub-atomic particles of an atom are:
(i) Protons
(ii) Electrons, and
(iii) Neutrons

Number of neutrons = Atomic mass - Number of protons
Therefore, the number of neutrons in the atom = 4 - 2 = 2

-> The total number of electrons in a carbon atom is 6. The distribution of electrons in carbon atom is given by:

First orbit or K-shell = 2 electrons
Second orbit or L-shell = 4 electrons

Or, we can write the distribution of electrons in a carbon atom as 2, 4.

-> The total number of electrons in a sodium atom is 11. The distribution of electrons in sodium atom is given by:

First orbit or K-shell = 2 electrons
Second orbit or L-shell = 8 electrons
Third orbit or M-shell = 1 electron

Or, we can write distribution of electrons in a sodium atom as 2, 8, 1.

The maximum capacity of K shell is 2 electrons and L shell can accommodate maximum 8 electrons in it. Therefore, there will be ten electrons in the atom.

If the number of electrons in the outermost shell of the atom of an element is less than or equal to 4, then the valency of the element is equal to the number of electrons in the outermost shell. On the other hand, if the number of electrons in the outermost shell of the atom of an element is greater than
4, then the valency of that element is determined by subtracting the number of electrons in the outermost shell from 8.
The distribution of electrons in chlorine, sulphur, and magnesium atoms are 2, 8, 7; 2, 8, 6 and 2, 8, 2 respectively.

Therefore, the number of electrons in the outer most shell of chlorine, sulphur, and magnesium atoms are 7, 6, and 2 respectively.

► Thus, the valency of chlorine = 8 -7 = 1

► The valency of sulphur = 8 - 6 = 2

► The valency of magnesium = 2

(i) The atomic number is equal to the number of protons. Therefore, the atomic number of the atom is 8.

(ii) Since the number of both electrons and protons is equal, therefore, the charge on the atom is 0.

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

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

Symbol	Proton	Neutron	Electron
H	1	0	1
D	1	1	1
T	1	2	1

¹²C₆ and ¹⁴C₆ are isotopes, have the same electronic configuration as (2, 4)²²Ne₁₀and ²²Ne₁₁ are isobars. They have different electronic configuration as given below:
²²Ne₁₀ – 2, 8
²²Ne₁₁ – 2, 8, 1

Particle	Nature of Charge	Mass	Location
Electron	Electrons are negatively charged.	9 x 10–31 kg	Extra nuclear part distributed in different shell or orbits.
Proton	Protons are positively charged.	1.672 x 10–27 kg (1 µ) (approx. 2000 times that of the electron)	Nucleus
Neutron	Neutrons are neutral.	Equal to mass of proton	Nucleus

The limitations of J.J. Thomson's model of the atom are:
→ It could not explain the result of scattering experiment performed by rutherford.
→ It did not have any experiment support.

The limitations of Rutherford's model of the atom are
→ It failed to explain the stability of an atom.
→ It doesn't explain the spectrum of hydrogen and other atoms.

→ The atom consists of a small positively charged nucleus at its center.
→ The whole mass of the atom is concentrated at the nucleus and the volume of the nucleus is much smaller than the volume of the atom.
→ All the protons and neutrons of the atom are contained in the nucleus.
→ Only certain orbits known as discrete orbits of electrons are allowed inside the atom.
→ While revolving in these discrete orbits electrons do not radiate energy. These orbits or cells are represented by the letters K, L, M, N etc. or the numbers, n = 1, 2, 3, 4, . . as shown in below figure.

Thomson’s model	Rutherford’s model	Bohr’s model
→ An atom consists of a positively charged sphere and the electrons are embedded in it. → The negative and positive charges are equal in magnitude. As a result the atom is electrically neutral.	→ An atom consists of a positively charged center in the atom called the nucleus. The mass of the atom is contributed mainly by the nucleus. →  The size of the nucleus is very small as compared to the size of the atom. → The electrons revolve around the nucleus in well-defined orbits.	→ Bohr agreed with almost all points as said by Rutherford except regarding the revolution of electrons for which he added that there are only certain orbits known as discrete orbits inside the atom in which electrons revolve around the nucleus. → While revolving in its discrete orbits the electrons do not radiate energy.

The rules for writing of the distribution of electrons in various shells for the first eighteen elements are given below.
→ If n gives the number of orbit or energy level, then 2n² gives the maximum number of electrons possible in a given orbit or energy level. Thus,
First orbit or K-shell will have 2 electrons,
Second orbit or L-shell will have 8 electrons,
Third orbit or M-shell will have 18 electrons.
→ If it is the outermost orbit, then it should have not more than 8 electrons.
→ There should be step-wise filling of electrons in different orbits, i.e., electrons are not accompanied in a given orbit if the earlier orbits or shells are incompletely filled.

The valency of an element is the combining capacity of that element. The valency of an element is determined by the number of valence electrons present in the atom of that element.→ Valency of Silicon: It has electronic configuration: 2,8,4
Thus, the valency of silicon is 4 as these electrons can be shared with others to complete octet.
→ Valency of Oxygen: It has electronic configuration: 2,6
Thus, the valency of oxygen is 2 as it will gain 2 electrons to complete its octet.

(i) Atomic number: The atomic number of an element is the total number of protons present in the atom of that element. For example, nitrogen has 7 protons in its atom. Thus, the atomic number of nitrogen is 7.

(ii) Mass number: The mass number of an element is the sum of the number of protons and neutrons present in the atom of that element. For example, the atom of boron has 5 protons and 6 neutrons. So, the mass number of boron is 5 + 6 = 11.

(iii) Isotopes: These are atoms of the same element having the same atomic number, but different mass numbers. For example, chlorine has two isotopes with atomic number 17 but mass numbers 35 and 37 represented by



(iv) Isobars: These are atoms having the same mass number, but different atomic numbers i.e., isobars are atoms of different elements having the same mass number. For example, Ne has atomic number 10 and sodium has atomic number 11 but both of them have mass numbers as 22 represented by -



Two uses of isotopes:
→ One isotope of uranium is used as a fuel in nuclear reactors.
→ One isotope of cobalt is used in the treatment of cancer.

The atomic number of sodium is 11. So, neutral sodium atom has 11 electrons and its electronic configuration is 2, 8, 1. But Na⁺ has 10 electrons. Out of 10, K-shell contains 2 and L-shell 8 electrons respectively. Thus, Na⁺ has completely filled K and L shells.

It is given that two isotopes of bromine are (49.7%) and (50.3%). Then, the average atomic mass of bromine atom is given by:

It is given that the average atomic mass of the sample of element X is 16.2 u.
 
Let the percentage of isotope be y %. Thus, the percentage of isotope will be (100 - y) %.

Therefore,



Therefore, the percentage of isotopeis 10%.

And, the percentage of isotopeis (100 - 10) % = 90%.

By Z = 3, we mean that the atomic number of the element is 3. Its electronic configuration is 2, 1. Hence, the valency of the element is 1 (since the outermost shell has only one electron).
Therefore, the element with Z = 3 is lithium.

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 of X = Number of protons + Number of neutrons

= 6 + 6
= 12 

Mass number of Y = Number of protons + Number of neutrons 
= 6 + 8
= 14

These two atomic species X and Y have the same atomic number, but different mass numbers. Hence, they are isotopes.

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

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

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

► True

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

► False

Rutherford's alpha-particle scattering experiment was responsible for the discovery of
(a) Atomic nucleus
(b) Electron
(c) Proton
(d) Neutron
ANS (a) Atomic nucleus

Isotopes of an element have
(a) the same physical properties
(b) different chemical properties
(c) different number of neutrons
(d) different atomic numbers
ANS (c) different number of neutrons

Number of valence electrons in Cl ^-ion are:
(a) 16
(b) 8
(c) 17
(d) 18
ANS (b) 8

Which one of the following is a correct electronic configuration of sodium?
(a) 2, 8
(b) 8, 2, 1
(c) 2, 1, 8
(d) 2, 8, 1
ANS (d) 2, 8, 1

Complete the following table.

Answer
 

1. Which of the following correctly represent the electronic distribution in the Mg atom?
(a) 3, 8, 1
(b) 2, 8, 2
(c) 1, 8, 3
(d) 8, 2, 2
Ans. (b) 2, 8, 2
Explanation: The first shell can have at the most two electrons in it. Hence option �b� is correct.

2. Rutherford�s �alpha (α) particles scattering experiment� resulted in to discovery of
(a) Electron
(b) Proton
(c) Nucleus in the atom
(d) Atomic mass
Ans. (c) Nucleus in the atom
Explanation: The observation that some alpha particles returned on their original path showed the presence of nucleus in the centre of an atom.

3. The number of electrons in an element X is 15 and the number of neutrons is 16. Which of the following is the correct representation of the element?

(
Ans. (a)

Explanation: Atomic number (no. of electrons) is written in subscript, while mass number is written in superscript; before the symbol of element.

4. Dalton�s atomic theory successfully explained
(i) Law of conservation of mass
(ii) Law of constant composition
(iii) Law of radioactivity
(iv) Law of multiple proportion
(a) (i), (ii) and (iii)
(b) (i), (iii) and (iv)
(c) (ii), (iii) and (iv)
(d) (i), (ii) and (iv)
Ans. (d) (i), (ii) and (iv)

Explanation: Dalton�s theory explained the law of conservation of mass, law of constant composition and law of multiple proportions. But the theory did not talk about law of ratio activity.

5. Which of the following statements about Rutherford�s model of atom are correct?
(i) considered the nucleus as positively charged
(ii) established that the α�particles are four times as heavy as a hydrogen atom
(iii) can be compared to solar system
(iv) was in agreement with Thomson�s model
(a) (i) and (iii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) only (i)
Ans. (a) (i) and (iii)
Explanation: Alpha particles are positively charged and hence were deflected by the nucleus. This showed that nucleus is positively charged. Rutherford also postulated that electrons are arranged around the nucleus; the way planets are arranged around the sun.

6. Which of the following are true for an element?
(i) Atomic number = number of protons + number of electrons
(ii) Mass number = number of protons + number of neutrons
(iii) Atomic mass = number of protons = number of neutrons
(iv) Atomic number = number of protons = number of electrons
(a) (i) and (ii)
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (ii) and (iv)
Ans. (d) (ii) and (iv)
Explanation: Atomic number = number of protons = number of electrons. Since electron has negligible mass; so, masses of protons and neutrons are taken into account for mass number.

7. In the Thomson�s model of atom, which of the following statements are correct?
(i) the mass of the atom is assumed to be uniformly distributed over the atom
(ii) the positive charge is assumed to be uniformly distributed over the atom
(iii) the electrons are uniformly distributed in the positively charged sphere
(iv) the electrons attract each other to stabilize the atom
(a) (i), (ii) and (iii)
(b) (i) and (iii)
(c) (i) and (iv)
(d) (i), (iii) and (iv)
Ans. (a) (i), (ii) and (iii)
Explanation: Thomson proposed that negatively charge electrons are stabilized by positively charged nucleus. Hence, option (iv) is not correct. Rest of the options are correct.

8. Rutherford�s α�particle scattering experiment showed that
(i) electrons have negative charge
(ii) the mass and positive charge of the atom is concentrated in the nucleus
(iii) neutron exists in the nucleus
(iv) most of the space in atom is empty
Which of the above statements are correct?
(a) (i) and (iii)
(b) (ii) and (iv)
(c) (i) and (iv)
(d) (iii) and (iv)
Ans. (b) (ii) and (iv)

9. The ion of an element has 3 positive charges. Mass number of the atom is 27 and the number of neutrons is 14. What is the number of electrons in the ion?
(a) 13
(b) 10
(c) 14
(d) 16
Ans. (b) 10
Explanation: Mass number = 27 and number of neutrons = 14
Hence, number of electrons in atom = 27 � 14 = 13
Now, ion has 3 positively charges, so number of electrons in the ion = 13 � 3 = 10

10. Identify the Mg2+ ion from the Fig.4.1 where, n and p represent the number of neutrons and protons respectively
(a) (b)(c)(d)

(d)
Explanation: Electronic configuration of Mg atom is: 2, 8, 2
So, electronic configuration of Mg2+ ion is; 2, 8.

11. In a sample of ethyl ethanoate (CH₃COOC₂H₅) the two oxygen atoms have the same number of electrons but different number of neutrons. Which of the following is the correct reason for it?
(a) One of the oxygen atoms has gained electrons
(b) One of the oxygen atoms has gained two neutrons
(c) The two oxygen atoms are isotopes
(d) The two oxygen atoms are isobars.
Ans. (c) The two oxygen atoms are isotopes

Explanation: Isotopes are elements with same atomic number but different mass numbers. The different in mass number is because of different number of neutrons in them.

12. Elements with valency 1 are
(a) always metals
(b) always metalloids
(c) either metals or non-metals
(d) always non-metals
Ans. (c) either metals or non-metals
Explanation: If an element show positive valency then it is a metal; otherwise it is a nonmetal.

13. The first model of an atom was given by
(a) N. Bohr
(b) E. Goldstein
(c) Rutherford
(d) J.J. Thomson
Ans. (d) J.J. Thomson

14. An atom with 3 protons and 4 neutrons will have a valency of
(a) 3
(b) 7
(c) 1
(d) 4
Ans. (c) 1
Explanation: Electronic configuration of this element: 2, 1
Since number of electron in outermost shell is 1, hence valency is 1.

15. The electron distribution in an aluminium atom is
(a) 2, 8, 3
(b) 2, 8, 2
(c) 8, 2, 3
(d) 2, 3, 8
Ans. (a) 2, 8, 3
Explanation: Atomic number of aluminium is 13 and the first shell can have at the most two electrons in it only. Hence option (a) is correct.

16. Which of the following in Fig. 4.2 do not represent Bohr�s model of an atom correctly?

(a) (i) and (ii)
(b) (ii) and (iii)
(c) (ii) and (iv)
(d) (i) and (iv)
Ans. (c) (ii) and (iv)
Explanation: The second shell cannot have more than 88 electrons and first shell cannot have more than 2 electrons.

17. Which of the following statement is always correct?
(a) An atom has equal number of electrons and protons.
(b) An atom has equal number of electrons and neutrons.
(c) An atom has equal number of protons and neutrons.
(d) An atom has equal number of electrons, protons and neutrons.
Ans. (a) An atom has equal number of electrons and protons.

18. Atomic models have been improved over the years. Arrange the following atomic models in the order of their chronological order
(i) Rutherford�s atomic model
(ii) Thomson�s atomic model
(iii) Bohr�s atomic model
(a) (i), (ii) and (iii)
(b) (ii), (iii) and (i)
(c) (ii), (i) and (iii)
(d) (iii), (ii) and (i)
Ans. (c) (ii), (i) and (iii)

Yes, it is true for hydrogen atom which is represented as ¹₁H

Discovery of electrons and protons

They are isotopes. Isotopes have same number of electrons in them. Hence, their valencies do not differ.

Gold is highly malleable. Hence, it can be made into very thin sheet. Rutherford wanted a metal sheet which could be as thin as possible. So, he selected gold foil for his alpha-ray scattering experiment.

Atom ‘a’ has zero valency; while atom ‘b’ has a valency of 1.

Single and positive charge (+1).

2, 8, 7. The L shell has eight electrons

This element needs to gain two electrons in order to acquire noble gas configuration. So, the charge on ion would be – 2.

What information do you get from the Fig. 4.4 about the atomic number, mass number and valency of atoms X, Y and Z? Give your answer in a tabular form.

Student is giving wrong statement. Number of protons can never be more than that of neutrons. It can be equal to or less than the number of neutrons. Number of electrons and protons is always equal in an atom.

Mass number = No. of protons + No. of neutrons = 31

∴ Number of neutrons = 31 – number of protons

= 31 – 15 = 16

Ans. (a) (iii) (b) (iv) (c) (i) (d) (ii) (e) (vi) (f) (vii) (g) (v)

Elements with same mass number and different atomic numbers are called isobars.

Complete the Table 4.1 on the basis of information available in the symbols given below

(a) ³⁵₁₇CI

(b) ¹²₆C

(c) ⁸¹₃₅Br

Helium atom has 2 electrons in its outermost shell and its duplet is complete. Hence the valency is zero.

(a) Rutherford’s α-particle scattering experiment led to the discovery of the __________.
Ans. atomic nucleus
(b) Isotopes have same __________ but different __________.
Ans. atomic number, mass number
(c) Neon and chlorine have atomic numbers 10 and 17 respectively. Their valencies will
be __________ and __________ respectively.
Ans. 0 and 1.
(d) The electronic configuration of silicon is __________ and that of Sulphur is __________.
Ans. Silicon—2, 8, 4, Sulphur— 2, 8, 6 

Valency is zero as K shell is completely filled.

Helium has two electrons in its only energy shell, while Argon and Neon have 8 electrons in their valence shells. As these have maximum number of electrons in their valence shells, they do not have any tendency to combine with other elements. Hence, they have a valency equal to zero.

(ii) If the atom is represented by the planet earth (R_e= 6.4 �10⁶ m) then the radius of the nucleus would be r_n = R_e /10⁵

r_n = 6.4 x 10⁶ m / 10⁵
= 6.5 x 10 m
= 64 m

Rutherford concluded from the α-particle scattering experiment that–
(i) Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected.
(ii) Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space. 
(iii) A very small fraction of α-particles were deflected by 180^o, indicating that all the positive charges and mass of the gold atom were concentrated in a very small volume within the atom.
From the data he also calculated that the radius of the nucleus is about 10⁵ times less than the radius of the atom.

Rutherford proposed a model in which electrons revolve around the nucleus in welldefined orbits. There is a positively charged centre in an atom called the nucleus. He also proposed that the size of the nucleus is very small as compared to the size of the atom and nearly all the mass of an atom is centred in the nucleus. Whereas, Thomson proposed the model of an atom to be similar to a christmas pudding. The electrons are studded like currants in a positively charged sphere like christmas pudding and the mass of the atom was supposed to be uniformly distributed.

The orbital revolution of the electron is not expected to be stable. Any particle in a circular orbit would undergo a acceleration and the charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know

The postulates put forth by Neils Bohr’s about the model of an atom:
(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.
(ii) While revolving in discrete orbits the electrons do not radiate energy.
These orbits are called energy levels. Energy levels in an atom are shown by circles.
These orbits are represented by the letters K, L, M, N, … or the numbers, n = 1, 2, 3, 4, …. 

Since the atomic number of sodium atom is 11, it has 11 electrons. A positively charged sodium ion (Na+) is formed by the removal of one electron from a sodium atom. So, a sodium ion has 11–1 = 10 electrons in it. Thus, electronic distribution of sodium ion will be 2, 8. The atomic number of an element is equal to the number of protons in its atom. Since, sodium atom and sodium ion contain the same number of protons, therefore, the atomic number of both is 11.

% of α-particles deflected more than 50⁰ =1% of α-particles.
 % of α-particles deflected less than 50⁰ =100–1 = 99%
 Number of α-particles bombarded = 1 mole = 6.022 ×10²³ particles
 Number of particles that deflected at an angles less than 50⁰ 

1. Electron was discovered by J.J.Thom son and proton by Goldstein.
2. The mass of proton was 2000 times as that of electron.
3. The mass of proton is taken as unit charge and its charge is +1, mass of electron is negligible and its charge is minus one.
4. J.J.Thomson proposed a model which states that an atom consist of a positively charged sphere and electrons are embedded in it.
5. He also stated that +ve and —ve charges are equal in magnitude and atom as a whole is electrically neutral.
6. Rutherford’s scattering experiment discovered fast moving (a) particles and also atomic nucleus.
7. Rutherford’s model states that atomic nucleus is +vely charge and lies in the centre, and electrons revolve around nucleus in well-defined orbits, size of nucleus is very small.
8. Rutherford could not explain the stability of an atom.
9. Neils Bohr suggested that electrons revolve in discrete orbits and does not radiate energy. If the atomic shells are complete then atoms are stable and less reactive.
10. J. Chadwick discovered subatomic particle with no charge called neutron.
11. Maximum number of electron present in a shell is given by formula 2n².
12. The maximum number of electrons that can be accommodated in outermost orbit is 8 is called octet.
13. To maintain octet atoms share, lose or gain the electrons.
14. Valency is the combining capacity of an atom.
15. Atomic number is the total number of protons present in nucleus of an atom.Represented by Z.
16. Mass number ¡s sum of total number of protons and neutrons present in nucleus of atom.
17. Isotopes are atoms of same elements which have different mass number, and isobars are atoms with some mass number but different atomic number.