2 Structure of Atom

Question 1

What is the Dalton’s Atomic Theory?

Answer 1

John Dalton considered that “ all matter was composed of smallest indivisible particle called atom.

Daltons Atomic Theory : This theory is based on law of mass conservation and law of definite proportions.

The salient feature’s of this theory are :-
(1) Every element is composed of extremely small particles called atoms.

(2) Atoms of a particular element are like but differ from atoms of other element.
(3) Atom of each element is an ultimate particle and it has a characteristic mass but is structureless
(4) Atoms are indestructible i.e. they can neither be created nor be destroyed.
(5) Atoms of different elements take part in chemical reaction to form molecule.

Question 2

The name negatron was given by _________.

Answer 2

Thomson

Question 3

The name electron was given by _________.

Answer 3

Stoney

Question 4

Define atomic number.

Answer 4

Atomic Number

It is represented by Z. The number of protons present in the nucleus is called atomic number of an element.

Question 5

What is the formula of calculating number of electrons?

Answer 5

For neutral atom : Number of protons = Number of electrons

For an ion : Number of electrons = Z - (charge on ion)

Z = Atomic Number or No. of Protons

Question 6

Define mass number

Answer 6

Mass Number (A)
It is represented by capital A. The sum of number of neutrons and protons is called the mass number of the element. It is also known as number of nucleons.

Question 7

What is formula of mass number?

Answer 7

A = No. of Protons + No. of Neutrons

Number of Neutrons = A - Z

Question 8

What are isotopes?

Answer 8

Isotopes : They are atoms of a given element which have the same atomic number but differ in their mass number.

Question 9

Which sub-atomic particle distinguishes an isotope?

Answer 9

Neutrons

Isotopes have the same number of protons but differ in the number of neutrons in the nucleus

Question 10

What are isobars?

Answer 10

Isobars are the atoms of different element which have the same mass number but different atomic number i.e they have different number of electron, protons & neutrons but sum of number of neutrons & protons remains same.

Question 11

What are isodiapheres?

Answer 11

Isodiapheres are the atoms of different element which have the same difference of the number of neutrons & protons.

Question 12

What are isotones?

Answer 12

They are the atoms of different element which have the same number of neutrons

Question 13

What are the other names for isotones?

Answer 13

Isotones/Isoneutronic Species/Isotonic

Question 14

What are isosters?

Answer 14

They are the molecules which have the same number of atoms & electrons

Question 15

What are isoelectronic species?

Answer 15

They are the atoms, molecules or ions which have the same number of electrons

Question 16

Isotopes possess _______ value of e/m

Answer 16

Different

Question 17

Isobars possess ________ chemical and physical properties.

Answer 17

Different

Question 18

How were electrons discovered?

Answer 18

Electrons were discovered using the Cathode Ray Experiment by JJ Thomson.

Question 19

Cathode rays depend on the nature of gas. True/False

Answer 19

False

Question 20

How were protons discovered?

Answer 20

Protons were discovered using the Anode Ray Experiment by E. Goldstein

Question 21

How were neutrons discovered?

Answer 21

James Chadwick discovered neutrons by bombarding alpha particles on a thin sheet of beryllium.

Question 22

Which experiment is associated with charge on electron?

Answer 22

Millikan Oil Drop Experiment

Question 23

What is the specific charge of electron?

Answer 23

Charge = - 1.6 x 10^-19 C
Mass = 9.1 x 10^-28 g

Specific Charge = 1.6 x 10^-19C/9.1 x 10^-28 g = 1.76 x 10^8 C/g

Question 24

What is the specific charge of proton?

Answer 24

Charge = 1.6 x 10^-19 C
Mass = 1.672 x 10^-24 g

Specific Charge = 1.6 x 10^-19 C/1.672 x 10^-24 g = 9.58 x 10^4 /g

Question 25

What is the specific charge of neutron?

Answer 25

Charge = 0
Mass = 1.675 x 10^-24 g

Specific Charge = 0/1.675 x 10^-24 g = 0

Question 26

Dalton’s Atomic Theory is based on which two laws?

Answer 26

Law of Conservation of Mass

Law of Definite Proportions

Question 27

What is the relation between Mass of Neutron, Proton & Electron?

Answer 27

Mass of Neutron = Mass of Proton = 1837 x Mass of Electron

Question 28

What is the formula for calculating percentage change in mass?

Answer 28

Percentage Change = |Final Mass - Initial Mass| x 100 / Initial Mass

Question 29

What are the characteristics of Thomson’s Model of an Atom?

Answer 29

Thomson was the first to propose a detailed model of the atom.

Thomson proposed that an atom consists of a uniform sphere of positive
charge in which the electrons are distributed more or less uniformly.

This model of atom is known as “Plum-Pudding model” or “Raisin Pudding Model” or “Watermelon Model”

Question 30

What are the drawbacks of Thomson’s Model?

Answer 30

Drawbacks :

An important drawback of this model is that the mass of the atoms is considered to be evenly spread over that
atom.

It is a static model. It does not reflect the movement of electron

Question 31

What were the observations of Rutherford’s Model?

Answer 31

Rutherford observed that -

(i) Most of the a-particles passed through the gold foil undeflected.
(ii) A small fraction of the a-particles was deflected by small angles.
(iii) A very few a-particles (~1 in 20,000) bounced back, that is, were deflected by nearly 180°

Question 32

What were the conclusions drawn from the Rutherford’s Model?

Answer 32

1) Since most of the a-particle went straight through the metal foil undeflected, it means
that there must be very large empty space
within the atom.

2) Since few of the a-particles were deflected
from their original paths through moderate
angles; it was concluded that whole of the
+ve charge is concentrated and the space
occupied by this positive charge is very small
in the atom

3) When a-particles come closer to this point, they suffer a force of repulsion and deviate from their paths.

4) The positively charged heavy mass which occupies only a small volume in an atom is called nucleus. It is
supposed to be present at the centre of the atom.

5) A very few of the a-particles suffered strong deflections or even returned on their path indicating that the nucleus is rigid and a-particles recoil due to direct collision with the heavy positively charged mass

Question 33

What are the applications of Rutherford’s Model?

Answer 33

(i) An atom consists of a heavy positively charged nucleus where all the protons and neutrons are present. Almost whole of the mass of the atom is
contributed by these nucleons. The magnitude of the +ve charge on the nucleus is different for different atoms.

(ii) The volume of the nucleus is very small and is only a minute fraction of the total volume of the atom.

(iii) There is an empty space around the nucleus called extra nuclear part. In this part electrons are present.
The number of electrons in an atom is always equal to number of protons present in the nucleus. As the
nuclear part of atom is responsible for the mass of the atom, the extra nuclear part is responsible for its
volume.

(iv) Electrons revolve around the nucleus in closed orbits with high speeds. The centrifugal force acting on the
revolving electron is being counterbalanced by the force of attraction between the electrons and the
nucleus.

Question 34

What is the relation between radius of atom and radius of nucleus?

Answer 34

Radius of Atom/Radius of Nucleus = 10^5

Question 35

What is the relation between volume of atom and volume of nucleus?

Answer 35

Volume of Atom/Volume of Nucleus = 10^15

Question 36

What is the formula of calculating radius of atom?

Answer 36

Radius = 1.33 x 10^-13 x A^1/3 cm

Question 37

What are the drawbacks of Rutherford’s Model?

Answer 37

1) This theory could not explain the stability of atom. According to Maxwell
electron looses its energy continuously in the form of electromagnetic radiations. As a result of this, the e- should loose energy at every turn and move closer and closer to the nucleus following a spiral path. The ultimate result will be that it will fall into the nucleus, thereby making the atom
unstable.

2) If the electrons loose energy continuously, the observed spectrum should be continuous but the actual observed spectrum consists of well defined lines of definite frequencies. Hence, the loss of energy by
electron is not continuous in an atom.

Question 38

Thomson’s model of atom explained electrical _________ of atom.

Answer 38

Electrical Neutrality

Question 39

Density of nucleus is dependent on the nature of element. (True/False)

Answer 39

False