# Atomic Structure NIOS Intext Questions With Answer

### National Institute of Open Schooling (NIOS)

Sr. Secondary, Module-2

Lession-2: Atomic Structure Intext Questions With Answer

## INTEXT QUESTIONS 2.1

### 1. Compare the mass of an electron with that of the proton.

**Answer:**Proton is heavier than electron. The ratio of their masses is

m

_{p}/ m

_{e}= 1.672 x 10

^{−27}kg / 9.109 x 10

^{−31}kg = 1836

### 2. What is a fundamental particle?

**Answer:**Fundamental particles are those which are present in every atom. Every atom has a nucleus in which positively charged particles protons and neutral particles neutrons are present and electrons revolving around nucleus. Thus, we can say proton, neutron and electron are fundamental particles.

### 3. What is the name given to neutral particles in the atom?

**Answer:**Neutron.

## INTEXT QUESTIONS 2.2

### 1. List the three constituent particles of an atom.

**Answer:**Electron, proton and neutron are the three constituent particles of an atom

### 2. What was the aim of Rutherford's α-rays scattering experiment?

**Answer:**The aim of Rutherford's experiment was to test the Thomson's plum-pudding atomic model.

### 3. Briefly describe Rutherford’s model of an atom.

**Answer:**According to Rutherford's model for atom, the positive charge of the atom and most of its mass is contained in the nucleus. The rest of the atom is empty space which contains the much smaller and negatively charged electrons.

### 4. On what basis was the Rutherford’s model rejected?

**Answer:**Rutherford's model was rejected because it could not explain the stability of the atom.

## INTEXT QUESTIONS 2.3

### 1. What is an electromagnetic radiation?

**Answer:**Electromagnetic radiation is a kind of energy which is transmitted through space in the form of electric and magnetic fields. It travels with the same speed of light and does not need any medium to travel.

### 2. List any three characteristics of electromagnetic radiation.

**Answer:**The different characteristics of electromagnetic radiation are-

i. Amplitude

ii. Wavelength

iii. Frequency

iv. Wave number

v. Velocity

### 3. What is wave number? How is it related to wave length?

**Answer:**The wave number is defined as the number of waves per centimeter. It is equal to the reciprocal of the wavelength.

### 4. What is the difference between a 'quantum' and a 'photon'?

**Answer:**A quantum of visible light is called photon. The energy of the quantum (or photon) is proportional to the frequency of radiation.

A photon has the properties of both wave like and particle like, but quantum does not. Quantum can be compared to a measure of quantity, but a photon is not about a measure of quantity.

## INTEXT QUESTIONS 2.4

### 1. What is the difference between a line spectrum and a continuous spectrum?

**Answer:**A line spectrum consists of a series of discrete lines of characteristic wavelengths while a continuous spectrum contains a broad band of radiations containing all possible wavelengths in the range i.e., the wavelengths of the radiation varies continuously.

### 2. What are the main postulates of Bohr's model?

**Answer:**The main postulates of Bohr's model are-

i. In an atom, negatively charged electrons revolve around the positively charged nucleus in a definite circular path called orbits or shells.

ii. Each orbit or shell has a fixed energy and these circular orbits are known as orbital shells.

iii. The energy levels are represented by an integer (n = 1, 2, 3…) known as the quantum number. This range of quantum number starts from nucleus side with n = 1 having the lowest energy level. The orbits n = 1, 2, 3, 4… are assigned as K, L, M, N…. shells and when an electron attains the lowest energy level, it is said to be in the ground state.

iv. The electrons in an atom move from a lower energy level to a higher energy level by gaining the required energy and an electron moves from a higher energy level to lower energy level by losing energy.

v. The angular momentum of an electron in an atom must have certain definite or discrete values and it should not have any value of its own. We can express angular momentum by only permissible values given in the expression-

mvr = nh/2π

### 3. How does the energy of a Bohr orbit vary with the principle quantum number 'n'.

**Answer:**The energy of a Bohr's orbit increases with an increase in the value of the principal quantum number, n. In fact it becomes lesser and lesser negative.

## INTEXT QUESTIONS 2.5

### 1. What do you understand by wave-particle duality?

**Answer:**The wave-particle duality refers to the fact that light and the material particles like electrons could sometimes behave as a particle and as a wave at other times.

### 2. Name the experiment that established the wave nature of electron.

**Answer:**The wave nature of electron was established by the diffraction of electron waves by the crystal lattice of nickel.

### 3. Compute the de-Broglie wavelength associated with an electron moving with a velocity of 100 km /second? (m_{e} = 9.1 x 10^{-31}kg).

**Answer:**Mass of the electron = 9.1 × 10

^{−31}kg

Speed or Velocity = 100 km s

^{−1}= 105 m s

^{−1}

we know that, the wavelength (λ) associated with the electron is h/mv

λ = h/mv

λ = 6.626 x 10

^{34}Js / (9.1 x 10

^{−31}kg x 10

^{5}m/s)

λ = 7.28 x 10

^{−9}m.

### 4. State Heisenberg's Uncertainty Principle?

**Answer:**According to Heisenberg's Uncertainty Principle it is not possible to measure both the position and momentum of a particle simultaneously accurately. More accurately we measure a particle's position, the less accurately we are able to measure it's momentum, and vice versa.

## INTEXT QUESTIONS 2.6

### 1. What do you understand by a Wave Function?

**Answer:**It is a mathematical function that describes the motion of an electron inside the atom. It contains all the information about the system and can be found by solving a wave equation called Schrödinger wave equation.

### 2. What is the difference between an orbit and an orbital?

**Answer:**An orbit refers to definite circular paths of fixed energy around a central stationary nucleus while an orbital refers to the three dimensional region of space around the nucleus where there is a probability of finding the electron.

### 3. What are quantum numbers? List different quantum numbers obtained from Schrödinger Wave Equation?

**Answer:**The quantum numbers are integers that characterize the wavefunctions. These are obtained in the process of solving Schrödinger wave equation and every electron in an atom has a different set of quantum numbers. The three quantum numbers obtained from Schrödinger Wave Equation are-

i. The principal quantum number, n

ii. Azimuthal quantum number, l

iii. The magnetic quantum number m

There is also a fourth quantum number called spin quantum number s.

### 4. Give the significance of the principal, azimuthal and magnetic quantum numbers?

**Answer:**The principal quantum number (n) represents the energy of the electron in a shell. The azimuthal quantum number (l) is related to the geometrical shape of the orbital and the magnetic quantum number (m) describes the orientation of the orbital in space.

## INTEXT QUESTIONS 2.7

### 1. What are the shapes of s, p and d orbitals?

**Answer:**s orbital : spherical

p orbitals : dumb-bell shaped

d orbitals : cloverleaf shaped.

### 2. Describe the shape of a 2s orbital. How is it different from that of a 1s orbital?

**Answer:**The 2s orbital is spherical in shape similar to the 1s orbital . However there are two differences. Firstly, the size of a 2s orbital is bigger as compared to a 1s orbital and secondly, it contains a spherical node.

### 3. What do you understand by

i. a spherical node?

ii. a nodal plane?

**Answer:**i. It is a spherical region of zero probability in an s orbital (other than 1s).

ii. It is a planar region in an orbital (other than s orbitals ) where the probability of finding the electron is zero.

### 4. How many spherical nodes will be there in 3s orbital ?

**Answer:**The 3s orbital will have two spherical nodes.

## INTEXT QUESTIONS 2.8

### 1. What do you understand by the electronic configuration of an atom?

**Answer:**The distribution of electrons in the shells and subshells of an atom is called Electronic Configuration.

### 2. What is Pauli's exclusion principle?

**Answer:**Pauli's principle states that in an atom no two electrons can have same set of the four quantum numbers

### 3. What is Aufbau principle? What are ( n + l ) rules?

**Answer:**Aufbau principle states that the electrons in an atom are filled in the increasing order of their energies which is determined by (n + l) rules.

There are two ( n+l ) rules. These are-

i. An orbital with a lower value for (n + l) is filled first.

ii. If the value of (n + l) is same for two orbitals then the orbital with lower value of n will be filled first

### 4. Which of the following orbitals will be filled first? i) 2p or 3s ii) 3d or 4s

**Answer:**i. 2p: (n + l) for 2p = 2 + 1 = 3

for 3s (n + l) = 3 + 0 = 3

Here n + l = 3 in both cases.So, according to rule-2, 2p orbital will fill first as its 'n' value is smaller. ii. 4s: (n + l) for 4s = 4 + 0 = 4

for 3d (n + l) = 3 + 2 = 5

According to rule 1, 4s orbital will fill first.

### 5. The electronic configuration of Cr is (Ar)3d^{5}4s^{1} not 3d^{4}4s^{2}.

**Answer:**Because half filled d-orbital is more stable as compared to partially filled orbital.