Living Science Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure

ICSE Living Science Chemistry for Class 8 Solutions Chapter 4 Atomic Structure

Check Your Progress (Page 41)

Answer these questions.
Question 1.
Name the three subatomic particles present in an atom.
Answer:
Electron, proton and neutron.

Question 2.
Who discovered electrons?
Answer:
JI. Thomson.

Question 3.
What is the positively-charged particle present in an atom called?
Answer:
Proton.

Question 4.
What are the protons and neutrons collectively known as?
Answer:
Nucleons.

Question 5.
What does the extra-nuclear region of an atom contain?
Answer:
Electrons.

Think And Answer (Page 43)

Name the element that has only two types of subatomic particles.
Answer:
Hydrogen, it contains one proton and one electron.

A. Tick the most appropriate answer

Question 1.
Which of the following scientists observed that cathode rays consist of negatively-charged particles?
a. John Dalton
b. J.J Thomson
c. E. Goldstein
d. James Chadwick
Answer:
c. E. Goldstein

Question 2.
Rutherford’s alpha-scattering experiment led to the discovery of which of the following?
a. nucleus
b. electrons
c. protons
d. neutrons
Answer:
a. nucleus

Question 3.
What is an electrode connected to the negative terminal of a voltage source called?
a. cathode
b. neutron
c. anode
d. proton
Answer:
a. cathode

Question 4.
What is an electrode connected to the positive terminal of a voltage source called?
a. cathode
b. neutron
c. anode
d. proton
Answer:
c. anode

Question 5.
What is the positively-charged core in the centre of an atom called?
a. neutron
b. anode
c. proton
d. nucleus
Answer:
d. nucleus

Question 6.
Which of the following symbols represents the atomic number of an element?
a. Z
b. p
c. e
d. A
Answer:
a. Z

Question 7.
Which of the following is not present in a hydrogen atom?
a. proton
b. neutron
c. nucleus
d. electron
Answer:
b. neutron

Question 8.
What is the maximum number of electrons that can be accommodated by the M shell?
a. 2
b. 8
c. 18
d. 32
Answer:
c. 18

B. Fill in the blanks.

Question 1.
An electron carries a ……………… charge.
Answer:
An electron carries a negative charge.

Question 2.
An atom is electrically ………………
Answer:
An atom is electrically neutral.

Question 3.
The mass of a ……………… is equal to that of hydrogen.
Answer:
The mass of a proton is equal to that of hydrogen.

Question 4.
The paths along which electrons revolve around the nucleus are called ………………
Answer:
The paths along which electrons revolve around the nucleus are called orbits.

Question 5.
A ……………… has mass approximately equal to that of a proton.

Answer:
A neutron has mass approximately equal to that of a proton.

Question 6.
The number of protons present in the nucleus of an atom is known as its ………………
Answer:
The number of protons present in the nucleus of an atom is kpown as its atomic number.

Question 7.
The outermost shell of an atom is also called the ……………… shell.
Answer:
The outermost shell of an atom is also called the valence shell.

C. Write true or false. Correct the false statements

Question 1.
Maharshi Kanada named the smallest particle of matter anu.
Answer:
True

Question 2.
All atoms of an element differ from each other.
Answer:
True

Question 3.
Atoms of different elements combine in fixed, small whole- number ratios to form compounds.
Answer:
True

Question 4.
An electrode connected to the negative terminal of a voltage source is called the anode.
Answer:
False; An electrode connected to the positive terminal of a voltage source is called the anode.

Question 5.
An electron is considered nearly massless.
Answer:
True

Question 6.
The positive charge of the nucleus is due to the neutrons present in it.
Answer:
False; The positive charge of the nucleus is due to the protons present in it.

Question 7.
The number of protons present in the nucleus of an atom is known as its mass number.
Answer:
False; The sum of the protons and neutrons present in the nucleus of an atom is called its mass number.

Question 8.
Thomson proposed that the nucleus of an atom contains protons and neutrons.
Answer:
False; Ernest Rutherford proposed that the nucleus of an atom contains protons and neutrons.

D. Name the following

Question 1.
A subatomic particle that carries a negative charge.
Answer:
Electrons

Question 2.
The model that states the atom is a sphere of positive charges and the electrons are spread out inside it.
Answer:
Plum pudding model

Question 3.
All the mass of an atom is concentrated in it.
Answer:
Nucleus

Question 4.
A subatomic particle that is electrically neutral.
Answer:
Neutrons

Question 5.
Arrangement of electrons is different shells of an atom.
Answer:
Electronic configuration of the element

Question 6.
The outermost shell of an atom.
Answer:
Valence shell

E. Match the columns
1. J.J. Thomson – a. proton
2. Ernest Rutherford – b. neutron
3. James Chadwick – c. atomic theory
4. John Dalton – d. nucleus
5. E. Goldstein – e. electron
Answer:
1. J.J Thomson – e. electron
2. Ernest Rutherford – d. nucleus
3. James Chadwick – b. neutron
4. John Dalton – c. atomic theory
5. E. Goldstein – a. proton

F. Answer the following in short

Question 1.
What are cathode rays? What is the nature of charge’on cathode rays?
Answer:
Cathode rays are streams of electrons emitted from and travelling away from the cathode. The cathode rays carry negative charge.

Question 2.
Name the three subatomic particles of an atom. Write their symbols and charges.
Answer:

Question 3.
What is a proton? How does it differ from a neutron?
Answe:
A proton is a subatomic particle with a positive electric charge of +le elementary charge and mass slightly less than that of a neutron. The difference between proton and neutron is that protons have an electrical charge but neutrons are electrically neutral.

Question 4.
What are shells? Write the maximum number of electrons that they can accommodate.
Answer:
According to Bohr’s atomic model, electrons revolve around the nucleus in orbits or shells that have a fixed or definite amount of energy. Shells are also called energy levels.

Question 5.
How is an element symbolically represented?
Answer:
The atom of an element is generally represented by the symbol of that element (say X). The atomic number (Z) is written on the left side of the symbol as a subscript, whereas the mass number (A) is written on the left side of the symbol as a superscript \({ }_{A}^{Z} X\)

Question 6.
Differentiate between the atomic number and the mass number.
Answer:
The number of protons present in the nucleus of an atom is known as its atomic number. The atomic number is represented by the symbol Z. The sum of the protons and neutrons present in the nucleus of an atom is called its mass number. The mass number is represented by the symbol A.

Question 7.
From the symbol \({ }_{4}^{2} X\), identify the element and write down its mass number and atomic number.
Answer:
The element is helium. Its mass number is 4 and atomic number is 2.

G. Answer the following in detail

Question 1.
State the principal ideas of Dalton’s atomic theory.
Answer:
The principal ideas of Dalton’s atomic theory are as follows.

• All elements are made up of very tiny, indivisible particles called atoms.
• Atoms can neither be created nor destroyed during chemical reactions.
• All atoms of an element are identical, but the atoms of an element differ from the atoms of other elements.
• The atoms of different elements combine in fixed, small whole-number ratios to form compounds.
• In chemical reactions, the atoms themselves do not change. Chemical reactions only rearrange the way the atoms are combined to form new compounds.

Question 2.
Describe Thomson’s plum pudding model. Which subatomic particle was not present in this model of the atom?
Answer:
Thomson’s model of the atom :
Dalton’s atomic theory suggested that the atom was indivisible and indestructible. But the discovery of two fundamental particles (electrons and protons) inside the atom led to the failure of this aspect of the theory. It was considered necessary to know how the electrons and protons are arranged within an atom. J. J. Thomson was the first to propose a model for the inner structure of an atom to explain the arrangement of protons and electrons in the atom.

Thomson proposed the plum pudding model of the atom in 1904. According to this model, the atom is a sphere of positive charges the electrons are spread out inside it like plums in a spherical pudding. The atom has an equal number of positive charges (protons) and negative charges(electrons). So, the atom as a whole is electrically neutral. The plum pudding model was later found to be incorrect when Rutherford proposed his atomic model.

Question 3.
State Rutherford’s observations based on his alpha-scattering experiment. What conclusions did he draw from the observations?
Answer:
Observations :
1. Most of the alpha particles passed through the foil without any deflection from their path.
2. Some of the alpha particles were sharply deflected at different angles after passing through the foil. A few of the particles turned back directly on their original path.

Conclusions :
On the basis of the above observations, Rutherford in 1911 made the following conclusions.
1. Most of the alpha particles passed straight through the foil, which suggested that the atom consists of a lot of empty space inside it.
2. Some of the alpha particles were deflected from their path at different angles, which suggested that the atom has a tiny but heavy, positively-charged alpha particles. The mass of the atom is concentrated in this tiny central core, which Rutherford called the nucleus. As a result of this experiment, the plum pudding model was replaced by the nuclear model of the atom.

Question 4.
Describe Rutherford’s nuclear model of the atom.
Answer:
Rutherford’s nuclear model of the atom On the basis of his experiment, Rutherford put forward the nuclear model of the atom. The model described the structure of the atom as follows.

• There is a positively-charged core in the centre of an atom called the nucleus. Nearly, all the mass of the atom is concentrated in the nucleus.
• The electrons revolve around the nucleus in well-defined circular paths called orbits, like planets revolving around the sun. Hence, this model is also called the planetary model of the atom.
• Since the atom is neutral, the number of electrons (negetively-charged particles) is equal to the number of protons (positively-charged particles) in the nucleus.
  

Question 5.
Describe’ Bohr’s atomic model.
Answer:
The Danish physicist, Niels Bohr, further developed Rutherford’s model. In 1913, he proposed his atomic model. According to Bohr’s model, electrons revolving around the nucleus occupy particular orbits, which are also called shells. He labelled the shells as K, L, M, N, …, starting from the innermost shell.

Each orbit or shell is associated with a fixed or definite amount of energy. Hence, these shells are called energy levels. The energy associated with energy levels increases as we go further away from the nucleus, that is, amount of energy of K < L < M < N…. ‘

Question 6.
What is the Bohr-Bury scheme of electronic configuration? Write down the rules that are observed according to the scheme for writing the number of electrons in different energy levels or shells.
Answer:
Niels Bohr and Charles Bury proposed certain rules for the distribution of electrons in different shells of atoms. This is known as the Bohr-Bury scheme of electronic configuration. The following rules are observed according to the Bohr-Bury scheme for writing the number of electrons in different energy levels or shells.

1. The maximum number of electrons present in a shell is given by the formula 2n², where n is the shell number or energy level number. K shell is represented by n = 1, L shell by n = 2, and so on.

Thus, the maximum number of electrons in the first four energy levels would be 2, 8, 18 and 32.

2. The outermost shell, called the valence shell, of an atom cannot accommodate more than 8 electrons, even if it has the capacity to accommodate more electrons. This means that the outermost shell of an atom can accommodate a maximum of 8 electrons expect in the case of the first shell (K shell), which cannot have more than 2 electrons.

Question 7.
Explain how the electrons are arranged in the shells of a potassium atom.
Answer:
Potassium (K) has 19 electrons. The first shell (K shell) accommodates 2 electrons and the second shell (L shell) accommodates 8 electrons. According to the 2n² rule, the remaining 9 electrons should occupy the third shell (M shell), which can hold up to 18 electrons. But, since the outermost shell of an atom cannot have more than 8 electrons, the third shell can accommodate only 8 electrons and the remaining electron occupies the fourth shell (N shell). Therefore, the electronic configuration of potassium is 2, 8, 8, 1.

H. Draw diagrams of the following.

Question 1.
Rutherford’s alpha-scattering experiment
Answer:
Observations :
1. Most of the alpha particles passed through the foil without any deflection from their path.
2. Some of the alpha particles were sharply deflected at different angles after passing through the foil. A few of the particles turned back directly on their original path.

Conclusions :
On the basis of the above observations, Rutherford in 1911 made the following conclusions.
1. Most of the alpha particles passed straight through the foil, which suggested that the atom consists of a lot of empty space inside it.
2. Some of the alpha particles were deflected from their path at different angles, which suggested that the atom has a tiny but heavy, positively-charged alpha particles. The mass of the atom is concentrated in this tiny central core, which Rutherford called the nucleus. As a result of this experiment, the plum pudding model was replaced by the nuclear model of the atom.

Question 2.
Bohr’s atomic model
Answer:
Bohr’s model of the atom

I. Complete the following table.

Answer:

J. Study the table and answer the questions given below.
Answer:

a. Write down the atomic number and mass number of each element.
Answer:

b. Identify elements X and Y
Answer:
X is chlorine.
Y is aluminium.

Think And Answer

Question 1.
The atom as a whole is electrically neutral. Why?
Answer:
Since the atom is neutral, the number of electrons (negatively- charged particles) is equal to the number of protons (positively- charged particles) in the nucleus.

Question 2.
Rutherford’s nuclear model of the atom is also called the planetary model of the atom. Explain.
Answer:
The electrons revolve around the nucleus in well-defined circular paths called orbits, like planets revolving around the sun. Hence, this model is also called the planetary model of the atom.

Question 3.
The shells surrounding the nucleus are called energy levels. Why?
Answer:
Each orbit or shell is associated with a fixed or definite amount of energy. Hence, these shells are called energy levels.

Question 4.
The electronic configuration of sodium is written as 2, 8, 1, but not 2, 9. Explain.
Answer:
Sodium (Na) has 11 electrons. Therefore, its electronic configuration is written as 2, 8, 1 and not 2, 9. This is because as per the rule, the second shell in any atom cannot accommodate more than 8 electrons in it and so the remaining electron moves into the third shell.

Living Science Chemistry Class 8 ICSE Solutions