Chapter 4 – Atomic Structure

Selina Concise Chemistry 2023 solutions for class 8 Chemistry. Chapter 4 – Atomic Structure is provided here with simple step-by-step explanations. These solutions for Atomic Structure are extremely popular among class 10 students. The Atomic Structure solution is handy for quickly completing your homework and preparing for exams.

Question 1

Fill in the blanks :

1. Dalton said that ____ could not be divided.
2. An ion which has a positive charge is called a ____
3. The outermost shell of an atom is known as ____
4. The ____ of an atom is very hard and dense.
5. Neutrons are particles having mass ____ equal to that of protons.
6. Isotopes are the atoms of ____ element having the ____ atomic number but ____ mass number.

Answer

1. atoms
2. cation
3. valence shell
4. nucleus
5. nearly
6. same, same, different

Question 2

Write ‘true’ or ‘false’ for the following statements:

1. An atom on the whole has a positive charge.
2. The maximum number of electrons in the first shell can be 8.
3. The central part of an atom is called the nucleus.

Answer

1. False (an atom, on the whole, is neutral, it has no net charge)
2. False (the maximum number of electrons in the first shell can be 2)
3. True (the central part of an atom is indeed called the nucleus)

Question 3

Give the following a suitable word/phrase.

1. The sub-atomic particle with a negative charge and negligible mass.
2. Protons and neutrons are present in the nucleus.
3. The electrons are present in the outermost shell. Atomic Structure
4. Arrangement of electrons in the shells of an atom.
5. The number of protons present in the nucleus of an atom.
6. The sum of the number of protons and neutrons of an atom.
7. Atoms of the same element with the same atomic number but a different mass number.
8. The smallest unit of an element which takes part in a chemical reaction.

Answer

1. Electron
2. Nucleus
3. Valence shell
4. Electronic configuration
5. Atomic number
6. Mass number
7. Isotopes
8. Atom

Question 4

Multiple Choice Questions

(a) The outermost shell of an atom is known as

1. Valency
2. valence electrons
3. nucleus
4. valence shell

(b) The number of valence electrons present in magnesium is

1. two
2. three
3. four
4.  five

(c) The sub-atomic particle with a positive charge is

1. proton
2. neutron
3. electron
4. nucleon

(d) If the atomic number of an atom is 17 and the mass number is 35, then the number of neutrons will be

1. 35
2. 17
3. 18
4. 52

(e) The number of electrons in an atom is equal to the number of

1. protons in a neutral atom
2. neutrons in a neutral atom
3. nucleons in a neutral atom
4. none of the above

(f) The sum of number of protons and number of neutrons present in the nucleus of an atom is called its

1. mass number
2. atomic number
3. number of electrons
4. all of the above

Answer

1. Valence shell
2. Two
3. Proton
4. 18
5. Protons in a neutral atom
6. Mass number

Question 5

Name three fundamental particles of an atom. Give the symbol with the charge, on each particle.

Answer

Question 6

Define the following terms:

1. Atomic number
2. Mass number
3. Nucleons
4. Valence shell

Answer

1. Atomic number: It is defined as the number of protons present in the nucleus of an atom. It is denoted by ‘Z’ and determines the identity of the atom.
2. Mass number: It is defined as the sum of the number of protons and the number of neutrons present in the nucleus of an atom. It is denoted by ‘A’.
3. Nucleons: Nucleons are the particles that are present in the nucleus of an atom. These particles include protons and neutrons.
4. Valence shell: It is the outermost electron shell of an atom, containing the valence electrons that participate in chemical bonding. The number of valence electrons determines the chemical properties of an element.

Question 7

Mention briefly the salient features of Dalton’s atomic theory (five points).

Answer

Dalton’s atomic theory was the first scientific theory about the nature of matter, proposed by John Dalton in the early 19th century.

The salient features of Dalton’s atomic theory are:

1. All matter is composed of tiny indivisible particles called atoms.
2. Atoms of the same element are identical in size, shape, and other properties, while atoms of different elements are different.
3. Atoms cannot be created or destroyed in a chemical reaction, only rearranged.
4. Compounds are formed when atoms of different elements combine in fixed ratios.
5. The relative number and arrangement of atoms in a compound determine its properties.

Question 8

1. What are the two main features of Rutherford’s atomic model?
2. State its one drawback.

Answer

(a) The two main features of Rutherford’s atomic model are:

1. The atom consists of a small, dense, positively charged nucleus at the centre, where nearly all of the mass of the atom is concentrated.
2. Electrons orbit the nucleus at a distance, with the majority of the atom being empty space.

(b) The model had one significant drawback. It could not explain the stability of an atom.

Question 9

What are the observations of the experiment done by Rutherford in order to determine the structure of an atom?

Answer

Ernest Rutherford’s experiment involved bombarding a thin gold foil with alpha particles (positively charged particles) and observing the deflection of these particles.

The observations from the experiment are:

1. Most of the alpha particles passed through the gold foil without any deflection.
2. A small fraction of alpha particles were deflected at different angles.
3. Very few alpha particles were deflected back towards the source.

Question 10

State the mass number, atomic number, number of neutrons and electronic configuration of the following atoms.

₆C¹², ₈O¹⁶, ₉F¹⁹, ₁₀Ne²⁰, ₁₃Al²⁷, ₁₇Cl³⁵

Also, draw atomic diagrams for each of them.

Answer

Carbon-12:

• Mass number = 12
• Atomic number = 6
• Number of neutrons = 6
• Electronic configuration = 2, 4

Oxygen-16:

• Mass number = 16
• Atomic number = 8
• Number of neutrons = 8
• Electronic configuration = 2, 6

Fluorine-19:

• Mass number = 19
• Atomic number = 9
• Number of neutrons = 10
• Electronic configuration = 2, 7

Neon-20:

• Mass number = 20
• Atomic number = 10
• Number of neutrons = 10
• Electronic configuration = 2, 8

Aluminum-27:

• Mass number = 27
• Atomic number = 13
• Number of neutrons = 14
• Electronic configuration = 2, 8, 3

Chlorine-35:

• Mass number = 35
• Atomic number = 17
• Number of neutrons = 18
• Electronic configuration = 2, 8, 7

Question 11

What is variable valency? Name two elements having variable valency and state their valencies.

Answer

Variable valency is the ability of an element to exhibit different valencies or combining capacities in different chemical compounds. This is due to the different numbers of electrons that an element can lose or gain in order to form chemical bonds with other elements.

Two elements having variable valency are:

1. Iron (Fe) – It can exhibit a valency of +2 and +3. In ferrous compounds, iron has a valency of +2, while in ferric compounds, it has a valency of +3.
2. Copper (Cu) – It can exhibit a valency of +1 and +2. In cuprous compounds, copper has a valency of +1, while in cupric compounds, it has a valency of +2.

Question 12

The atomic number and the mass number of sodium are 11 and 23 respectively. What information is conveyed by this statement?

Answer

The statement “The atomic number and the mass number of sodium are 11 and 23 respectively” conveys the following information:

1. The atomic number of sodium is 11, which means that there are 11 protons in the nucleus of an atom of sodium.
2. The mass number of sodium is 23, which means that the total number of protons and neutrons in the nucleus of an atom of sodium is 23.
3. Since the number of protons in an atom determines its identity, we can conclude that the element is sodium.

Thus, this statement provides us with important information about the identity, atomic structure, and composition of an atom of sodium.

Question 13

Draw the diagrams representing the atomic structures of the following:

1. Nitrogen
2. Neon

Answer

1. The diagrams representing the atomic structures of nitrogen
   
2. The diagrams representing the atomic structures of neon
   

Question 14

Explain the rule with an example according to which electrons are filled in various energy levels.

Answer

1. The maximum capacity of a shell to accommodate electrons is given by the general formula 2n², where n is the serial number of a shell.
2. The maximum number of electrons possible in the outermost shell is 8, and that in the penultimate shell is 18.
3. It is not necessary for an orbit to become completed before another is formed. In fact, a new orbit is formed when the outermost shell attains 8 electrons.

Question 15

The atom of an element is made up of 4 protons, 5 neutrons and 4 electrons. What is its atomic number and mass number?

Answer

The atomic number of an element is equal to the number of protons in the nucleus of its atom. Given that the atom of the element has 4 protons, its atomic number is 4.

The mass number of an element is the sum of the number of protons and neutrons in the nucleus of its atom. Given that the atom of the element has 4 protons and 5 neutrons, its mass number is 4 + 5 = 9.

Therefore, the atomic number of the element is 4 and its mass number is 9.

Question 16

1. What are the two main parts of which an atom is made?
2. Where is the nucleus of an atom situated?
3. What are the orbits or shells of an atom?

Answer

(a) An atom is made up of two main parts:

1. Nucleus: It is the central part of an atom that contains protons and neutrons.
2. Electrons: They are negatively charged particles that revolve around the nucleus in shells or orbits.

(b) The nucleus of an atom is situated at the centre of the atom.

(c) The orbits or shells of an atom are the regions around the nucleus where electrons revolve around the nucleus.

Question 17

What are isotopes? How does the existence of isotopes contradict Dalton’s atomic theory?

Answer

Isotopes are atoms of the same element that have the same number of protons (atomic number) but different numbers of neutrons (and therefore different mass numbers).

The existence of isotopes contradicts Dalton’s atomic theory, which stated that all atoms of a given element have the same mass and other properties. However, isotopes have different masses due to the varying number of neutrons, which means that not all atoms of the same element are identical in mass and properties, as Dalton’s theory assumed.

Question 18

Complete the table below by identifying A, B, C, D, E and F.

Fluorine

• Symbol – ₉F¹⁹
• Number of protons – 9
• Number of neutrons – A
•  Number of electrons – B

Aluminium

• Symbol – C
• Number of protons – D
• Number of neutrons – 14
•  Number of electrons – 13

Potassium 

• Symbol – ₁₉K³⁹
• Number of protons – E
• Number of neutrons – F
• Number of electrons – 19

Answer

A = 10 Neutrons

B = 9 electron

C = Al

D = 13

E = 19

F = 20 Neutrons