Ch.5: Inside the Atom

Inside the Atom | Class 8 | Science | Chapter 5 | Maharashtra State Board

Explore the evolution of atomic theory, from ancient philosophers to modern science. Understand key concepts such as atoms, molecules, subatomic particles, and various atomic models presented by Dalton, Thomson, Rutherford, and Bohr.

Questions & Answers

1. Answer the following

a. What is the difference in the atomic models of Thomson and Rutherford?

Ans.

	Thomson's Model (Plum Pudding Model)	Rutherford's Model (Nuclear Model)
1.	Structure: The atom is a positively charged sphere with electrons embedded within it, similar to plums in a pudding.	Structure: The atom has a small, dense, positively charged nucleus at the center, with electrons orbiting around it, similar to planets around the sun.
2.	Charge Distribution: The positive charge is spread uniformly, with electrons scattered throughout.	Charge Distribution: The nucleus contains almost all the positive charge and mass, while electrons occupy the surrounding space, leaving most of the atom as empty space.
3.	Atomic Neutrality: The total positive and negative charges balance each other, making the atom neutral.	Atomic Neutrality: The atom is electrically neutral because the nucleus's positive charge is balanced by the electrons' negative charge.

b. What is meant by valency of an element? What is the relationship between the number of valence electron and valency?

Ans.

Valency of an Element: Valency is the ability of an atom to combine with other atoms. It is determined by the number of electrons an atom can lose, gain, or share to achieve a full outer shell (usually eight electrons, known as the octet rule).

Relationship Between Valence Electrons and Valency:

• For Atoms with 1 to 4 Valence Electrons: The valency is typically equal to the number of valence electrons. For example, an atom with 3 valence electrons has a valency of 3.

• For Atoms with 5 to 7 Valence Electrons: The valency is calculated as 8 minus the number of valence electrons. For example, an atom with 6 valence electrons has a valency of 2.

• For Atoms with 8 Valence Electrons: The valency is zero because the atom is stable and does not need to gain, lose, or share electrons.

c. What is meant by atomic mass i number? Explain how the atomic number and mass number of carbon are 6 and 12 respectively.

Ans.

Atomic Mass and Atomic Number:

• Atomic Number: The atomic number of an element is the number of protons in the nucleus of its atom. It uniquely identifies an element and determines its position in the periodic table.

• Mass Number: The mass number is the total number of protons and neutrons in an atom's nucleus. It gives an approximation of the atom's mass.

Carbon's Atomic Number and Mass Number:

• Atomic Number of Carbon (6): Carbon has 6 protons in its nucleus, so its atomic number is 6.

• Mass Number of Carbon (12): Carbon has 6 protons and 6 neutrons in its nucleus. Adding these together gives a mass number of 12 (6 protons + 6 neutrons = 12).

Thus, carbon's atomic number is 6, and its mass number is 12.

d. What is meant by subatomic particle? Give brief information of three subatomic particles with reference to electrical charge, mass and location.

Ans.

Subatomic Particles:

Subatomic particles are the smaller particles that make up an atom. The three main subatomic particles are protons, neutrons, and electrons.

1. Protons:

Electrical Charge: Positive charge (+1).

Mass: Relatively heavy, about 1 atomic mass unit (amu).

Location: Found in the nucleus at the center of the atom.

2. Neutrons:

Electrical Charge: No charge (neutral).

Mass: Similar to protons, about 1 atomic mass unit (amu).

Location: Also located in the nucleus alongside protons.

3. Electrons:

Electrical Charge: Negative charge (-1).

Mass: Very light, about 1/1836 of a proton's mass.

Location: Move in orbits or electron clouds around the nucleus

2. Give scientific reasons:

a. All the mass of an atom is concentrated in the nucleus.

Ans.

All the mass of an atom is concentrated in the nucleus because the nucleus contains protons and neutrons, which are much heavier than electrons. Electrons have negligible mass compared to protons and neutrons, so their contribution to the atom's overall mass is minimal. Since protons and neutrons are located in the nucleus, nearly all the atom's mass is concentrated there.

b. Atom is electrically neutral.

Ans.

An atom is electrically neutral because it has an equal number of protons (positively charged) and electrons (negatively charged). The positive charges of the protons are exactly balanced by the negative charges of the electrons, resulting in no overall charge.

c. Atomic mass number is a whole number.

Ans.

The atomic mass number is a whole number because it represents the total count of protons and neutrons in an atom's nucleus, and both protons and neutrons are whole particles. Since you can only have whole numbers of these particles, the atomic mass number is always an integer.

d. Atoms are stable though negatively charged electron are revolving within it.

Ans.

Atoms are stable because the electrostatic attraction between negatively charged electrons and positively charged protons keeps the electrons in orbit around the nucleus. Quantum mechanics dictates that electrons occupy specific energy levels, which are stable due to the balance of forces. The centripetal force from the electron's motion balances the electrostatic attraction, preventing them from collapsing into the nucleus. Additionally, the Pauli exclusion principle helps stabilize atoms by ensuring that electrons occupy distinct quantum states.

3. Define the following terms.

a. Atom:

Ans.

An atom is the smallest unit of matter that retains the properties of an element. It consists of a central nucleus, containing positively charged protons and neutral neutrons, surrounded by negatively charged electrons that orbit the nucleus in specific energy levels.

b. Isotope

Ans.

Isotopes are variants of a chemical element that have the same number of protons but different numbers of neutrons in their nuclei, resulting in different atomic masses. Despite having different masses, isotopes of an element have similar chemical properties.

c. Atomic number

Ans.

The atomic number is the number of protons in the nucleus of an atom, which determines the element's identity and its position in the periodic table.

d. Atomic mass number

Ans.

The atomic mass number, also known as the mass number, is the total number of protons and neutrons in an atom's nucleus.

e. Moderator in nuclear reactor

Ans.

In a nuclear reactor, a moderator is a material used to slow down fast neutrons, making them more effective at sustaining a nuclear chain reaction by increasing the likelihood of neutron capture by fuel nuclei. Common moderators include water, heavy water, and graphite.

4. Draw a neat labelled diagram.

a. Explain Rutherford’s scattering experiment.

Ans.

Rutherford's Scattering Experiment

Objective:

To explore atomic structure by observing alpha particle scattering.

Setup:

• A beam of alpha particles was directed at a thin gold foil.

• A fluorescent screen surrounded the foil to detect scattered particles.

Observations:

1. Most Particles Passed Through: Indicating atoms are mostly empty space.

2. Some Deflected: Showing a dense, positively charged region in the atom.

3. Few Backscattered: Suggesting a small, dense nucleus.

Conclusion:

Nuclear Model of the Atom: Atoms have a small, dense nucleus containing most of the mass, with electrons orbiting in the surrounding empty space. This model replaced the earlier "plum pudding" model and changed the understanding of atomic structure.

b. Thomson’s atomic model.

Ans.

Thomson’s plum pudding model of atom:

1. According to Thomson’s model the positive charge is distributed throughout the atom and the negatively charged electron: are embedded in a gel of positive charge (a plum pudding model).

2. The distributed positive charge is balanced by the negative charge on the electrons. Therefore the atom becomes electrically neutral.

c. Diagrammatic sketch of electronic configuration of magnesium (Atomic number 12).

Ans.

d. Diagrammatic sketch of electronic configuration of argon (Atomic number 18).

Ans.

5. Fill in the blanks.

1. Electron, proton, neutron are the types of ………… in an atom.

Ans. Electron, proton, neutron are the types of subatomic particles in an atom.

2. An electron carries a ……………. charge.

Ans. An electron carries a negative charge.

3. The electron shell ………….. is nearest to the nucleus.

Ans. The electron shell K is nearest to the nucleus.

4. The electronic configuration of magnesium is 2, 8, 2. From this it is understood that the valence shell of Magnesium is …………….. .

Ans. The electronic configuration of magnesium is 2, 8, 2. From this it is understood that the valence shell of Magnesium is M.

5. The valency of hydrogen is one as per the molecular formula H2O. Therefore valency of ‘Fe’ turns out to be ………….. as per the formula Fe2O3.

Ans. The valency of hydrogen is ‘one’ as per the molecular formula H2O. Therefore valency of ‘Fe’ turns out to be 3 as per the formula Fe2O3.

6. Match the pairs.

	Column I	Column II
1.	Proton	a. Negatively charged
2.	Electron	b. Neutral
3.	Neutron	c. Positively charged

Ans.

	Column I	Column II
1.	Proton	c. Positively charged
2.	Electron	a. Negatively charged
3.	Neutron	b. Neutral

7. Deduce from the datum provided.

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E X T R A

1. Rewrite the sentences after filling the blanks:

1. An atom is electrically ……………. .

Ans. An atom is electrically neutral.

2. Except hydrogen, the nuclei of all atoms contain ………….. .

Ans. Except hydrogen, the nuclei of all atoms contain neutrons.

3. An atom has 11 protons and …………. neutrons and hence its atomic mass number is 23.

Ans. An atom has 11 protons and 12 neutrons and hence its atomic mass number is 23.

4. The element ………….. has two electrons in the K shell, but it is a noble gas.

Ans. The element helium has two electrons in the K shell, but it is a noble gas.

5. Isotopes of the element have the same ……….. properties.

Ans. Isotopes of the element have the same chemical properties.

6. Electrons must absorb …………….. to transit between orbits.

Ans. Electrons must absorb energy to transit between orbits.

7. ……….. discovered the electron.

Ans. J. J. Thomson discovered the electron.

8. ∝ – particles have ………….. charge.

Ans. ∝ – particles have positive charge.

9. Electrons revolve around the …………….. in certain discrete orbits.

Ans. Electrons revolve around the nucleus in certain discrete orbits.

10. The shell-wise distribution of electrons is called the ………………… .

Ans. The shell-wise distribution of electrons is called the electronic configuration.

11. Democritus termed the smallest particles of matter as …………… .

Ans. Democritus termed the smallest particles of matter as atoms.

12. ………………. discovered neutron.

Ans. James Chadwick discovered neutron.

13. Electrons revolve around the nucleus in paths called ………….. .

Ans. Electrons revolve around the nucleus in paths called orbits.

14. The second shell has the capacity of ……………… .

Ans. The second shell has the capacity of 8 electrons.

15. Electrons in the ………….. shell have minimum energy.

Ans. Electrons in the K or first shell have minimum energy.

16. Electrons in an atom occupy shells in increasing order of …………… .

Ans. Electrons in an atom occupy shells in increasing order of energy.

17. Atomic masses are measured in a unit called …………… .

Ans. Atomic masses are measured in a unit called dalton.

18. According to the atomic model of …………….., electrons are embedded in a gel of positive charge.

Ans. According to the atomic model of Thomson, electrons are embedded in a gel of positive charge.

19. All atoms, except ………….. contain neutrons in their nuclei.

Ans. All atoms, except normal hydrogen contain neutrons in their nuclei.

20. The maximum capacity of the M shell is ………………… electrons.

Ans. The maximum capacity of the M shell is 18 electrons.

21. The atom of the element ……………. has eight electrons in the outermost shell.

Ans. The atom of the element neon has eight electrons in the outermost shell.

2. Rewrite the following statements selecting the correct options:

1. The symbol A is used to denote the …………. .

(a) atomic number

(b) atomic radius

(c) atomic mass number

(d) atomic mass

Ans. (c) atomic mass number

2. The existence of isotopes is due to the presence of different number of …………….. .(a) electrons

(b) protons

(c) neutrons

(d) positrons

Ans. (c) neutrons

3. Isotopes of an element have the same number of …………….. .

(a) neutrons

(b) nucleons

(c) electrons

(d) atoms

Ans. (c) electrons

4. The great Indian philosopher ………….. proposed that matter is made up of invisible tiny particles.

(a) Aryabhatta

(b) Kanad

(c) Bhaskaracharya

(d) Chanakya

Ans. (b) Kanad

5. The maximum capacity of the M shell is …………… electrons.

(a) 2

(b) 8

(c) 18

(d) 32

Ans. (c) 18

6. The mass of the electron is …………….. times less than that of a hydrogen atom.

(a) 1800

(b) 8100

(c) 1550

(d) 1600

Ans. (a) 1800

7. The L shell is the valence shell in …………. .

(a) hydrogen

(b) chlorine

(c) oxygen

(d) sodium

Ans. (c) oxygen

8. The M shell is the valence shell in ……………….. .

(a) fluorine

(b) neon

(c) carbon

(d) chlorine

Ans. (d) chlorine

9. The N shell is the valence shell in ………….. .

(a) fluorine

(b) chlorine

(c) bromine

(d) helium

Ans. (c) bromine

10. The maximum capacity of the N shell is ……………. electrons.

(a) 2

(b) 32

(c) 18

(d) 8

Ans. (b) 32

11. The maximum number of electrons that can be accommodated in the third orbit is ………… .

(a) 3

(b) 8

(c) 32

(d) 18

Ans. (d) 18

12. Rutherford alpha-particle scattering experiment was responsible for the discovery of the ………………. .

(a) atomic nucleus

(b) proton

(c) electron

(d) atomic mass

Ans. (d) atomic mass

13. Isotopes of element have …………….. .

(a) different atomic numbers and different atomic mass

(b) different atomic numbers but the same atomic mass number

(c) the same atomic number but different atomic mass numbers

(d) the same atomic number and the same atomic mass number

Ans. (c) the same atomic number but different atomic mass numbers

14. The nucleus of an atom contains 19 protons and 21 neutrons. The atomic mass number of the element is ………….. .

(a) 19

(b) 21

(c) 40

(d) 39

Ans. (c) 40

15. The nucleus of an atom contains 18 protons and 22 neutrons. The atomic number of the element is ……………… .

(a) 18

(b) 40

(c) 22

(d) 4

Ans. (a) 18

16. When writing the symbol of its …………… and ………….. are written.

(a) atomic number, atomic mass number

(b) protons, electrons

(c) protons, neutrons

(d) atomic number, electrons

Ans. (a) atomic number, atomic mass number

17. The particles in the atomic nucleus are …………….. .

(a) protons and electrons

(b) electrons

(c) electrons and neutrons

(d) protons and neutrons

Ans. (d) protons and neutrons

3. State whether the following statements are True or False:

1. An atom as a whole is electrically neutral

Ans. True.

2. The mass of an atom is distributed evenly within it.

Ans. False. (The mass of atom is not distributed evenly, practically all the mass is concentrated in the nucleus.)

3. The electron has the same mass as that of the proton.

Ans. False. (The mass of the electron is much less than that of the proton.)

4. The electron in the K shell has maximum energy.

Ans. False. (The electrons in the K shell has minimum energy.)

5. Isotopes have same atomic number but different atomic mass number.

Ans. True.

6. Matter is composed of molecules and molecules are made of atoms.

Ans. True.

7. In India, total 22 nuclear reactors in eight places are functioning.

Ans. True.

8. The number of electrons in a given orbit is given by the formula n.

Ans. False. (The number of electrons in a given orbit is given by the formula 2n²)

9. Atomic masses are measured in a unit called the dalton (u).

Ans. True.

10. The chemical properties of isotopes are different.

Ans. False. (The chemical properties of isotopes are the same.)

11. The maximum capacity of the N shell is 18 electrons.

Ans. False. (The maximum capacity of the N shell is 32 electrons.)

12. Deuterium is an isotope of hydrogen.

Ans. True.

13. Isotopes are used in the treatment of cancer.

Ans. True.

14. Atoms of all elements, except normal hydrogen contain neutrons.

Ans. True.

15. Electrons moving in different orbits possess the same amount of energy.

Ans. False. (Electrons moving in different orbits possess different amounts of energy.

16. Rutherford discovered the neutron.

Ans. False. (Chadwick discovered the neutron.)

17. The nth orbit contains at the most 2n² electrons.

Ans. True.

18. Electrons have different energies according to their orbits.

Ans. True.

19. The capacity of the second orbit is 18 electrons.

Ans. False. (The capacity of the second orbit is 8 electrons.)

20. The radioactive isotope Sodium-24 is used in the medical treatment of cancer.

Ans. False. (The radioactive isotope Sodium-24 is used for detection of cracks in the underground pipes.)

21. Uranium-235 is used in the production of electricity.

Ans. True.

4. Answer the following questions in one sentence each:

1. Name the particles which are present in the nucleus of an atom.

Ans. Protons and neutrons are present in the nucleus of an atom.

2. State the relation between the number of protons, the number of neutrons and the atomic mass number (A) of an element.

Ans. Atomic mass = Number of + Number of number (A) protons (p) neutrons (n)

3. Chlorine contains 17 protons and 18 neutrons. What is its atomic mass number

Ans. The atomic mass number of chlorine is 35.

4. Carbon contains 6 protons and 6 neutrons. State its atomic number and atomic mass number.

Ans. The atomic number of carbon is 6 and the atomic mass number is 12.

5. State one use of isotopes of cobalt.

Ans. Isotopes of Cobalt-60 are used in the treatment of cancer.

6. State one use of isotopes of i uranium.

Ans. Isotopes of uranium are used for production of electrical energy.

7. Write the electronic configuration of oxygen.

Ans. Electronic configuration of oxygen is 2, 6.

8. Write the electronic configuration of chlorine.

Ans. Electronic configuration of chlorine is 2, 8, 7.

9. State the number of electrons in the L shell of lithium.

Ans. There is one electron in the L shell of lithium.

10. State the number of electrons in the M shell of argon.

Ans. There are 8 electrons in the M shell of argon.

11. State the number of electrons in the K shell of  helium.

Ans. There are two electrons in the K shell of helium.

12. Name isotopes of hydrogen.

Ans. Hydrogen, Deuterium and Tritium are the isotopes of hydrogen.

14. Name two elements in which the K and L shells of an atom are completely filled with electrons.

Ans. The elements are neon and argon.

15. From the symbol 168O, state the electronic configuration of oxygen and the atomic mass number of isotope oxygen.

Ans. Electronic configuration of oxygen : 2, 6. Atomic mass number of oxygen : 16.

16. The atomic mass number of an element is 18, and the element contains 8 electrons. What is the number of protons and neutrons in it?

Ans. Number of protons : 8, Number of neutrons : 10.

17. An atom contains 2 protons, 2 electrons and 3 neutrons. State its atomic number and atomic mass number.

Ans. Atomic number: 2, Atomic mass number: 5.

18. How many electrons could there be in the outermost orbit of an element whose valency is 3?

Ans. If the valency of an element is 3 then there is a possibility of 3 or 5 electrons in the outermost orbit.

19. Which element is used as fuel in atomic reactors?

Ans. Isotopes of uranium are used as fuel in atomic reactors.

20. Name the place and the first nuclear reactor in India.

Ans. Apsara at Bhabha Atomic Research Centre in Mumbai is the first nuclear reactor in India.

5. Answer the following questions:

1. Explain Dalton’s atomic theory.ORWrite the postulates of Dalton’s atomic theory.

Ans. Dalton’s atomic theory:

1.  According to Dalton’s atomic theory matter is made up of atoms and atoms are indivisible and indestructible.

2.  All atoms of an element are alike while different element have different atom with different mass.

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