Std 8 ch 5 THE STRUCTURE OF AN ATOM

5. THE STRUCTURE OF AN ATOM

5 THE STRUCTURE OF AN ATOM

points to remember :

1. Dalton’s Atomic model : Dalton proposed that matter is made

up of very small particles. These small particles are called atoms.

An atom is a hard and solid ball. Being the smallest unit of

matter, it is indivisible.

2. Thomson’s atomic model : Thomson proposed that the atom is

a homogeneous sphere of positive charge in which electrons are

embedded. Electrons are negatively charged particles. As the

positive and negative charges are equal, the atom as a whole

does not have any resultant charge.

3. Rutherford’s atomic model : The centre of the atom is called

the nucleus and it has positive charge. Most of the mass of the

atom is concentrated in the nucleus. Electrons bear negative

charge. They revolve around the nucleus in specific orbits. The

nucleus is very very small as compared to the size of the atom.

4. The structure of the atom : The fundamental particles in atom

are the protons, neutrons and electrons. The number of protons

and neutrons together in the nucleus of an atom are called

nucleons. The proton is positively charged particle. The neutron

is electrically neutral. The electron is negatively charged

particle. Electrons revolve around the nucleus of the atom in

specific orbits.

5. Atomic number (Z) : The number of electrons or protons in an

atom is called the atomic number (Z).

6. Atomic mass number (A) : The sum of the number of protons

and neutrons in the nucleus of the atom is called the atomic mass

number (A).

7. Isotopes : Atoms of the same element having the same atomic

number, but different atomic mass numbers are called isotopes.

The nuclei of these atoms of different isotopes of an element have

the same number of protons but different number of neutrons.

For example : Hydrogen has three isotopes. Generally, all

isotopes of an element have the same chemical properties.

8. Formation of ions : The electrons in an atom revolve around

the nucleus in specific orbits. If the outermost orbit of an atom of

an element is incomplete, during a chemical reaction, the atom

gains or loses electrons. This produces ions.

9. Valency : The capacity of an element to combine with another

element is called its valency. The valency of hydrogen is one.

The valency of any element is compared with that of hydrogen.

The valency of an element is always a whole number.

Questions and Answers

Q. 1 Answer the following questions :

(1) Explain Dalton’s atomic theory.

Ans. (1) Dalton proposed that matter is

made up of very small particles. These small

particles are called atoms. (2) An atom is a

hard and solid ball. (3) Being the smallest

unit of matter, it is indivisible.

(2) What are the drawbacks of Dalton’s atomic theory.

Ans. Dalton’s theory does not mention anything about the

positive and negative charges on an atom. Hence, many of the

properties of substances cannot be explained.

(3) Explain Thomson’s atomic

model.

Ans. (1) Thomson proposed that the

atom is a homogeneous sphere of positive

charge in which electrons are embedded.

(2) Electrons are negatively charged

particles. (3) As the positive and negative

charges are equal, the atom as a whole

does not have any resultant charge.

(4) Which experiment did Ernest Rutherford conduct

about alpha rays ?

Ans. Ernest Rutherford bombarded a very thin sheet of gold

with positively charged alpha (_) rays. He studied the properties of

these rays.

(5) What were the observations of Rutherford’s

experiment of alpha rays ?

Ans. Rutherford bombarded a very thin sheet of gold with

positively charged alpha rays. He observed the following things

during his experiment :

(1) Most of the alpha rays passed through the gold sheet

without colliding with any obstacles. (2) A few alpha rays were

turned back from the thin sheet.

(6) What are the conclusions drawn from the alpha ray

experiment performed by Rutherford ?

Ans. On the basis of the experiment of the alpha rays

experiment, the following conclusions can be drawn :

(1) Most of the alpha rays pass through a thin sheet of gold

without any deviation. This shows that an atom must be hollow

with a lot of empty space in it. (2) Alpha (_) particles are positively

charged. Some alpha particles collide with the nucleus and due to

the electric repulsion turn back. This shows that the region in

which positive charge of the atom is concentrated must be very

small as compared to the size of the atom.

(7) Explain Rutherford’s atomic model.

Ans. (1) The centre of the atom is

called the nucleus and it has positive

charge. (2) Most of the mass of the

atom is concentrated in the nucleus.

(3) Electrons bear negative charge.

They revolve around the nucleus in

specific orbits. (4) The nucleus is very

very small as compared to the size of

the atom.

* (8) Name the fundamental particles in an atom. State their

characteristics.

Ans. The fundamental particles in atom are the protons,

neutrons and electrons.

Proton (p_) : The proton is the positively charged particle

denoted by the symbol p. It is located in the nucleus of an atom.

54 NAVNEET GENERAL SCIENCE DIGEST : STANDARD VIII

Neutron (n) : The Neutron is electrically neutral denoted by

the symbol n. It is located in the nucleus of an atom. Atoms of all the

elements, except hydrogen, contain neutrons. The mass of a

neutron is approximately equal to that of a proton.

Electron (e_) : The electron is negatively charged particle

denoted by the symbol e_. The charge on the electron (e) is treated

as one elementary unit of negative charge. Electrons revolve

around the nucleus of the atom in specific orbits. The electrons

have specific energy according to their orbits in which they

revolve. The mass of an electron is negligible as compared to the

mass of a proton or a neutron.

Answers to the Questions given on page 49 of the textbook

* (1) Is the structure of the atom the same as the

structure of the solar system?

Ans. It is not true that atomic structure is the same as the

structure of the solar system. The only similarity is that

electrons revolve around the nucleus just as planets revolve

around the sun.

* (2) The planets of the solar system revolve around the

sun due to gravitational forces. Which force acts in the

atom?

Ans. The force acting in the atom is the electric force of

attraction between positively charged nucleus and the

negatively charged electrons.

* (3) What is the function of the neutrons in the

nucleus?

Ans. The neutrons keep the positively charged protons

together in the nucleus.

(9) Define : (1) Atomic number (2) Atomic mass number.

Ans. (1) Atomic number : The number of electrons or

protons in an atom is called the atomic number. It is denoted by Z.

(2) Atomic mass number : The sum of the number of protons

and neutrons in the nucleus of the atom is called the atomic mass

number. It is denoted by A.

* (10) What is meant by atomic mass number?

For the answer, refer to Q. 1 (9) (2).

Answers to the Questions given on page 50 of the textbook

(1) Look at the atomic structure

of oxygen is shown in the figure.

What is the atomic number Z equal

to ?

Ans. There are 8 protons in the

nucleus of oxygen. The number of

electrons revolving around the nucleus

is 8. Hence, its atomic number is 8.

* (2) If the atomic number of carbon is (Z_6), what is

the number of the electrons revolving in its atom?

Ans. The atomic number of carbon is (Z_6), therefore,

the number of electrons revolving around the carbon nucleus

is 6.

* (3) Why are atomic numbers and atomic mass

numbers always whole numbers?

Ans. The atomic number gives the number of protons in

the nucleus and the atomic mass number gives the number of

protons and neutrons in the nucleus. Therefore, atomic

numbers and atomic mass numbers are always whole

numbers.

* (4) A sulphur atom contains 16 protons and

16 neutrons. Give its atomic number and atomic mass

number.

Ans. The atomic number of sulphur is 16 and the atomic

mass number is 32.

(11) Define isotopes.

Ans. Atoms of the same element having the same atomic

number, but different atomic mass numbers are called isotopes.

(12) Give two examples of isotopes.

Ans. (1) Isotopes of carbon : They are

(2) Isotopes of chlorine : They are

* (What is the characteristic property of the element

hydrogen?

Ans. Hydrogen has three isotopes. They are called hydrogen,

deuterium and tritium. Each has one proton in the nucleus. There

is no neutron in the nucleus of normal hydrogen.

e–

1p+

e–

Hydrogen __

H Deuterium __

H Tritium _

Fig. 5.5 : Isotopes of Hydrogen

There is one neutron in the nucleus of deuterium and two

neutrons in the nucleus of tritium.

These isotopes have generally the same chemical properties.

(14) Explain the formation of ions with suitable examples.

Ans. The electrons revolve around the nucleus in specific

orbits. The electrons in the outermost orbit take part in chemical

reactions. If the outermost orbit is incomplete i.e., it has capacity to

hold more electrons, then there is the possibility of the give and

take or sharing of electrons during a chemical reaction. Thus, ions

are formed. During a chemical reaction, metals have a tendency to

give electrons and non-metals have a tendency to receive electrons.

Sodium atom has 11 electrons of which two are in the first orbit,

8 in the second and one in the third orbit, which is the outermost

orbit. Sodium has incomplete outermost orbit. During a chemical

reaction, sodium atom gives away the outermost electron. The

number of protons (11) in the nucleus becomes greater than the

number of electrons (10) by one. Sodium acquires positive charge.

Thus, sodium ion is formed.

Na IIIIIIIJ Na__e_

Chlorine atom has 17 electrons of which 2 are in the first orbit,

8 in the second and 7 in the third orbit, which is the outermost orbit.

Chlorine has incomplete outermost orbit. To complete the orbit,

chlorine must receive one electron. When it happens, then the

number of electrons (18) in the nucleus becomes more than the

number of protons (17) by one. Chlorine acquires negative charge.

Thus, chloride ion is formed.

Cl_e_ IIIIIJ Cl_

* (15) What is meant by valency ?

Ans. The capacity of an element to combine with another

element is called its valency. The valency of hydrogen is one.

valency of any element is compared with that of hydrogen. Sodium

and potassium have valency one and it is equal to that of hydrogen.

Many elements have valency more than one. Oxygen and calcium

have valency 2. The valency of nitrogen is 3 and that of silicon is 4.

The valency of an element is always a whole number.

Answers to the Questions given on page 53 of the textbook

* (1) The gas argon does not take part in chemical

reactions. What do you think is its valency ?

Ans. Argon does not take part in chemical reaction its

valency is zero.

* (2) How many electrons could there be in the

outermost orbit of the atom of an element whose valency

is 2 ?

Ans. If the valency of an element is 2 then there is

possibility of two or six electrons in the outermost orbit.

(16) Potassium and chlorine have valency one. Write the

molecular formula of potassium chloride.

Ans. Molecular formula of potassium chloride : KCl.

(17) From the molecular formula of aluminium chloride

state the valencies of aluminium and chlorine.

Ans. In aluminium chloride

), the valency of aluminium

is 3 and that of chlorine is 1.

(18) Write the molecular formulae of the following

compounds : (1) Hydrogen chloride (2) Sodium chloride

(3) Calcium chloride (4) Silicon dioxide.

Ans. (1) Hydrogen chloride : HCl (2) Sodium chloride : NaCl

(3) Calcium chloride : CaCl2

(4) Silicon dioxide : SiO2

Answers to the Questions given on page 54 of the textbook

* (1) The valency of carbon is 4 and that of oxygen is 2.

Write the molecular formula of carbon dioxide.

Ans. The molecular formula of carbon dioxide : CO2

* (2) From the formula of water, H2O

O, work out the

valencies of hydrogen and oxygen.

Ans. From the molecular formula of water (H

O), the

valency of hydrogen is one and that of oxygen is 2.

Q. 2 Write short notes on the following :

(1) Isotopes (2) Formation of ions.

Ans. (1) Isotopes : Atoms of the same element having the

same atomic number, but different atomic mass numbers are called

isotopes. The nuclei of these atoms of different isotopes of an

element have the same number of protons but different number of

neutrons. For example : Isotopes of Carbon

C. They have

six protons but number of neutrons are 6 and 8. Isotopes of an

element have the same chemical properties. Isotopes of some

elements show special properties, therefore, they have some

practical applications. Examples : (1) Isotopes of uranium are used

as fuel in atomic reactors. (2) Isotopes of cobalt are used in the

treatment of cancer. (3) Isotopes of iodine are used in the treatment

of goitre.

(2) Formation of ions : See the answer to Q. 1 (14).

* Q. 3 Give scientific reasons :

(1) In Rutherford’s experiment, some alpha rays collide

with the gold sheet and are turned back.

Ans. (1) All the positive charges in an atom is concentrated in

a very small region, called the nucleus, at the centre of the atom.

(2) Electrons revolve around the nucleus, but most of the atom is

empty. (3) Alpha rays are positively charged. (4) When a thin sheet

of gold is bombarded by alpha rays, most of the rays passed through

it without deviation as the atom is almost empty. (5) But some

alpha particles collide with the positive nucleus and due to the

electric repulsion, turn back. Hence, in Rutherford’s experiment,

some alpha rays collide with the gold sheet and are turned back.

(2) During ion formation, an electric charge develops on

the (atoms of) elements.

Ans. (1) If the outermost orbit of an atom of an element is

incomplete, during a chemical reaction, the atom gains or loses

electrons forming an ion. (2) If the atom gain electrons, the number

of electrons becomes greater than the number of protons and hence

the ion carries a negative charge. (3) If the atom loses electrons, the

number of electrons becomes less than the number of protons and

hence, the ions carry a positive charge. Thus, during ion formation,

an electric charge develops on the elements.

(3) Why does ionization take place ?

Ans. (1) The electrons in an atom revolve around the nucleus

in specific orbits. (2) If the outermost orbit of an atom of an element

is incomplete, the atom gains or loses electrons during a chemical

reaction. (3) This produces ions. When some compounds are

dissolved in water, ionization takes place.

* (4) All the mass of an atom is concentrated in the nucleus.

Ans. (1) The nucleus of an atom contains protons and

neutrons. (2) The electrons revolve around the nucleus. (3) The

mass of an electron is negligible compared to that of a proton or a

neutron. (4) The mass of an atom depends on the number of protons

and neutrons. Hence, all the mass of an atom is concentrated in the

nucleus.

Q. 4 Distinguish between the following :

* (1) Thomson’s atomic model and Rutherford’s atomic

model :

Ans. Thomson’s atomic

model

Rutherford’s atomic

model

1. Atom is a homogeneous 1. The positive charge is in the

nucleus of the atom. sphere of positive charge.

2. In homogeneous sphere, the 2. The negatively charged

electrons revolve around the nucleus in the specific orbits.

negatively charged electrons

are embedded.

(2) Proton and Neutron :

Ans. Proton Neutron

1. The Proton is the positively 1. The Neutron does not have

any charge

charged particle.

2. In an atom, the number of 2. In general, the number of

neutrons is not equal to the protons is always equal to the

number of electrons number of electrons

(3) Neutron and Electron :

Ans. Neutron Electron

1. Neutron is an electrically

neutral particle.

1. Electron is a negatively

charged particle.

2. Neutrons are present in the

nucleus of an atom.

2. Electrons revolve around the

nucleus of the atom in

specific orbits.

(4) Proton and Electron :

Ans. Proton Electron

1. Proton is a positively charged

particle.

1. Electron is a negatively

charged particle.

2. Protons are present in the

nucleus of an atom.

2. Electrons revolve around the

nucleus of the atom in

specific orbits.

(5) Atomic number and Atomic mass number :

Ans. Atomic number Atomic mass number

1. The number of electrons or

protons in an atom is called

the atomic number.

1. The sum of the number of

protons and neutrons in the

nucleus of the atoms is called

the atomic mass number.

2. All the isotopes of an element

have the same atomic

number.

2. Different isotopes of the same

element have different atomic

mass numbers.

3. Atomic number is

represented by the letter Z.

3. The atomic mass number is

represented by the letter A.

Q. 5 Answer the following questions in one sentence

each :

(1) What are nucleons?

Ans. The number of protons and neutrons together in the

nucleus of an atom are called nucleons.

(2) In helium, there are two electrons revolving around

the nucleus. What is the atomic number of helium?

Ans. The atomic number of helium is 2.

(3) Give the relation between protons (p), neutrons (n) and

atomic mass number (A) of an element.

Ans.

Atomic mass number_ Number of Protons _ Number of

Neutrons

_ A _ p _ n

(4) Sodium contains 11 protons and 12 neutrons. What is

its atomic mass number?

Ans. Atomic mass number of sodium is 23.

(5) Lithium contains 3 protons and 4 neutrons. Give its

atomic number and atomic mass number.

Ans. The atomic number of Lithium is 3 and atomic mass

number is 7.

(6) State the use of isotopes of cobalt.

Ans. Isotopes of cobalt are used in the treatment of cancer.

(7) Which element is used as fuel in atomic reactors ?

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

(8) State the number of electrons in the outermost orbit

of magnesium.

Ans. There are two electrons in the outermost orbit of

magnesium.

(9) 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.

(10) An element contains seven electrons in the outermost

orbit. What could be its valency ?

Ans. An element contains seven electrons in the outermost

orbit. The valency of the element is one.

(11) Name three elements with valency one.

Ans. Elements with valency one : Hydrogen, chlorine, sodium.

(12) Name two elements with valency two.

Ans. Elements with valency two : Magnesium, oxygen.

(13) Name two elements with valency four.

Ans. Elements with valency four : Silicon, carbon.

Q. 6 State whether the following statements are True or

False . If a statement is false, correct it and rewrite :

(1) The mass of a neutron is approximately equal to that of a

proton.

(2) The electrons have specific energy depending upon the

orbit in which they revolve.

(3) Most of the mass of the atom is concentrated in the nucleus.

(4) Thomson proposed the fundamental theory about the

composition of matter for the first time.

(5) The mass of an atom depends on the number of protons and

electrons.

(6) Generally, all isotopes of an element have the same

chemical properties.

(7) Electrons revolve around the nucleus of the atom in specific

orbits.

(8) The deuterium and tritium have 2 and 3 protons in their

nuclei respectively.

(9) Calcium has a combining capacity twice that of hydrogen.

Hence, the valency of calcium is 2.

(10) Neutrons are present in the nucleus of an atom.

Ans. (1) True. (2) True. (3) True. (4) False. Dalton proposed

the fundamental theory about the composition of matter for the first

time. (5) False. The mass of an atom depends on the number

of protons and neutrons. (6) True. (7) True. (8) False. The

deuterium and tritium have one proton in their nuclei. (9) True.

(10) True.

Q. 7 Fill in the blanks :

*1. (1) If the valency of carbon is 4 and that of hydrogen is 1 then

the molecular formula of methane is ……… .

(2) There are two electrons in the outermost orbit of the

magnesium atom. Hence, the valency of magnesium is

……… .

(3) _

Cl and ……… are isotopes of chlorine.

(4) Isotopes of uranium are used as ……… in atomic reactors.

Ans. (1) CH

(2) 2 (3) _

Cl (4) fuel.

-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

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

(2) An atom is electrically ……… .

(3) The nucleus of helium contains ……… neutrons.

(4) The particles in the nucleus of an atom are called ……… .

Ans. (1) neutrons (2) neutral (3) 2 (4) nucleons.

-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

3. (1) __

C and ……… are isotopes of carbon.

(2) Isotopes of iodine are used in the treatment of ……….

(3) The valency of nitrogen is ……… .

5. THE STRUCTURE OF AN ATOM 63

(4) The atomic number Z is equal to number of………or……….

Ans. (1) __

C (2) goitre (3) 3 (4) protons or electrons.

Q. 8 Choose the correct alternatives and complete the

following statements :

(1) Matter is made up of very small particles called ……….

(a) electrons (b) nucleus (c) atoms (d) neutrons

(2) The fundamental particles present in the nucleus of the

atom are ……… .

(a) protons and electrons (b) electrons

(c) electrons and neutrons (d) protons and neutrons

(3) The nucleus of an atom contains 11 protons and 12 neutrons.

The atomic mass number of the element is ………

(a) 11 (b) 23 (c) 12 (d) 24

(4) The nucleus of an atom contains 2 protons and 2 neutrons.

The atomic number (Z) of the element is ………

(a) 2 (b) 4 (c) 1 (d) 0

(5) The atom contains 16 electrons of which 2 are in the first

orbit, 8 in the second and 6 in the third orbit. Then the

valency of the atom is …….

(a) 2 (b) 6 (c) 8 (d) 1

(6) An element contains 5 electrons in the outermost orbit.

The valency of the element is ……… .

(a) 5 (b) 8 (c) 1 (d) 3

(7) The valency of magnesium is 2 and that of chlorine is 1.

The molecular formula of magnesium chloride is ……….

(a) Mg

(b) MgCl (c) MgCl

(d) MgCl

(8) In aluminium chloride (AlCl

), the valency of aluminium

is ……… and that of chlorine is ……… .

(a) 2,1 (b) 3,1 (c) 1,3 (d) 1,2

(9) When writing the symbol of __

O, its ……… and ……… are

written.

(a) atomic number, atomic mass number

(b) protons, electrons

(c) protons, neutrons

(d) atomic number, electrons

Ans. (1) atoms (2) protons and neutrons (3) 23 (4) 2 (5) 2

(6) 3 (7) MgCl

(8) 3,1 (9) atomic number and atomic mass

number.

Q. 9 Match the following :

* [1] ‘A’ ‘B’

(1) Isotopes of cobalt

(2) Valency of nitrogen

(3) Non-metal

(4) Metal

(5) Valency

(a) Three

(b) Treatment of cancer

(d) Receives electrons

(e) Loses electrons

Ans. (1) Isotopes of cobalt – Treatment of cancer (2) Valency of

nitrogen – Three (3) Non-metal– Receives electrons (4) Metal –

Loses electrons (5) Valency – Electrons in the outermost orbit.

[2] ‘A’ ‘B’

(1) Atom

(2) Proton

(3) Isotopes of iodine

(4) Atomic mass number

(5) Isotopes

(a) Treatment of goitre

(b) Protons_Neutrons

(d) Electrically neutral

(e) Positively charged

Ans. (1) Atom– Electrically neutral (2) Proton – Positively

charged (3) Isotopes of iodine – Treatment of goitre (4) Atomic

mass number– Protons_Neutrons (5) Isotopes – Different number

of neutrons.

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David Bangaru

Std 8 ch 5 THE STRUCTURE OF AN ATOM

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