Journey Inside the Atom

Answer:
No. Atoms are not indivisible. They are made up of smaller particles called subatomic particles. The three main subatomic particles are:

• Electrons - negatively charged
• Protons - positively charged
• Neutrons - have no charge

Answer:
According to Bohr's model, electrons move around the nucleus only in certain fixed energy levels or shells.

While an electron moves in an allowed shell, it does not lose energy. Therefore, it does not spiral inward and fall into the nucleus.

Answer:
Scientists modified atomic models because new experiments gave new evidence about atoms.

Older models could not explain all observations. Therefore, scientists improved the models whenever new discoveries were made.

For example:

Dalton → Thomson → Rutherford → Bohr → Modern atomic model

This shows that scientific knowledge improves with new experiments and evidence.

Answer:
An atom can be divided further. It contains smaller particles such as electrons, protons and neutrons.

Answer:
Atoms are mainly made up of three subatomic particles:

1. Protons
2. Neutrons
3. Electrons

Protons and neutrons are present inside the nucleus, while electrons are found around the nucleus.

Answer:
An atom would have:

• a very small and dense nucleus at the centre,
• protons and neutrons inside the nucleus,
• electrons present around the nucleus in different energy levels,
• a large amount of empty space.

Answer:
The number of protons makes one element different from another.

The number of protons is called the atomic number.

For example:

• Hydrogen has 1 proton.
• Helium has 2 protons.
• Carbon has 6 protons.

Therefore, each element has a unique atomic number.

(i) the positive charge on the clay is lesser than the total negative charge of the beads?

Answer:
The atom would have more negative charge than positive charge.

Therefore, it would become negatively charged and would not represent a neutral atom.

(ii) by mistake, the clay itself carries a bit of negative charge? Would your model still represent a neutral atom?

Answer:
No.

The clay is supposed to represent the positive charge of the atom. If the clay also carries negative charge, the positive and negative charges will not balance properly.

Therefore, the model would not represent a neutral atom unless the total positive and negative charges were exactly equal.

Answer:
Yes, an orange or lemon can be used as a simple comparison.

In Thomson's model:

• the soft pulp can represent the positive charge,
• the seeds can represent the electrons spread inside it.

However, the comparison is not perfect because:

• real fruit pulp does not carry positive charge,
• seeds are much larger than electrons,
• an actual atom does not really look like a fruit.

So, the comparison only helps us understand Thomson's basic idea.

Answer:
Thomson found that the nature of cathode rays was the same even when he changed:

• the material of the cathode, and
• the gas inside the tube.

This showed that the particles in cathode rays were present in all kinds of atoms.

These particles were called electrons.

Given:

Diameter of one atom

= 10−10 m

Thickness of paper

= 0.1 mm

= 10−4 m

Answer:

Number of atoms

Therefore, about one million atoms would have to be stacked together to make a sheet 0.1 mm thick.

Answer:
If the gold foil were made thicker:

1. Fewer alpha particles would pass straight through.
2. More alpha particles would be deflected.
3. Some particles might be deflected through larger angles.
4. The chance of an alpha particle coming close to a nucleus would increase.

Simple diagram

The thicker foil gives the alpha particles more chances to interact with nuclei.

Answer:
Alpha particles are positively charged, so they are repelled by the positively charged nucleus.

If negatively charged particles were used instead, they would be attracted towards the positively charged nucleus.

Therefore, their paths and pattern of deflection would be different from those of alpha particles.

Answer:
According to Thomson's model, positive charge was spread evenly throughout the atom.

If this were true, alpha particles should not bounce back strongly.

However, a few alpha particles bounced back. This showed that:

• the positive charge is concentrated in a very small region,
• this region is very dense,
• most of the atom's mass is present there.

This small central region was called the nucleus.

Therefore, Thomson's model could not explain the result.

Answer:
One possible question is:

“How did you realise from the gold foil experiment that most of an atom is empty space?”

Another acceptable answer can also be written because this is an open-ended question.

Reason (R): According to Thomson’s model, electrons are embedded in a uniformly distributed positive charge sphere.

Choose the correct option:

(i) Both A and R are true, and R is the correct explanation of A.

(ii) Both A and R are true, but R is not the correct explanation of A.

(iii) A is true, but R is false.

(iv) A is false, but R is true.

Answer:
(ii) Both A and R are true, but R is not the correct explanation of A.

Reason: Rutherford concluded that most of the mass and positive charge are concentrated in the nucleus from the gold foil experiment.

Thomson's model is correctly described in the Reason, but it does not explain Rutherford's conclusion.

Answer:
Bohr said that electrons move only in certain fixed shells or stationary energy levels.

When an electron moves in one of these allowed shells, it does not lose energy.

Therefore, the electron does not fall into the nucleus and the atom remains stable.

Answer:
According to Bohr, an electron in an allowed stationary energy level has a fixed amount of energy.

It does not lose energy while remaining in that shell.

Answer:
The names came from the work of physicist Charles Barkla.

During X-ray experiments, Barkla named the first observed X-ray line K. He did not start from A because he wanted to leave space for any earlier series that might later be discovered.

Bohr later used the same names:

K, L, M, N...

for electron shells.

Answer:
The mass of an atom comes mainly from the:

• protons, and
• neutrons

present inside its nucleus.

The mass of electrons is very small and can normally be ignored.

Answer:
Yes. The particle is called the neutron.

A neutron:

• has almost the same mass as a proton,
• has no electrical charge,
• is found inside the nucleus.

James Chadwick discovered the neutron in 1932.

Answer:
Protons do repel each other because they have the same positive charge.

However, inside the nucleus, a very strong force called the nuclear force holds the particles together.

Neutrons also help in keeping the nucleus stable. Heavier nuclei therefore generally contain more neutrons.

Answer:
If atoms had no empty space, atoms would be much smaller.

Therefore:

• objects made from them would occupy much less space,
• matter would become extremely compact and dense.

Most of the size of an atom comes from the large space between the nucleus and electrons.

Answer:
Dhruva is associated with the Bhabha Atomic Research Centre (BARC), Mumbai.

Scientists use neutron-scattering experiments there to study different materials.

According to the chapter, such research has helped scientists understand materials such as:

• superconductors,
• battery electrodes,
• drug molecules,
• industrial alloys.

This research can help in the development of:

• better medicines,
• better energy-storage systems,
• improved industrial materials.

Answer:
Example:

Suppose my name is Aarav.

I can name the new element Aaravium and give it the symbol Aa.

The symbol follows the rules because:

• the first letter A is a capital letter,
• the second letter a is a small letter.

Note: Students can make their own answer using their own name.

Answer:
If scientists used different symbols for the same element:

1. There would be a lot of confusion.
2. Chemical formulas could be misunderstood.
3. Scientists from different countries would find communication difficult.
4. Mistakes could occur in experiments and calculations.

Therefore, internationally accepted symbols are necessary.

Answer:
Yes.

Every element has its own fixed atomic number, which is equal to its number of protons.

Therefore, the atomic number uniquely identifies an element.

Answer:

For lithium:

• Number of protons = 3
• Number of neutrons = 4
• Atomic number = 3

Answer:

Atomic number = 26

Therefore,

Number of protons = 26

For a neutral atom:

Number of electrons = 26

Mass number = number of nucleons = 56

Final Answer:

• Protons = 26
• Electrons = 26
• Neutrons = 30

Answer:

Answer:
21 neutrons

Answer:

Atomic number = number of protons = 17

Neutrons = 18

Answer:
Mass number = 35

Answer:

For a neutral atom:

Number of protons = number of electrons

Therefore,

Protons = 11

Mass number = 23

Answer:
12 neutrons

Answer:
The first shell or K-shell can hold a maximum of 2 electrons.

Therefore, both electrons of helium will be present in the K-shell.

Electronic configuration of helium = 2.

(i) 126⁢𝐶

Atomic number = 6

Electronic configuration = 2, 4

Answer:
4 electrons

(ii) 199⁢𝐹

Atomic number = 9

Electronic configuration = 2, 7

Answer:
7 electrons

(iii) 2814⁢𝑆⁢𝑖

Atomic number = 14

Electronic configuration = 2, 8, 4

Answer:
4 electrons

Answer:

Answer:

Number of protons = 11

Therefore, atomic number = 11.

The element with atomic number 11 is sodium (Na).

Mass number = 23

Answer:
The element is sodium and it has 12 neutrons.

One more riddle

Q. I have 8 protons and a mass number of 16. I am necessary for breathing. Who am I and how many neutrons do I have?

Answer:
I am oxygen.

I have 8 neutrons.

Answer:

In NH₃, one nitrogen atom combines with 3 hydrogen atoms.

Therefore:

Valency of nitrogen = 3

In MgCl₂, one magnesium atom combines with 2 chlorine atoms.

Therefore:

Valency of magnesium = 2

Answer:
No.

Atoms having a complete outermost shell are already stable.

They normally do not need to gain, lose or share electrons.

Their valency is generally zero.

Examples:

• Neon = 2, 8
• Argon = 2, 8, 8

Helium is also stable with 2 electrons in its first shell.

Answer:

Answer:
No.

Atoms of the same element can have different numbers of neutrons.

Such atoms are called isotopes.

Answer:
Isotopes of an element have:

• similar chemical properties because they have the same number of electrons,
• different physical properties because their masses are different.

For example, their melting points and boiling points may be different.

Answer:
All three isotopes of hydrogen have one proton.

Since the atoms are neutral, each also has one electron.

Therefore:

• Protium = 1 electron
• Deuterium = 1 electron
• Tritium = 1 electron

Answer:
Isotopes have the same:

• atomic number,
• number of electrons,
• electronic configuration,
• number of valence electrons.

Chemical properties mainly depend on the electrons, especially the valence electrons.

Therefore, isotopes of the same element have similar chemical properties.

Answer:

Formula:

Carbon-12

6 neutrons

Carbon-13

7 neutrons

Carbon-14

8 neutrons

Answer:
Neither value alone gives the correct average mass because both isotopes occur naturally.

About:

• 75% is chlorine-35,
• 25% is chlorine-37.

Therefore:

Answer:
Average atomic mass of chlorine = 35.5 u

This does not mean that one chlorine atom has a mass of 35.5 u. It is the weighted average of a large number of chlorine atoms.

Answer:

Both atoms have 11 protons.

Therefore:

Atomic number of both = 11

First atom

Protons = 11

Neutrons = 12

Second atom

Protons = 11

Neutrons = 13

So:

• Atomic numbers are the same: 11
• Mass numbers are different: 23 and 24

They are the same element, because their atomic numbers are the same.

They are isotopes of the same element.

Answer:

Therefore:

Answer:
Approximately 80.0 u

Answer:
Such atoms are called isobars.

Isobars are atoms of different elements having:

• the same mass number,
• different atomic numbers.

Example:

• Calcium-40: atomic number 20
• Potassium-40: atomic number 19
• Argon-40: atomic number 18

All have mass number 40, so they are isobars.

(i) The experiment clearly showed the existence of neutrons in the nucleus.

(ii) The results disproved the plum pudding model and led to the idea of a nucleus at the centre of the atom.

(iii) The large deflection of a few alpha particles indicated that most of the mass of the atom and positive charge are packed into a tiny centre.

(iv) The way alpha particles were deflected showed that electrons move around the nucleus.

Answer:

The correct statements are (ii) and (iii).

(i) Incorrect

Rutherford's experiment did not discover neutrons.

James Chadwick discovered the neutron later.

(ii) Correct

The experiment showed that Thomson's plum pudding model could not explain the large deflections of alpha particles.

It led Rutherford to propose a small central nucleus.

(iii) Correct

A few alpha particles were strongly deflected or bounced back.

This showed that most of the positive charge and mass are concentrated in a very small, dense nucleus.

(iv) Incorrect

The scattering experiment showed the presence of the nucleus. It did not directly show that electrons were moving around it.

(i) Electrons lose energy while moving in fixed orbits and slowly fall into the nucleus.

Answer:
Incorrect.

According to Bohr, electrons do not lose energy while moving in their allowed fixed shells.

(ii) Electrons can exist anywhere around the nucleus with no fixed energy.

Answer:
Incorrect.

According to Bohr, electrons can exist only in certain fixed energy levels or shells.

(iii) Electrons revolve around the nucleus in orbits of fixed energy without losing energy.

Answer:
Correct.

This is one of the main ideas of Bohr's atomic model.

(iv) Electrons can be found between energy levels as they move around the nucleus.

Answer:
Incorrect.

Bohr proposed that electrons cannot remain between two allowed energy levels.

They can move from one level to another only by absorbing or releasing a fixed amount of energy.

Explain the relation between the following:

(i) Y and Z

Answer:

For Y:

For Z:

Both have atomic number 17, but their mass numbers are different.

Therefore, Y and Z are isotopes of the same element.

(ii) Z and X

Answer:

For Z:

For X:

They have the same mass number 37, but different atomic numbers.

Therefore, Z and X are isobars.

Answer:
Rutherford concluded that the positive charge of an atom is:

• concentrated in a very small region,
• located at the centre of the atom,
• very dense,
• and contains most of the atom's mass.

This central region is called the nucleus.

(i) Bohr’s model proposed that electrons move in fixed orbits around the nucleus, each with a definite energy.

(ii) Thomson’s model depicted the atom as a ‘plum pudding’ with electrons embedded in a sphere of positive charge.

(iii) Rutherford’s model proposed that atoms have a dense central nucleus.

(iv) Dalton’s model described atoms as indivisible particles.

Answer:

The correct chronological order is:

1. Dalton's model - (iv)

Dalton said atoms were indivisible particles.

2. Thomson's model - (ii)

After discovering electrons, Thomson proposed that electrons were embedded in a sphere of positive charge.

3. Rutherford's model - (iii)

Rutherford showed that the atom has a small and dense nucleus.

4. Bohr's model - (i)

Bohr proposed that electrons move around the nucleus in fixed energy levels.

Answer:
Electrons have a negative charge, while protons in the nucleus have a positive charge.

Opposite charges attract each other.

Therefore, the electrostatic attraction between the negatively charged electrons and the positively charged nucleus keeps electrons connected to the atom.

According to Bohr's model, electrons remain in fixed energy levels around the nucleus.

Reason (R): The number of electrons is equal to the number of protons in an atom.

Choose the correct option:

(i) Both A and R are true, and R is the correct explanation of A.

(ii) Both A and R are true, but R is not the correct explanation of A.

(iii) A is true, but R is false.

(iv) A is false, but R is true.

Answer:
(ii) Both A and R are true, but R is not the correct explanation of A.

Explanation: Discovery of electrons, protons and neutrons helped scientists understand the structure of atoms.

It is also true that a neutral atom has equal numbers of protons and electrons.

However, this fact alone does not explain why the discovery of subatomic particles helped us understand atomic structure.

Answer:

Atomic number = 12

Therefore:

(i) Number of protons

(ii) Number of neutrons

(iii) Number of electrons

For a neutral atom:

Electronic arrangement

• K-shell = 2 electrons
• L-shell = 8 electrons
• M-shell = 2 electrons

Final Answer:

• Protons = 12
• Neutrons = 12
• Electrons = 12
• Electronic configuration = 2, 8, 2

(i) Name of the element

(ii) Symbol

(iii) Total number of electrons

(iv) Number of valence electrons

(v) Valency of the element

(vi) Number of protons

(vii) Atomic number

Answer:

From the electron arrangements shown in Fig. 8.17:

The diagrams on page 159 show the electronic arrangements 2,1; 2,5; 2,8,3; and 2,7 respectively.

Answer:
In Rutherford's model, a moving electron should continuously lose energy.

If it lost energy, it should:

1. move closer and closer to the nucleus,
2. finally fall into the nucleus,
3. cause the atom to collapse.

But atoms do not collapse.

Therefore, Rutherford's model could not explain atomic stability.

Bohr proposed that electrons move in special fixed energy levels.

While moving in these allowed shells, electrons do not lose energy.

Therefore, Bohr's model could explain why atoms remain stable.

Answer:

Number of electrons = 31

For a neutral atom:

Number of protons = 31

Mass number = 70

Answer:
39 neutrons

(i) the number of neutrons

Answer:
118 neutrons

(ii) the number of electrons

For a neutral atom:

Answer:
79 electrons

Answer:

(i) How many electrons and protons does element X have?

Mass number = 35

Neutrons = 18

For a neutral atom:

Electrons = 17

Answer:
17 protons and 17 electrons

(ii) What is its atomic number?

Atomic number = number of protons

Answer:
17

(iii) Identify the element X.

The element with atomic number 17 is chlorine.

Answer:
Chlorine (Cl)

(iv) Write its electronic configuration.

Chlorine has 17 electrons.

Answer:
2, 8, 7

(v) How many valence electrons does it have?

The outermost shell contains 7 electrons.

Answer:
7 valence electrons

(vi) What will be the mass number if two neutrons are added to its nucleus?

Original mass number = 35

Two neutrons are added.

Answer:
37

(vii) What will be the relation of X with the new atom?

The number of protons remains 17, but the number of neutrons changes.

Therefore:

• atomic number remains the same,
• mass number changes from 35 to 37.

So the two atoms are isotopes of chlorine.

Answer:
They are isotopes of the same element.

(i) Atomic number

Answer:
The atomic number will not change.

Atomic number depends only on the number of protons.

There are still 12 protons.

(ii) Atomic mass

Answer:
The atomic mass will increase because the new negatively charged particles are much heavier than normal electrons.

(iii) Mass number

Answer:
The mass number will not change.

Mass number counts only:

Therefore:

(iv) Overall charge

Answer:
The overall charge will remain neutral.

There are:

• 12 positively charged protons,
• 12 negatively charged replacement particles.

The positive and negative charges still balance each other.

Answer:
Overall charge = zero or neutral.

Answer:

Make cards with clues like these:

Card 1

Clues:

• Atomic number = 8
• Mass number = 16
• Protons = 8
• Electrons = 8
• Neutrons = 8
• Valency = 2

Answer:
Oxygen (O)

Card 2

Clues:

• Atomic number = 11
• Mass number = 23
• Protons = 11
• Neutrons = 12
• Electrons = 11
• Valency = 1

Answer:
Sodium (Na)

Card 3

Clues:

• Atomic number = 6
• Mass number = 12
• Electrons = 6
• Neutrons = 6
• Valency = 4

Answer:
Carbon (C)

Card 4

Clues:

• Atomic number = 17
• Mass number = 35
• Protons = 17
• Neutrons = 18
• Valency = 1

Answer:
Chlorine (Cl)

Students can take turns picking cards and identifying the element.

Answer:

Atoms in Everyday Life

Atoms and their properties are very useful in our daily lives.

1. Healthcare

Some radioactive isotopes are used in medicine.

• Cobalt-60 is used in radiation treatment for cancer.
• Iodine-131 is used in the treatment of thyroid diseases, including thyroid cancer.

2. Energy

Uranium-235 is used as a fuel in nuclear reactors.

Nuclear power plants use atomic energy to generate electricity.

3. Agriculture

Atomic research is also used to support developments in agriculture. Scientific institutions study the peaceful uses of atomic energy in different fields, including agriculture.

4. Technology

Neutron-scattering experiments help scientists study:

• battery materials,
• superconductors,
• industrial alloys,
• drug molecules.

Modern microscopes can even produce images showing materials at the atomic level.

Conclusion

Understanding atoms has helped humans develop better medicines, energy systems and new technologies.

Answer:

Characters

• Narrator
• Acharya Kanada
• John Dalton
• J. J. Thomson
• Ernest Rutherford
• Niels Bohr
• James Chadwick

Short Role-play

Narrator: For thousands of years, people have wondered what matter is made of.

Acharya Kanada: I believe that if matter is divided again and again, we finally reach very tiny particles called parmanus.

Dalton: I proposed a scientific atomic theory. I said that matter is made of tiny particles called atoms.

Thomson: I discovered the electron. Therefore, atoms cannot be indivisible. I suggested that electrons are present inside a positively charged sphere.

Rutherford: My gold foil experiment showed that most of an atom is empty space. I discovered that positive charge and most of the mass are concentrated in a tiny nucleus.

Bohr: I explained that electrons move in fixed energy levels or shells around the nucleus. They do not lose energy while remaining in these shells.

Chadwick: I discovered the neutron, a particle with no electrical charge present in the nucleus.

Narrator: Each discovery improved our understanding of the atom. Even today, scientists continue to learn more about atomic structure.

Answer:
The animation can be made in five stages:

Scene 1 - Dalton's Model

Show an atom as a small solid sphere.

Scene 2 - Thomson's Model

Show a positively charged sphere with small electrons inside it.

Scene 3 - Rutherford's Model

Show:

• a tiny central nucleus,
• electrons moving around it,
• large empty space.

Scene 4 - Bohr's Model

Show electrons moving in fixed shells:

• K
• L
• M
• N

Scene 5 - Modern Model

Show electrons as an electron cloud around the nucleus instead of fixed circular paths.

Add the name of each scientist and his main contribution.

Answer:
I watched an educational documentary on “The Structure of the Atom.”

Its main topic was how scientists gradually discovered the structure of the atom and developed different atomic models.

Answer:
It taught me that an atom contains:

• a nucleus at the centre,
• protons and neutrons inside the nucleus,
• electrons around the nucleus.

It also showed how atomic models changed from Dalton's model to Thomson's, Rutherford's and Bohr's models.

Answer:

John Dalton: Proposed an early scientific atomic theory.

J. J. Thomson: Discovered the electron and proposed the plum pudding model.

Ernest Rutherford: Discovered the nucleus and proposed the nuclear model.

Niels Bohr: Proposed fixed energy levels for electrons.

James Chadwick: Discovered the neutron.

Answer:
I found Rutherford's gold foil experiment most interesting because a few alpha particles bounced back and helped scientists discover the nucleus.

One question I still have is:

“What exactly happens inside the nucleus at the smallest level?”

Answer:
We can choose hydrogen, carbon and sodium.

Simple representation

Students can copy these values onto graph paper and draw similar vertical bars.

Answer:
Scientists have learnt a great deal about atoms, but our understanding is still developing.

Earlier models were changed when new experiments gave better evidence.

Even Bohr's model was later improved by the quantum mechanical model.

Therefore, science continues to explore what happens inside atoms, and new discoveries may improve our understanding further.