Journey Inside the Atom

Are atoms the smallest indivisible particles?

Answer

No, atoms are not the smallest indivisible particles. For a long time, atoms were believed to be indivisible. However, the discovery of radioactivity and of subatomic particles showed that atoms are made up of still smaller particles — electrons, protons and neutrons. Hence, atoms can be divided further into these subatomic particles.

Why do electrons not fall into the nucleus even though they are attracted to protons in it?

Answer

According to Bohr's model of the atom, electrons revolve around the nucleus only in certain fixed orbits called stationary states (or shells), and each shell has a definite amount of energy. While moving in such a fixed shell, an electron does not lose energy. Since the electron does not lose energy, it does not spiral inward, and therefore it does not fall into the nucleus.

Why did scientists keep modifying atomic models?

Answer

As new experiments were performed and new evidence came to light, the existing atomic models could not explain all the observations. Each new model was proposed to overcome the limitations of the earlier one. For example, Thomson's model could not explain the gold foil experiment, and Rutherford's model could not explain the stability of the atom. So scientists kept modifying the models to match new experimental results. This shows how science progresses step by step through curiosity, questioning and experimentation.

Suppose you made up your own 'atom', as Thomson described, using clay for the positive charge and small beads for the electrons spread through it. What will happen if:

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

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

Answer

(i) If the positive charge on the clay is lesser than the total negative charge of the beads, the charge will not be fully balanced. The model will have a net negative charge, so it will not represent a neutral atom — it would behave like a negatively charged ion.

(ii) If the clay itself carries a bit of negative charge, then there is no positive charge left to balance the negative charge of the beads. The whole model would be negatively charged. So, no, it would not represent a neutral atom, because in a neutral atom the total positive charge must exactly balance the total negative charge.

Could an orange or a lemon, which also contain seeds inside soft pulp, be a good comparison? In what ways does it match Thomson's idea and where does it fall short?

Answer

An orange or a lemon can be used as a partial comparison for Thomson's model.

Just as the seeds are embedded inside the soft pulp, the electrons are pictured as being embedded inside the sphere of positive charge. The roughly round shape also resembles the spherical atom.

In an orange or a lemon, the seeds are clustered together in separate segments rather than being spread evenly throughout the pulp. In Thomson's model, the electrons are supposed to be distributed uniformly within the positive charge. Also, the pulp is not a uniformly positively charged matter. So the comparison does not correctly show the even distribution of electrons in the positive sphere.

Why did Thomson conclude that electrons are present in all atoms?

Answer

Thomson observed that the nature of cathode rays was independent of the material of the cathode and the nature of the gas filled in the cathode ray tube. Whatever cathode material or gas was used, the same negatively charged particles (electrons) were produced. This showed that electrons are a fundamental component present in every element, and hence Thomson concluded that electrons are present in all atoms.

What do you think would happen if α-particles were replaced with negatively charged particles in Rutherford's gold foil experiment?

Answer

Negatively charged particles would be attracted towards the positively charged nucleus instead of being repelled by it. So, instead of bouncing back, the particles passing close to a nucleus would be pulled in and deflected towards the nucleus. The pattern of scattering would therefore be different from that of the positively charged α-particles, which are repelled by the nucleus.

Rutherford found that a few α-particles bounced back sharply. How does this single surprising result completely rule out Thomson's 'plum pudding model' of the atom?

Answer

In Thomson's plum pudding model, the positive charge was spread out evenly throughout the whole atom. Such a spread-out, weak positive charge could never push a fast-moving, heavy α-particle straight back. The fact that a few α-particles bounced back sharply means they must have struck a very small region in which all the positive charge and most of the mass were concentrated. This concentrated centre (the nucleus) cannot exist in Thomson's model, where the charge is uniformly distributed. Hence, this single observation completely ruled out the plum pudding model.

If you could ask Rutherford one question about his work, what would it be?

Answer

One question about Rutherford's work would be —
"When you saw that a few α-particles bounced straight back, did you immediately think of a tiny dense nucleus, or did you first suspect an error in the experiment?"

Assertion (A): Rutherford concluded that most of the mass of an atom is concentrated in a small region at the centre called the nucleus.

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

Choose the correct option:

1. Both A and R are true, and R is the correct explanation of A.
2. Both A and R are true, but R is not the correct explanation of A.
3. A is true, but R is false.
4. A is false, but R is true.

Answer

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

Explanation

Assertion (A) is true because from the gold foil experiment, Rutherford concluded that most of the mass of an atom (along with its positive charge) is concentrated in a tiny central region called the nucleus.

Reason (R) is true as it correctly describes Thomson's model, in which electrons are embedded in a uniformly distributed sphere of positive charge.

However, R is not the correct explanation of A because Rutherford's conclusion came from the large-angle deflection and bouncing back of α-particles, not from Thomson's description of the atom. So, both statements are true, but R does not explain A is the correct option.

Imagine you are a scientist who has discovered a new element. Name this element after yourself and justify that the symbol you have chosen follows the IUPAC rules.

Answer

Suppose a scientist named Rohan discovers a new element and names it Rohanium.

A suitable symbol could be Rh — but since Rh is already used for Rhodium, a better choice would be Ro.

This symbol follows the IUPAC rules because:

1. It is formed from the letters of the name of the element.
2. The first letter, R, is written in capital (uppercase).
3. The second letter, o, is written in small (lowercase).
4. The symbol is short and does not clash with the symbol of any existing element.

What problems could arise if every scientist used different symbols for the same element?

Answer

If every scientist used different symbols for the same element, it would lead to a lot of confusion. Some problems that could arise are:

1. Scientists from different countries and languages would not understand each other's work clearly.
2. Chemical formulae and equations would have different forms, making them difficult to read and compare.
3. Errors and misunderstandings would occur while sharing research, recording data and teaching.

This is why IUPAC approves standard, internationally recognised symbols, so that scientists all over the world can communicate clearly regardless of language barriers.

An atom with an atomic number of 26 has 56 nucleons. Find out its number of electrons, protons and neutrons.

Answer

Given,

Atomic number (Z) = 26 and number of nucleons (mass number, A) = 56

Number of protons = Atomic number = 26

Number of electrons = Number of protons = 26 (since the atom is neutral)

Number of neutrons = Mass number − Number of protons = 56 − 26 = 30

Hence, the atom has 26 electrons, 26 protons and 30 neutrons.

The nucleus of an atom contains 20 protons. If its mass number is 41, find the number of neutrons in it.

Answer

Given,

Number of protons = 20 and mass number (A) = 41

Number of neutrons = Mass number − Number of protons = 41 − 20 = 21

Hence, the number of neutrons in the atom is 21.

An atom has 18 neutrons and an atomic number of 17. What is its mass number?

Answer

Given,

Number of neutrons = 18 and atomic number (Z) = number of protons = 17

Mass number = Number of protons + Number of neutrons = 17 + 18 = 35

Hence, the mass number of the atom is 35.

An atom ²³A has 11 electrons. Find the number of neutrons in it.

Answer

Given,

Mass number (A) = 23 and number of electrons = 11

Number of protons = Number of electrons = 11 (since the atom is neutral)

Number of neutrons = Mass number − Number of protons = 23 − 11 = 12

Hence, the number of neutrons in the atom is 12.

Identify the number of electrons in the outermost shell of the following elements:

(i) ¹²₆C

(ii) ¹⁹₉F

(iii) ²⁸₁₄Si

Answer

(i) ¹²₆C: Atomic number = 6, so it has 6 electrons.

Electronic configuration = 2, 4

Number of electrons in the outermost shell = 4

(ii) ¹⁹₉F: Atomic number = 9, so it has 9 electrons.

Electronic configuration = 2, 7

Number of electrons in the outermost shell = 7

(iii) ²⁸₁₄Si: Atomic number = 14, so it has 14 electrons.

Electronic configuration = 2, 8, 4

Number of electrons in the outermost shell = 4

Write the electronic configuration of the elements having atomic numbers 12, 16 and 18.

Answer

The electrons are filled in the shells (K, L, M, ...) following the rules that a shell can hold a maximum of 2n² electrons and that the outermost shell can have at most 8 electrons.

Atomic number 12 (Magnesium): 2, 8, 2

Atomic number 16 (Sulfur): 2, 8, 6

Atomic number 18 (Argon): 2, 8, 8

Solve this riddle: I am an atom with a mass number of 23 and 11 protons. I am a soft metal and react vigorously with water. Who am I and how many neutrons do I have? You can also create one such riddle.

Answer

Given,

Mass number (A) = 23 and number of protons = 11

Since the number of protons is 11, the atomic number is 11. The element with atomic number 11 is Sodium (Na), a soft metal that reacts vigorously with water.

Number of neutrons = Mass number − Number of protons = 23 − 11 = 12

Hence, the atom is Sodium (Na) and it has 12 neutrons.

Sample riddle: "I have a mass number of 4 and 2 protons. I am a very light gas used in balloons. Who am I?" — Answer: Helium. It has 2 neutrons.

Two different atoms have 11 protons each, but one has 12 neutrons, and the other has 13 neutrons. How do their atomic numbers and mass numbers compare? Are they the same element or different elements?

Answer

For both atoms, the number of protons is 11, so both have the same atomic number = 11.

Mass numbers:

First atom: 11 + 12 = 23

Second atom: 11 + 13 = 24

So the two atoms have the same atomic number (11) but different mass numbers (23 and 24).

Since the atomic number determines the identity of an element, both atoms belong to the same element (Sodium). They are isotopes of each other.

If a bromine atom is available in the form of, say two isotopes, ⁷⁹₃₅Br (49.7%) and ⁸¹₃₅Br (50.3%), calculate the average atomic mass of the bromine atom.

Answer

The average atomic mass is the weighted average of the masses of the isotopes, calculated using their percentage relative abundances.

$\text{Average atomic mass} = \left(79 \times \dfrac{49.7}{100}\right) + \left(81 \times \dfrac{50.3}{100}\right)$

= 39.263 + 40.743

= 80.006 u

Hence, the average atomic mass of the bromine atom is approximately 80 u.

Choose the correct options and explain the reason for the correct and incorrect options in the context of Ernest Rutherford's gold foil experiment:

(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 options are (ii) and (iii).

(i) Incorrect: The gold foil experiment did not show the existence of neutrons. The neutron was discovered much later, in 1932, by James Chadwick. The gold foil experiment dealt only with the positive charge and the mass of the atom.

(ii) Correct: Thomson's plum pudding model could not explain the large deflections and bouncing back of α-particles. These results disproved that model and led Rutherford to the idea of a dense nucleus at the centre of the atom.

(iii) Correct: The large deflection (and bouncing back) of a few α-particles could only happen if all the positive charge and most of the mass of the atom were concentrated in a very tiny central region, the nucleus.

(iv) Incorrect: The experiment gave information about the positive charge and the nucleus of the atom. It did not show how or where the electrons move around the nucleus.

Which of the following statements are correct or incorrect according to the Bohr's atomic model? Give a reason for each statement.

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

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

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

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

Answer

(i) Incorrect: According to Bohr, while moving in a fixed orbit (stationary state) an electron does not lose energy. Therefore it does not slowly fall into the nucleus. This idea of losing energy was the problem with Rutherford's model, which Bohr's model solved.

(ii) Incorrect: Electrons cannot exist anywhere around the nucleus. They can revolve only in certain allowed orbits (shells), and each orbit has a definite, fixed amount of energy.

(iii) Correct: This is exactly Bohr's postulate of stationary states. Electrons revolve around the nucleus in fixed orbits of definite energy, and while doing so they do not lose energy. This explains the stability of the atom.

(iv) Incorrect: Electrons can revolve only in the allowed shells and not in between them. An electron can move from one shell to another only by absorbing or releasing a fixed amount of energy, but it is never found between two energy levels.

The composition of the nuclei of three atomic species X, Y, and Z are given as follows.

Explain the relation between the following:

(i) Y and Z

(ii) Z and X

Answer

First, let us find the atomic number and mass number of each species.

X: protons = 18, neutrons = 19, so atomic number = 18 and mass number = 18 + 19 = 37

Y: protons = 17, neutrons = 18, so atomic number = 17 and mass number = 17 + 18 = 35

Z: protons = 17, neutrons = 20, so atomic number = 17 and mass number = 17 + 20 = 37

(i) Y and Z: Both have the same atomic number (17) but different mass numbers (35 and 37). Atoms of the same element having the same atomic number but different mass numbers are called isotopes. Hence, Y and Z are isotopes.

(ii) Z and X: Both have the same mass number (37) but different atomic numbers (17 and 18). Atoms of different elements having the same mass number but different atomic numbers are called isobars. Hence, Z and X are isobars.

What conclusion did Rutherford draw about the position and characteristics of the atom's positively charged part based on the few alpha particles that bounced back or were deflected at large angles in the gold foil experiment?

Answer

From the few α-particles that bounced back or were deflected at large angles, Rutherford concluded that the positively charged part of the atom is:

1. Concentrated in a very small central region of the atom, called the nucleus, and not spread throughout the atom.
2. Dense and heavy, as it contains all the positive charge and most of the mass of the atom.
3. Extremely small in size compared to the atom (about one lakh times smaller than the atom), so that most of the atom is empty space, which is why most α-particles passed through undeflected.

Explain and arrange the following statements in the correct chronological order to show how atomic models have evolved over time.

(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: (iv) → (ii) → (iii) → (i)

(iv) Dalton's model (1808): Atoms were described as the smallest, indivisible particles of matter.

(ii) Thomson's model: After the discovery of the electron, Thomson described the atom as a sphere of positive charge with electrons embedded in it (the plum pudding model).

(iii) Rutherford's model: The gold foil experiment showed that the atom has a dense, positively charged nucleus at its centre, with electrons revolving around it.

(i) Bohr's model (1913): Electrons move around the nucleus in fixed orbits (shells), each having a definite energy, which explained the stability of the atom.

Electrons move around the nucleus in orbits. Why do they not fly away from the atom? Explain what keeps them attracted to the nucleus.

Answer

The nucleus of an atom is positively charged (because of the protons in it), while the electrons are negatively charged. As unlike charges attract each other, there is an electrostatic force of attraction between the positively charged nucleus and the negatively charged electrons.

This attractive force pulls the electrons towards the nucleus and keeps them bound within the atom. It balances their tendency to move away while they revolve in their orbits. Because of this force of attraction, the electrons keep revolving around the nucleus instead of flying away from the atom.

Assertion (A): The discovery of subatomic particles helped in understanding the atomic structure.

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

Choose the correct option:

1. Both A and R are true, and R is the correct explanation of A.
2. Both A and R are true, but R is not the correct explanation of A.
3. A is true, but R is false.
4. A is false, but R is true.

Answer

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

Explanation

Assertion (A) is true. The discovery of subatomic particles — electrons, protons and neutrons — helped scientists understand how an atom is built and how it behaves.

Reason (R) is also true. In a neutral atom, the number of electrons is indeed equal to the number of protons, which keeps the atom electrically neutral.

However, R is not the correct explanation of A. The fact that electrons equal protons explains the electrical neutrality of the atom, but it is not the reason why the discovery of subatomic particles helped in understanding atomic structure.

Hence, both statements are true, but R does not explain A is the correct option.

Magnesium is essential for many biological processes, including muscle contraction. For an atom of magnesium with a mass number of 24 and atomic number 12, determine the number of (i) protons, (ii) neutrons, (iii) electrons, and also illustrate the arrangement of electrons in a magnesium atom.

Answer

Given,

Mass number (A) = 24 and atomic number (Z) = 12

(i) Number of protons = Atomic number = 12

(ii) Number of neutrons = Mass number − Number of protons = 24 − 12 = 12

(iii) Number of electrons = Number of protons = 12 (since the atom is neutral)

Arrangement of electrons (electronic configuration) of magnesium = 2, 8, 2

That is, the K-shell has 2 electrons, the L-shell has 8 electrons, and the M-shell has 2 electrons.

Find the following information for the elements shown in Fig. 8.17:

(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

By counting the electrons in each shell, the electronic configuration of each atom can be found, and from this all the required information is obtained.

(a) Electronic configuration = 2, 1

(i) Name of the element : Lithium

(ii) Symbol : Li

(iii) Total number of electrons : 3

(iv) Number of valence electrons : 1

(v) Valency : 1 (loses 1 electron)

(vi) Number of protons : 3

(vii) Atomic number : 3

(b) Electronic configuration = 2, 5

(i) Name of the element : Nitrogen

(ii) Symbol : N

(iii) Total number of electrons : 7

(iv) Number of valence electrons : 5

(v) Valency : 3 (gains 3 electrons to complete the octet)

(vi) Number of protons : 7

(vii) Atomic number : 7

(c) Electronic configuration = 2, 8, 3

(i) Name of the element : Aluminium

(ii) Symbol : Al

(iii) Total number of electrons : 13

(iv) Number of valence electrons : 3

(v) Valency : 3 (loses 3 electrons)

(vi) Number of protons : 13

(vii) Atomic number : 13

(d) Electronic configuration = 2, 7

(i) Name of the element : Fluorine

(ii) Symbol : F

(iii) Total number of electrons : 9

(iv) Number of valence electrons : 7

(v) Valency : 1 (gains 1 electron to complete the octet)

(vi) Number of protons : 9

(vii) Atomic number : 9

Both Rutherford's and Bohr's models have electrons orbiting the nucleus. Why did Rutherford's model fail to explain atomic stability, while Bohr's model succeeded?

Answer

Rutherford's model: In this model, electrons revolve around the nucleus in circular paths. A charged particle moving in a circular path is constantly changing its direction, which means it is accelerating. An accelerating charged electron should continuously lose energy in the form of radiation, spiral inward, and finally fall into the nucleus. If this happened, the atom would collapse. Since atoms are actually stable, Rutherford's model could not explain atomic stability.

Bohr's model: Bohr solved this problem by introducing the idea of stationary states. He proposed that electrons revolve only in certain fixed orbits (shells), each having a definite energy, and that while moving in such an orbit an electron does not lose energy. Because the electron does not lose energy, it does not spiral into the nucleus. In this way, Bohr's model successfully explained the stability of the atom.

An atom ⁷⁰X has 31 electrons. How many neutrons are there in its nucleus?

Answer

Given,

Mass number (A) = 70 and number of electrons = 31

Number of protons = Number of electrons = 31 (since the atom is neutral)

Number of neutrons = Mass number − Number of protons = 70 − 31 = 39

Hence, the number of neutrons in the nucleus is 39.

An atom has 79 protons and a mass number of 197. Calculate (i) the number of neutrons, and (ii) the number of electrons.

Answer

Given,

Number of protons = 79 and mass number (A) = 197

(i) Number of neutrons = Mass number − Number of protons = 197 − 79 = 118

(ii) Number of electrons = Number of protons = 79 (since the atom is neutral)

Complete the Table 8.5:

Answer

Using the relations: Atomic number = Number of protons = Number of electrons, and Mass number = Number of protons + Number of neutrons, the table is completed as follows.

Aman was discussing the structure of atom with his classmates. During the discussion, he learnt that an element X has a mass number of 35 and contains 18 neutrons. Based on this information, answer the following questions:

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

(ii) What is its atomic number?

(iii) Identify the element X.

(iv) Write its electronic configuration.

(v) How many valence electrons does it have?

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

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

Answer

Given,

Mass number (A) = 35 and number of neutrons = 18

(i) Number of protons = Mass number − Number of neutrons = 35 − 18 = 17

Number of electrons = Number of protons = 17

So, element X has 17 protons and 17 electrons.

(ii) Atomic number = Number of protons = 17

(iii) The element with atomic number 17 is Chlorine (Cl).

(iv) Electronic configuration = 2, 8, 7

(v) Number of valence electrons = 7 (electrons in the outermost shell)

(vi) New mass number = 35 + 2 = 37 (the number of protons stays the same; only neutrons increase)

(vii) The new atom has the same atomic number (17) but a different mass number (37). Hence, X and the new atom are isotopes of each other.

In an atom, there are 12 protons and 12 neutrons in the nucleus. Now, imagine that all the electrons are replaced with some hypothetical particles that have the same charge as electrons but are 500 times heavier. What effect will this replacement have on the atom's:

(i) Atomic number

(ii) Atomic mass

(iii) Mass number

(iv) Overall charge

Answer

The atom has 12 protons and 12 neutrons, so it has 12 electrons (atomic number 12). The replacement changes only the electrons; the nucleus (protons and neutrons) stays the same.

(i) Atomic number: No change. The atomic number depends only on the number of protons, which is still 12. So the atomic number remains 12.

(ii) Atomic mass : It will increase slightly. Normally the mass of electrons is so small that it is ignored. But the new particles are 500 times heavier than electrons, so their mass can no longer be ignored and they add a small but measurable amount to the mass of the atom. Hence, the atomic mass increases.

(iii) Mass number: No change. The mass number counts only the nucleons (protons + neutrons) in the nucleus = 12 + 12 = 24. Since the nucleus is unchanged, the mass number remains 24.

(iv) Overall charge: No change. The new particles carry the same charge as electrons (each −1) and their number is still 12. This balances the 12 protons (+12). So the atom stays electrically neutral, and its overall charge is zero.