Journey Inside the Atom — NCERT Solutions

No. The chapter says atoms were once thought to be indivisible, but later experiments showed that atoms are made of smaller subatomic particles like electrons, protons, and neutrons. So atoms are not the smallest indivisible particles.

In Bohr's model, electrons move in fixed energy levels or shells called stationary states. While moving in one allowed shell, an electron does not lose energy, so it does not spiral into the positively charged nucleus. Rutherford's model could not explain this stability, but Bohr's model did.

Scientists kept modifying atomic models because new experiments gave new evidence that earlier models could not explain. For example, the gold foil experiment showed that Thomson's model was incorrect, and Rutherford's model still could not explain atomic stability. So models were improved step by step as science advanced.

If the positive charge in the clay is less than the total negative charge of the beads, the model would not be neutral; it would have a net negative charge. If the clay itself carries a little negative charge, that would reduce the positive charge further, so it would again fail to represent a neutral atom. A neutral atom needs total positive and negative charges to be equal.

Yes, an orange or a lemon can be a rough comparison because, like Thomson's idea, it has seeds embedded inside soft pulp. The pulp can represent the positive charge and the seeds the electrons. But it falls short because in Thomson's model the electrons are spread throughout the positive sphere, not just placed in one region, and the comparison does not explain the exact balance of charge in an atom.

Thomson concluded that electrons are present in all atoms because the nature of cathode rays was independent of the material of the cathode and the gas in the tube. This showed that the negatively charged particles were a fundamental component of all atoms, not just of one particular substance.

If negatively charged particles were used instead of alpha particles, they would be repelled by the electrons in the atom and attracted toward the positive nucleus. Their path would be affected very differently from alpha particles. Since Rutherford's experiment depended on the behavior of positively charged alpha particles, replacing them with negative particles would not give the same scattering observations.

A few alpha particles bouncing back showed that the atom could not be a uniform sphere of positive charge as Thomson proposed. If positive charge were spread evenly, no alpha particle would have met a very concentrated, massive centre and bounced back sharply. The result proved that most of the atom is empty space and that nearly all the positive charge and mass are concentrated in a tiny nucleus. That contradicts the plum pudding model completely.

A good question would be: How did you infer that the atom has a tiny, dense nucleus from the scattering of only a few alpha particles? This would help understand the reasoning behind Rutherford's conclusion.

Both statements are true. Rutherford did conclude that most of the mass is concentrated in a small central nucleus, and Thomson's model did describe electrons embedded in a uniformly distributed positive sphere. However, R is not the correct explanation of A; Rutherford's conclusion came from the gold foil experiment, not from Thomson's description. So the correct option is (ii).

One possible name is Chatgptium with symbol Cg. This follows IUPAC-style rules because the symbol has two letters, the first letter is capital and the second is lowercase. If I chose a symbol, I would make sure it uses the element name properly and follows the convention of writing symbols with a capital first letter and a lowercase second letter.

If different scientists used different symbols for the same element, it would create confusion and make communication difficult. Scientists around the world would not know whether the same symbol referred to the same element, so it would be hard to compare results, write formulas, and share research clearly.

Atomic number $Z = 26$, so the atom has 26 protons. Since the atom is neutral, it also has 26 electrons. The mass number (nucleons) is 56, so

$$\text{neutrons} = 56 - 26 = 30.$$

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

Mass number $A = 41$ and protons $= 20$.

Using

$$\text{neutrons} = A - Z = 41 - 20 = 21,$$

the number of neutrons is 21.

Atomic number $Z = 17$, so protons = 17. Neutrons = 18.

Mass number

$$A = p + n = 17 + 18 = 35.$$

So the mass number is 35.

The atom has mass number $A = 23$ and 11 electrons. Since the atom is neutral, it has 11 protons. Then

$$\text{neutrons} = A - Z = 23 - 11 = 12.$$

So the number of neutrons is 12.

Use the electronic configurations from the chapter:

• Carbon, $Z=6$: $2,4$ so outermost shell has 4 electrons.
• Fluorine, $Z=9$: $2,7$ so outermost shell has 7 electrons.
• Silicon, $Z=14$: $2,8,4$ so outermost shell has 4 electrons.

So the numbers are 4, 7, 4.

From the table in the chapter:

• Atomic number 12 (magnesium) has electronic configuration 2, 8, 2.
• Atomic number 16 (sulfur) has electronic configuration 2, 8, 6.
• Atomic number 18 (argon) has electronic configuration 2, 8, 8.

An atom with 11 protons has atomic number 11, which is sodium (Na). Its mass number is 23, so

$$\text{neutrons} = 23 - 11 = 12.$$

So the atom is sodium and it has 12 neutrons. A suitable riddle of this type could be made using mass number, atomic number, and a property of the element.

Both atoms have the same number of protons, so their atomic number is the same: 11. But their mass numbers are different:

• First atom: $11 + 12 = 23$
• Second atom: $11 + 13 = 24$

Since they have the same atomic number but different mass numbers, they are isotopes of the same element, not different elements.

Use the weighted average:

$$\text{Average atomic mass} = 79\times \frac{49.7}{100} + 81\times \frac{50.3}{100}$$

$$= 79(0.497) + 81(0.503)$$

$$= 39.263 + 40.743$$

$$= 80.006\,\text{u} \approx 80.0\,\text{u}$$

So the average atomic mass is about 80.0 u. Using rounding, this is commonly written as 79.99 u or 80.0 u.

The question asks for the correct option to the Rutherford gold foil assertion-reason item. Rutherford concluded that most of the mass of an atom is concentrated in a small central nucleus, and the statement about Thomson's model is also true. However, the reason is not the correct explanation of the assertion. Therefore the correct option is (ii). If the printed option list is the standard one from the chapter, mark (ii) as correct.

According to Bohr's atomic model:

• (i) Incorrect — electrons do not lose energy while moving in fixed orbits, so they do not slowly fall into the nucleus.
• (ii) Incorrect — electrons cannot exist anywhere around the nucleus with no fixed energy; they occupy fixed energy levels.
• (iii) Correct — electrons revolve around the nucleus in orbits of fixed energy without losing energy.
• (iv) Incorrect — electrons are not found between energy levels; they can exist only in the allowed shells.

So only statement (iii) is correct.

The relation is:

• Y and Z: They have the same number of protons, 17, but different numbers of neutrons, 18 and 20. So they are isotopes of the same element.
• Z and X: X has 18 protons and Z has 17 protons, but both have the same mass number:
• X: $18 + 19 = 37$
• Z: $17 + 20 = 37$

So they are isobars.

The correct chronological order is:

1. Dalton's model — atoms were indivisible particles.
2. Thomson's model — atom as a sphere of positive charge with electrons embedded in it.
3. Rutherford's model — atom has a dense central nucleus and electrons around it.
4. Bohr's model — electrons move in fixed orbits with definite energy.

So the order is (iv) → (ii) → (iii) → (i).

Rutherford's gold foil experiment showed the presence of a small, dense nucleus and that most of the atom is empty space. It did not show the existence of neutrons; neutrons were discovered later by James Chadwick in 1932.

The scattering pattern showed the existence of a small nucleus and that most of the atom is empty space. It did not directly show that electrons move around the nucleus; that was part of Rutherford's model, but the experiment itself did not prove it.

In Bohr's model, electrons move in fixed energy orbits and do not lose energy while in those allowed shells. So they do not slowly fall into the nucleus.

According to Bohr's model, electrons cannot exist anywhere around the nucleus. They can exist only in fixed shells or energy levels.

Bohr stated that electrons revolve around the nucleus in allowed fixed shells or stationary states and do not lose energy while staying in those orbits.