STRUCTURE OF THE ATOM-Exercise

Q1. Compare the properties of electrons, protons and neutrons.

Electrons, protons, and neutrons are the three key subatomic particles that make up every atom, and each has its own distinct set of properties:

Electrons are incredibly light and carry a negative charge. They zoom around the nucleus in various shells or energy levels, acting as the "glue" that allows atoms to bond and interact. Their mass is negligible compared to the other particles, being about 1/1836 that of a proton.

Protons are much heavier than electrons and have a positive charge. They reside in the nucleus at the atom's center and determine an atom’s identity; changing the number of protons means changing the element itself. Protons’ mass is about the same as neutrons.

Neutrons are also found in the nucleus, and their mass is nearly equal to that of protons, though slightly heavier. However, they have no charge; they are neutral. Neutrons help stabilize the nucleus by reducing the repulsion between positively charged protons.

In short, electrons are small, fast, and negatively charged; protons are heavy and positive; neutrons are heavy and neutral. Their location, charge, and mass define their role in the atom’s structure and chemistry.

Q2. What are the limitations of J.J. Thomson’s model of the atom?

• Thomson’s model could not explain how the positive charge keeps the negatively charged electrons together—atomic stability remained unexplained.
• The existence of a central nucleus was missing; it wrongly assumed positive charge spread throughout the atom instead of being concentrated.
• It could not explain the results of Rutherford’s gold foil experiment, especially the observation that some alpha particles were deflected at large angles.
• The model gave no explanation for how atoms produce specific spectral lines, since electron arrangement and movement were not addressed.
• There was no real experimental support for the detailed “plum pudding” structure, making it unable to match later discoveries.

In summary, J.J. Thomson’s atomic model raised important questions but did not fully reveal the atom’s true structure.

Q3. What are the limitations of Rutherford’s model of the atom?

Rutherford’s model of the atom was a ground-breaking step in atomic theory, but it came with key limitations:

• It could not explain why electrons, which are accelerating as they move in circular orbits around the nucleus, do not lose energy and spiral into the nucleus. According to classical physics, this should happen, but atoms are stable in reality.
• The model failed to explain the unique spectral lines observed when elements emit or absorb light. It had no mechanism for why atoms only give off certain frequencies of light.
• Rutherford’s model did not propose how electrons are arranged or how they move around the nucleus. There was no description of possible energy levels or quantized orbits.
• The model did not account for the structure of the nucleus itself; it could not explain the presence or role of neutrons, which were discovered later.

In summary, while Rutherford’s model revealed the existence of a dense, positively charged nucleus, it left important questions about atomic stability, electron arrangement, and atomic spectra unanswered.

Q4. Describe Bohr’s model of the atom.

Bohr’s model describes the atom as a tiny, positively charged nucleus surrounded by electrons moving in fixed circular orbits, or shells, at specific distances. Each orbit represents a distinct energy level, and electrons can jump between these levels by absorbing or releasing energy. In Bohr’s view, electrons do not radiate energy while in stable orbits, which explains why atoms are stable and why only certain frequencies of light are emitted or absorbed by atoms.

Q5. Compare all the proposed models of an atom given in this chapter.

Q6. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

To distribute electrons in various shells for the first eighteen elements, follow these basic rules:

Fill the innermost shell first (the K-shell), and move outward.

The maximum number of electrons a shell can hold is \[2n^2\] where \(n\) is the shell number: K-shell (1st) can hold 2, L-shell (2nd) can hold 8, M-shell (3rd) can hold 18.

For the first eighteen elements, electrons fill shells in this order: K (max 2), then L (max 8), and then M (rest of electrons up to 8, since only 18 elements are considered).

The outer shells never start filling until the previous ones reach their maximum.

If the number of electrons left for a shell is less than its capacity, fill it only with the remaining electrons.

Q7. Define valency by taking examples of silicon and oxygen.

Valency is the combining power of an atom—basically, the number of bonds it can make with other atoms so it can achieve a stable outer shell.

Take silicon as an example. It has four electrons in its outermost shell. To get a full shell (like a noble gas), silicon can either share or gain four electrons. That makes its valency 4.

Now look at oxygen. Oxygen’s outer shell contains six electrons, but it needs two more to complete its octet. That means oxygen’s valency is 2, because it either gains or shares two electrons in chemical bonding.

So, in short: the valency of an element is the number of electrons it must gain, lose, or share to reach a stable, full shell—4 for silicon and 2 for oxygen.

Q8. Explain with examples
(i) Atomic number,
(ii) Mass number,
(iii) Isotopes and
(iv) Isobars.
Give any two uses of isotopes.

Atomic Number

The atomic number is the unique count of protons present in the nucleus of an atom. It determines what element the atom is. For example, every carbon atom has 6 protons, so carbon’s atomic number is 6. Likewise, oxygen has an atomic number of 8.

Mass Number

Mass number is the sum of the total number of protons and neutrons in the nucleus. For example, if a carbon atom has 6 protons and 6 neutrons, its mass number is 12 (6 + 6). For sodium, with 11 protons and 12 neutrons, the mass number is 23 (11 + 12).

Isotopes

Isotopes are atoms of the same element—that is, same atomic number—but with different mass numbers. This means they have the same number of protons but vary in neutrons. For instance, carbon-12 (6 protons, 6 neutrons) and carbon-14 (6 protons, 8 neutrons) are both isotopes of carbon.

Isobars

Isobars are atoms of different elements that have the same mass number, but different atomic numbers. That means they have the same total number of protons and neutrons, but a different number of each. As an example, argon-40 (18 protons, 22 neutrons) and calcium-40 (20 protons, 20 neutrons) are isobars—they both have a mass number of 40, but are different elements.

Two Uses of Isotopes

• Some isotopes are used in medicine, like cobalt-60, which helps treat cancer through targeted radiation therapy.
• Isotopes such as carbon-14 are used in archaeology for radiocarbon dating, helping scientists determine the ages of ancient artifacts and fossils.

Q9. \(\mathrm{Na^+}\) has completely filled K and L shells. Explain.

The sodium ion, \(\mathrm{Na^+}\) is formed when a neutral sodium atom loses one electron from its outermost shell.

Normally, a sodium atom has 11 electrons distributed as 2 in the K shell, 8 in the L shell, and 1 in the M shell.

When it loses this single outer electron to achieve a stable configuration, only 10 electrons remain—2 in the K shell and 8 in the L shell.

Both shells are now completely filled, just like in the noble gas neon.

This filled arrangement makes the \(\mathrm{Na^+}\) ion highly stable and less chemically reactive, which is why it plays a crucial role in biological and chemical processes.

Q10. If bromine atom is available in the form of, say, two isotopes \(^{79}_{35}\mathrm{Br}\) (49.7%) and \(^{81}_{35}\mathrm{Br}\) (50.3%), calculate the average atomic mass of bromine atom.

Solution: Avearge Atomic Mass of Br \[ \begin{aligned} &79\times 49.7 \% + 81\times 50.3\%\\\\ =&\frac{79\times 49.7 + 81\times 50.3}{100}\\\\ =&\frac{3926.30+4074.3}{100}\\\\ =&\frac{8000.6}{100}\\\\ =&80.006 \end{aligned} \]

Q11. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes \(^{16}_8\mathrm{X}\) and \(^{18}_8\mathrm{X}\) in the sample?

Solution:
Avearge Atomic Mass of \(\scriptsize ^{16}_8\mathrm{X}\) =16.2
Let %age of \(\scriptsize ^{16}_8\mathrm{X}\) be \(x\%\)
therefore,
%age of \(\scriptsize ^{18}_8\mathrm{X}\) will be \(\scriptsize 100-x\%\)
\[ \tiny \begin{aligned} 16.2&=(16\times x \%) + 18\times (100-x\%)\\\\ 16.2&=\frac{16x+ 1800-18x}{100}\\\\ \implies1800-2x&=16.2\times 100\\\\ -2x&=1620-1800\\\\ 2x&=180\\ x&=90\% \end{aligned} \] Therefore,
\(^{16}_8\mathrm{X}\)=90%
\(^{18}_8\mathrm{X}\)=10%

Q12. If Z = 3, what would be the valency of the element? Also, name the element.

If the atomic number (Z) is 3, the element is lithium.

To find the valency, write its electronic configuration:
Lithium has 3 electrons: 2 fill the K shell, and 1 goes into the L shell (2,1).

Valency is determined by the number of electrons it needs to lose or gain to achieve a full outer shell. Lithium has one electron in its outer shell, so it loses that electron to become stable.

Thus, lithium’s valency is 1.
Element Name: Lithium.

Q13. The composition of the nuclei of two atomic species X and Y are given as under \[ \begin{array}{|r|c|c|} \hline &\mathrm{X}&\mathrm{Y}\\\hline \text{Protons}&6&6\\\hline \text{Neutrons}&6&8\\\hline \end{array}\] Give the mass numbers of X and Y. What is the relation between the two species?

Solution:
To find the mass numbers, add the number of protons and neutrons for each:
For species X: 6 protons + 6 neutrons = 12

For species Y: 6 protons + 8 neutrons = 14

Mass numbers:
X: 12
Y: 14

Both X and Y have the same number of protons, which means they are the same element (carbon). However, their mass numbers are different because they have different numbers of neutrons. Such species are called isotopes. So, X and Y are isotopes of carbon.

Q14. For the following statements, write T for True and F for False. (a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
(c) The mass of an electron is about \(\frac{1}{2000}\) times that of proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
Put tick (ü) against correct choice and cross (×) against wrong choice in questions 15, 16 and 17

Solution:
(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.False
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.False
(c) The mass of an electron is about \(\frac{1}{2000}\) times that of proton.True
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.False

Q15. Rutherford’s alpha-particle scattering experiment was responsible for the discovery of
(a) Atomic Nucleus
(b) Electron
(c) Proton
(d) Neutron

Solution:
Rutherford’s alpha-particle scattering experiment was responsible for the discovery of:

(a) Atomic Nucleus

The experiment showed that atoms have a tiny, dense central core—the nucleus—which contains the atom’s positive charge and most of its mass.

Q16. Isotopes of an element have
(a) the same physical properties
(b) different chemical properties
(c) different number of neutrons
(d) different atomic numbers.

Solution:
The correct answer is:

(c) different number of neutrons

Isotopes of an element always have the same number of protons (and hence the same atomic number), so they are the same element. However, what sets them apart is the number of neutrons in their nuclei, which gives them different mass numbers and sometimes different physical properties. Their chemical properties remain nearly identical.

Q17. Number of valence electrons in \(Cl^–\) ion are:
(a) 16
(b) 8
(c) 17
(d) 18

Solution
The number of valence electrons in \(Cl^–\) ion is:

(b) 8

When chlorine gains one electron, it achieves a stable octet in its outermost shell, so the chloride ion has 8 valence electrons.

Q18. Which one of the following is a correct electronic configuration of sodium?
(a) 2,8
(b) 8,2,1
(c) 2,1,8
(d) 2,8,1.

The correct electronic configuration of sodium is:

(d) 2,8,1

Sodium has 11 electrons arranged as 2 in the first shell (K), 8 in the second shell (L), and 1 in the third shell (M).

Q19.