Answer:

(a) Postulates of Bohr's model of the atom:

1. Electrons revolve around the nucleus in specific, well-defined circular orbits called energy levels or electron shells.

2. Each electron shell has a fixed energy level, and electrons can only exist in these energy levels; they cannot exist in intermediate states.

3. Electrons can move from one energy level to another by absorbing or emitting energy in discrete packets or quanta. When an electron absorbs energy, it jumps to a higher energy level (excited state), and when it emits energy, it falls back to a lower energy level (ground state).

4. The energy of the electron in a particular energy level is quantized; it means that only certain energy levels are allowed for electrons in an atom.

The stability of the atom is explained by Bohr's model in the following way:

According to Bohr's model, electrons can only exist in certain discrete energy levels. The lowest energy level is called the ground state. When an electron is in the ground state, it is stable and does not lose energy. However, if an external energy source (e.g., heat or light) is applied to the atom, an electron can absorb this energy and move to a higher energy level (excited state). The excited state is temporary, and the electron tends to return to its original lower energy level by emitting the absorbed energy in the form of photons (light) or other forms of radiation. This process of absorption and emission of energy allows the atom to maintain stability and remain in its ground state.

(b) Difference between Bohr circular orbit and energy shells of an atom:

- Bohr Circular Orbit: In Bohr's model, electrons are assumed to move in specific circular orbits around the nucleus. Each circular orbit corresponds to a particular energy level, and an electron can transition between these orbits by absorbing or emitting energy. However, this circular orbit concept was later replaced by the more accurate description of electrons occupying electron clouds or regions of high electron probability, known as atomic orbitals, in quantum mechanics.

- Energy Shells: Energy shells, also called electron shells or energy levels, are the allowed energy states of electrons in an atom. These shells are represented by principal quantum numbers (n), where n = 1, 2, 3, and so on. The energy level with n = 1 is closest to the nucleus and has the lowest energy, while energy levels with higher values of n are progressively farther from the nucleus and have higher energies. Each energy shell can contain a specific maximum number of electrons (2n^2).

(c) The atomic number of element 'A' is 10. To distribute the number of electrons in its different energy shells, we follow the rules based on the electron configuration.

The electron configuration for element 'A' (atomic number 10) is: 1s^2 2s^2 2p^6

Distributing electrons in energy shells:

- The first energy shell (n = 1) can hold a maximum of 2 electrons (1s^2).

- The second energy shell (n = 2) can hold a maximum of 8 electrons (2s^2 2p^6).

Now, let's find the valency of element 'A':

The valency of an element is determined by the number of electrons in its outermost energy shell. In this case, the outermost energy shell is the second shell (n = 2), which contains 8 electrons.

Valency = Number of electrons in the outermost shell = 8

Element 'A' has 8 electrons in its outermost shell, so its valency is 8. Since it has a completely filled outermost shell, it is stable and less likely to react with other elements. Elements tend to react to achieve a stable electron configuration, and element 'A' already has a stable electron configuration in its outermost shell (full octet), making it relatively unreactive.

Verified answer

[tex]\huge{\underline{\underline{\pmb{\mathtt{\pink{A}\red{n}\orange{s}\purple{w}\green{e}\blue{R}}}}}}[/tex]

(a) Postulates of Bohr's Model of Atom and Explanation of Atom's Stability:

1. Postulates of Bohr's Model:

  a. Electrons revolve around the nucleus in fixed circular orbits or energy levels.

  b. Electrons can only exist in specific energy levels, and each energy level has a fixed energy value or energy state.

  c. Electrons can move from one energy level to another by either absorbing or emitting energy in the form of photons.

  d. Electrons in the lowest energy level closest to the nucleus have the least energy, while those in higher energy levels have increasing energy.

2. Explanation of Atom's Stability:

  Bohr's model explains the stability of the atom by proposing that electrons can only exist in specific, discrete energy levels. When electrons occupy these allowed energy levels, they are in their most stable states. Electrons tend to occupy the lowest energy level available (closest to the nucleus) before filling higher energy levels. This leads to stable electron configurations that minimize the atom's energy and make it less likely to lose or gain electrons spontaneously. Stable electron configurations correspond to noble gas configurations, which are highly stable and non-reactive.

(b) Difference between Bohr Circular Orbit and Energy Shells:

Bohr Circular Orbit:

- Bohr's model postulated that electrons revolve around the nucleus in fixed circular orbits, much like planets orbiting around the sun.

- Each circular orbit represents a specific energy level, and the electrons in these orbits have discrete energy values.

- The orbits are quantized, meaning electrons can only exist in certain permitted orbits with defined radii and energies.

Energy Shells:

- Energy shells are the regions around the nucleus where electrons are likely to be found.

- Each energy shell corresponds to a set of energy levels. For example, the first shell can have energy levels 1s, 2s, 3s, etc., while the second shell can have energy levels 2p, 3p, 4p, etc.

- Each shell can hold a specific number of electrons. The first shell (n = 1) can hold a maximum of 2 electrons, the second shell (n = 2) can hold a maximum of 8 electrons, and so on.

(c) Distribution of Electrons and Valency of Element 'A':

The atomic number of element 'A' is 10, which means it has 10 protons and 10 electrons.

Distribution of Electrons in Different Energy Shells:

- The first shell (n = 1) can hold a maximum of 2 electrons.

- The second shell (n = 2) can hold a maximum of 8 electrons.

Therefore, the electron configuration of element 'A' is: 1s² 2s² 2p⁶

Valency of Element 'A':

The valency of an element is determined by the number of electrons in its outermost energy shell. In this case, element 'A' has 2 electrons in its outermost shell (2s²), so its valency is 2.

Will Element 'A' React with Other Elements?

Yes, element 'A' will react with other elements to achieve a stable electron configuration. It has 2 electrons in its outermost shell, which means it needs to gain 6 electrons to achieve a stable electron configuration similar to the nearest noble gas, neon (1s² 2s² 2p⁶). Element 'A' is likely to form chemical bonds with other elements to gain these missing electrons and attain a stable configuration, becoming a chemically stable compound.