G11 C2 Atomic Structure Short Question Answer

Chapter 2 Atomic Structure

Short Answer Questions

(i) What is the importance of Bohr's Atomic model in modern atomic structure?

Bohr's model introduced the concept of quantized energy levels and electron orbits. While superseded by quantum mechanics, it laid the foundation for understanding atomic spectra and the discrete nature of energy within atoms, a core principle of modern atomic theory.

(ii) Explain the charge and mass of fundamental sub-atomic particles.

Protons have a positive charge (+1) and a mass of approximately 1 amu. Neutrons are neutral (no charge) with a similar mass to protons. Electrons have a negative charge (-1) and a much smaller, negligible mass.

(iii) Explain the periodic trends of atomic radius with justification.

Atomic radius generally decreases across a period due to increasing effective nuclear charge pulling electrons closer. It increases down a group because new electron shells are added, increasing the distance and shielding from the nucleus, despite the increase in nuclear charge.

(iv) How does shielding effect change the radius of an atom in a group from top to bottom?

As you move down a group, the number of electron shells increases. The inner-shell electrons shield the outer-shell electrons from the nucleus's full positive charge. This increased shielding weakens the attraction, causing the atomic radius to increase.

(v) Why is the cation always smaller than the parent atom, and anion is bigger than the parent atom?

A cation forms by losing electrons, reducing electron-electron repulsion and increasing the effective nuclear charge on the remaining electrons, pulling them closer. An anion gains electrons, increasing repulsion and expanding the electron cloud, making it larger than the parent atom.

(vi) Explain how different spectral series originate in hydrogen spectrum?

Spectral series originate from electrons transitioning between different energy levels. When an electron falls from a higher energy level to a lower one, it emits a photon. The specific series (e.g., Lyman, Balmer) is defined by the final energy level of the electron's transition.

(vii) Explain magnetic quantum number in detail. Why do s orbital have only one value of the magnetic quantum number?

The magnetic quantum number ($m_l$) describes the orientation of an orbital in space. For an s orbital, which is spherical and has a single orientation, its value is always 0. The s orbital is symmetric and does not have sub-orientations.

(viii) Why it is so that two electrons with same spin cannot reside in an orbital?

This is due to the Pauli Exclusion Principle. It states that no two electrons in an atom can have the same set of four quantum numbers ($n, l, m_l, m_s$). To be in the same orbital, they must have opposite spins ($+1/2$ and $-1/2$).

(ix) Why 3d orbital has greater energy than 4s orbital? Explain (n+l) rule.

According to the (n+l) rule, orbitals are filled in increasing order of their (n+l) value. For 4s, n=4 and l=0, so n+l=4. For 3d, n=3 and l=2, so n+l=5. Since 3d has a higher (n+l) value, it has a greater energy.

(x) Calculate the average atomic mass of magnesium keeping in view the relative abundance of its isotopes?

The average atomic mass is the weighted average of the masses of its isotopes. For magnesium: Mg-24 (78.99%), Mg-25 (10.00%), Mg-26 (11.01%). Average mass = $(24 \times 0.7899) + (25 \times 0.1000) + (26 \times 0.1101) = 18.9576 + 2.5 + 2.8626 = 24.3202 \text{ amu}$.

(xi) What is effective nuclear charge?

Effective nuclear charge ($Z_{eff}$) is the net positive charge experienced by an electron in a multi-electron atom. It is the attractive force from the nucleus minus the shielding or repulsive effects of other inner-shell electrons.

(xii) What is the importance of electronic configuration in semiconductor materials?

Electronic configuration determines a material's electrical conductivity. In semiconductors like silicon, it defines the narrow energy gap between valence and conduction bands. This small gap allows for controlled electron movement, making them ideal for electronic devices.

(xiii) Why is there a large I.E gape between second and third values in Mg atoms?

Magnesium has two valence electrons. The first and second ionization energies remove these easily. The third ionization energy requires removing an electron from a stable, filled inner shell, which is very close to the nucleus and requires a significantly larger amount of energy.

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