Atomic Structure and Chemical Bonding

1. Observation: Most $\alpha$-particles ($> 99.9\%$) passed through the gold foil in straight lines undeflected.
   Conclusion: Most of the interior volume of an atom is completely empty space.
   2. Observation: A small fraction of $\alpha$-particles were deflected through large angles.
   Conclusion: The entire positive charge of the atom is concentrated in a tiny central core called the Nucleus.
   3. Observation: Very few particles ($1$ in $20,000$) bounced completely backward ($180^\circ$).
   Conclusion: The nucleus contains virtually the entire mass of the atom and is extremely dense and rigid.

• Isotopes: Atoms of the same element having the same atomic number ($Z$) but different mass numbers ($A$) due to different numbers of neutrons. Examples: Carbon isotopes: $^{12}_6\text{C}$ and $^{14}_6\text{C}$ (or Chlorine: $^{35}_{17}\text{Cl}$ and $^{37}_{17}\text{Cl}$).
• Isobars: Atoms of different elements having the same mass number ($A$) but different atomic numbers ($Z$). Examples: Argon and Calcium: $^{40}_{18}\text{Ar}$ and $^{40}_{20}\text{Ca}$.

• Electronic configuration of Calcium ($Z = 20$): $2, 8, 8, 2$. It has $2$ valence electrons.
• Electronic configuration of Oxygen ($Z = 8$): $2, 6$. It needs $2$ electrons to complete its octet.
• Calcium transfers its $2$ valence electrons completely to oxygen:

$$\text{Ca } (2,8,8,2) o \text{Ca}^{2+} (2,8,8) \quad [\text{Stable Argon duplet/octet}]$$

$$\text{O } (2,6) o \text{O}^{2-} (2,8) \quad [\text{Stable Neon octet}]$$

• The oppositely charged ions attract via strong electrostatic forces:

$$\mathbf{\text{Ca}^{2+} + \text{O}^{2-} \to \text{CaO}}$$

• Nitrogen ($Z = 7$) has electronic configuration $2, 5$. Each nitrogen atom requires $3$ electrons to complete its stable octet.
• Two nitrogen atoms mutually share three pairs of electrons (total 6 shared electrons):

$$:\text{N} \;\vdots\vdots\; \text{N}: \implies \mathbf{:\text{N} \equiv \text{N}:}$$

• Each nitrogen atom contributes $3$ electrons to the bond, and keeps $1$ unshared lone pair ($2$ electrons).
• A Triple Covalent Bond is formed between the two nitrogen atoms.

• In the solid state, electrovalent compounds (like $\text{NaCl}$) exist as a rigid, tightly bound three-dimensional crystal lattice held by strong electrostatic forces. The ions are locked in fixed positions and cannot move freely, so they do not conduct electricity.
• In the molten state (melted) or aqueous solution (dissolved in water), thermal energy or high dielectric water breaks the lattice, setting the ions free.
• These freely mobile cations and anions migrate toward the oppositely charged electrodes when a potential difference is applied, conducting electric current.

• Covalent compounds consist of discrete, neutral molecules.
• The forces holding different molecules together are weak intermolecular forces (Van der Waals forces).
• Only a small amount of thermal kinetic energy is required to overcome these weak intermolecular attractions and separate the molecules.
• Hence, they melt and boil at significantly lower temperatures than ionic compounds, which require breaking immensely powerful electrostatic bonds.

• (i) Atomic Number ($Z$):

$$Z = A - n = 39 - 20 = \mathbf{19}$$

• (ii) Valency:
Electronic configuration for $Z = 19$: $2, 8, 8, 1$ (Potassium, $\text{K}$).
Since it has $1$ valence electron which it readily donates to achieve an octet, its valency is $1$ (Monovalent).

• In an ammonia molecule ($\text{NH}_3$), the nitrogen atom has completed its octet with three single covalent bonds to hydrogen atoms and possesses one unshared pair of electrons (lone pair): $:\text{NH}_3$.
• When ammonia reacts with a hydrogen ion ($\text{H}^+$, which has zero electrons), the nitrogen atom donates its entire lone pair of electrons to be shared mutually with the $\text{H}^+$ ion.
• A Coordinate Covalent Bond is formed (represented as $\text{N} \to \text{H}^+$):

$$\mathbf{\text{H}_3\text{N}: + \text{H}^+ \to [\text{H}_3\text{N} \to \text{H}]^+ = \text{NH}_4^+}$$