Structure of Atom

Definition

Atoms are not indivisible. They are composed of even smaller particles known as subatomic particles, which include electrons, protons, and neutrons.

Discovery of Particles

• Electron ($e^-$): Discovered by J.J. Thomson via the Cathode Ray Tube experiment. These are negatively charged particles orbiting the nucleus.
• Proton ($p^+$): Discovered through canal ray experiments. These are positively charged particles located in the nucleus.
• Neutron ($n^0$): Discovered by James Chadwick. These are neutral particles found inside the nucleus.

Key Points Table

Examples

• An atom with 6 protons, 6 neutrons, and 6 electrons is Carbon.
• If you add a neutron to a Carbon atom, it becomes an isotope of Carbon.

Atomic Number ($Z$) and Mass Number ($A$)

• Atomic Number ($Z$): The total number of protons. It defines the element.
• Mass Number ($A$): The sum of protons and neutrons ($A = Z + n$).

Isotopes vs. Isobars

Isotopes: Same atomic number, different mass number (e.g., Hydrogen: Protium, Deuterium, Tritium).

Isobars: Different atomic number, same mass number (e.g., Calcium-40 and Argon-40).

Historical Models

• Thomson’s Model: "Plum Pudding Model" - positive sphere with embedded electrons.
• Rutherford’s Model: "Nuclear Model" - discovered the nucleus via Gold Foil experiment.
• Bohr’s Model: "Planetary Model" - electrons move in fixed orbits (energy levels).

Comparison Table

Quantum Numbers (The Electron's "Address")

1. Principal ($n$): Main energy level/shell.
2. Azimuthal ($l$): Shape of orbital ($s, p, d, f$).
3. Magnetic ($m_l$): Orientation in space.
4. Spin ($m_s$): Direction of spin ($+1/2$ or $-1/2$).

• Aufbau Principle: Fill lowest energy orbitals first.
• Pauli’s Exclusion Principle: Max 2 electrons per orbital with opposite spins.
• Hund’s Rule: Electrons occupy empty orbitals singly before pairing up.

Problem 1: Finding Subatomic Particles

An element has an Atomic Number of 11 and a Mass Number of 23. Calculate protons, electrons, and neutrons.

Solution:
1. Protons = Z = 11
2. Electrons = 11 (neutral atom)
3. Neutrons = A - Z = 23 - 11 = 12
        

Problem 2: Electronic Configuration

Write the electronic configuration for Phosphorus ($Z=15$).

Solution:
Order: 1s, 2s, 2p, 3s, 3p
Result: 1s² 2s² 2p⁶ 3s² 3p³
        

In nuclear research, scientists observe Uranium isotopes emitting particles to reach stability. Because Uranium has 92 protons, electromagnetic repulsion is massive. This creates instability, causing the atom to undergo decay and change its atomic number...

1. Who discovered the neutron?
   a) J.J. Thomson b) Rutherford c) Chadwick d) Bohr
2. Isotopes have the same:
   a) Mass number b) Atomic number c) Number of neutrons d) Physical properties
3. The shape of an 's' orbital is:
   a) Dumbbell b) Spherical c) Double dumbbell d) Complex
4. Which principle states electrons occupy the lowest energy level first?
   a) Pauli's b) Hund's c) Aufbau d) Heisenberg's
5. The maximum number of electrons in a 'p' subshell is:
   a) 2 b) 6 c) 10 d) 14
6. Cathode rays are composed of:
   a) Protons b) Neutrons c) Electrons d) Positrons
7. An element with $Z=17$ belongs to which group?
   a) Noble gases b) Halogens c) Alkali metals d) Transition metals
8. Isobars have different:
   a) Mass number b) Atomic number c) Protons d) Electrons
9. The principal quantum number $n=3$ represents which shell?
   a) K b) L c) M d) N
10. Which is more stable: $d^5$ or $d^4$?
    a) $d^4$ b) $d^5$ c) Both equal d) Neither

Answers: 1(c), 2(b), 3(b), 4(c), 5(b), 6(c), 7(b), 8(b), 9(c), 10(b)

1. Explain why the electronic configuration of Chromium is $[Ar] 4s^1 3d^5$ instead of $[Ar] 4s^2 3d^4$.
   Ans: Because a half-filled $d$-subshell ($d^5$) provides extra stability due to symmetrical distribution and higher exchange energy.
2. Differentiate between Isotopes and Isobars with examples.
   Ans: Isotopes have the same $Z$ but different $A$ (e.g., $^{12}C, ^{14}C$). Isobars have the same $A$ but different $Z$ (e.g., $^{40}Ar, ^{40}Ca$).
3. What were the limitations of Rutherford's model?
   Ans: It couldn't explain atomic stability (why electrons don't spiral into the nucleus) or discrete atomic spectra.
4. State Hund's Rule of Maximum Multiplicity.
   Ans: Electrons don't pair up in orbitals of the same subshell until each orbital is singly occupied.
5. Define Quantum Numbers. Name the four types.
   Ans: Sets of numbers describing an electron's position and energy. Types: Principal, Azimuthal, Magnetic, Spin.

• Subatomic Particles: "e-P-N" $\rightarrow$ Electron (Negative), Proton (Positive), Neutron (Neutral).
• Quantum Numbers Order: "P-A-M-S" $\rightarrow$ Principal, Azimuthal, Magnetic, Spin.
• Hund's Rule: Remember "Singletons First"—don't pair up until you have to.
• Aufbau: Always build from the "ground floor" (lowest energy) up.