Chapter 3: Atomic Structure

Interactive Study Notes Preview

Chapter Overview & SLOs

What is atomic structure? Atomic structure refers to the arrangement of subatomic particles within an atom. An atom consists of a central nucleus (containing protons and neutrons) surrounded by electrons in specific energy levels.

What are the subatomic particles?

• Protons ($p^+$): Positively charged particles in the nucleus. Number of protons = atomic number ($Z$).
• Neutrons ($n^0$): Neutral particles in the nucleus. Number of neutrons = mass number ($A$) - atomic number ($Z$).
• Electrons ($e^-$): Negatively charged particles orbiting the nucleus in shells. Number of electrons = number of protons (in neutral atoms).

How has our understanding of the atom evolved? Atomic theory progressed through several key models:

• Dalton's model: Solid indivisible spheres (early 1800s).
• Thomson's model (Plum Pudding): Electrons embedded in a positive sphere (1897).
• Rutherford's nuclear model (1911): Gold foil experiment revealed a small, dense, positively charged nucleus. Most alpha particles passed through, some were deflected, very few bounced back. Limitations: Could not explain stability of electron orbits (electrons should spiral into nucleus).
• Bohr's model (1913): Introduced fixed energy levels (orbits) for electrons. Electrons move in quantized shells and do not radiate energy while in these stable orbits. Solved the stability issue of Rutherford's model. Only explains hydrogen-like atoms.

What is electronic configuration? Electronic configuration is the distribution of electrons in different shells and subshells around the nucleus.

• Shells (K, L, M, N): Maximum electrons per shell = $2n^2$ (where $n$ = shell number). K shell ($n=1$): 2 electrons, L shell ($n=2$): 8 electrons, M shell ($n=3$): 18 electrons, N shell ($n=4$): 32 electrons.
• Subshells (s, p, d, f): Electrons fill subshells in order of increasing energy: $1s^2$, $2s^2$, $2p^6$, $3s^2$, $3p^6$, $4s^2$, $3d^{10}$, etc. (Aufbau principle).
• For the first 18 elements: K (2), L (8), M (8) for valence shell never exceeds 8.

What are isotopes? Isotopes are atoms of the same element with the same atomic number ($Z$) but different mass numbers ($A$) due to different numbers of neutrons.

• Isotopes of Hydrogen:
  • Protium ($^1_1H$): 1 proton, 0 neutrons (99.985%)
  • Deuterium ($^2_1H$): 1 proton, 1 neutron (0.015%)
  • Tritium ($^3_1H$): 1 proton, 2 neutrons (radioactive)
• Isotopes of Carbon: Carbon-12 ($^{12}_6C$), Carbon-13 ($^{13}_6C$), Carbon-14 ($^{14}_6C$) — radioactive, used in carbon dating.
• Isotopes of Uranium: Uranium-235 ($^{235}_{92}U$) — fissile, used in nuclear reactors and weapons; Uranium-238 ($^{238}_{92}U$).

How are isotopes used in real life?

• Carbon-14 dating: Determines age of archaeological artifacts (up to ~50,000 years).
• Radioactive isotopes in medicine: Radiotherapy for cancer treatment (Cobalt-60, Iodine-131), medical imaging (Technetium-99m).
• Nuclear power generation: Uranium-235 fission produces heat for electricity generation.

How was the neutron discovered? James Chadwick (1932) bombarded beryllium with alpha particles, producing a neutral radiation that could knock out protons from paraffin wax. This led to the discovery of the neutron, explaining the missing mass in the nucleus.

How do we calculate subatomic particles?

• Atomic number $Z$ = number of protons = number of electrons (in neutral atom)
• Mass number $A$ = number of protons + number of neutrons
• Number of neutrons = $A - Z$

Example: For an atom of Carbon-14 ($^{14}_6C$):

• Protons = 6
• Electrons = 6 (neutral)
• Neutrons = 14 - 6 = 8

These notes are strictly aligned with the Student Learning Outcomes (SLOs) for the FBISE 2026 annual examination.

• How do we compare atomic models? Differentiate between Rutherford's planetary model (nucleus with electrons orbiting, but could not explain stability) and Bohr's quantized energy levels (fixed orbits where electrons do not radiate energy), focusing on how Bohr resolved the stability issues of Rutherford's atom.
• How do we write electronic configurations? Apply the principle of increasing energy levels (Aufbau principle) and the $2n^2$ rule to determine the electron arrangement for the first 18 elements of the periodic table, using K, L, M shells with correct maximum electron capacities (2, 8, 8 for valence shell).
• How do we identify isotopes and their uses? Define isotopes for hydrogen (protium, deuterium, tritium), carbon (C-12, C-14), and uranium (U-235, U-238), and explain their applications in diagnosing diseases (radiotherapy, medical imaging), carbon dating (archaeology), and generating nuclear power.
• How do we calculate subatomic particle counts? Use the atomic number ($Z$) and mass number ($A$) to find the number of protons ($Z$), neutrons ($A - Z$), and electrons ($Z$ for neutral atoms) in any given atom or ion.

Frequently Asked Questions (FAQ)