VCE Chemistry (Year 11) · Unit 1 — Atomic Structure · 12m read

Atomic Structure and Electron Configuration

Ever wondered why some elements are metals and others are gases? It all comes down to how electrons arrange themselves around an atom's nucleus. You'll learn about protons, neutrons, and electrons—the tiny players that make up every substance—and discover how to map out electron configurations (the seating chart showing where each electron lives). Understanding atomic structure is your foundation for predicting how atoms bond, react, and behave, which matters for everything from VCE exams to designing new materials in real life.

Learning objective: Describe atomic structure and write electron configurations.

By the end of this lesson you can:

• I explain the subatomic particles.
• I write electron configurations in shells and subshells.
• I distinguish isotopes.

Electron shells are hotel floors. Electrons fill the lowest floors first. Subshells (s, p, d) are rooms of different sizes on each floor.

Atoms and electrons

SUBATOMIC PARTICLES:
• PROTONS — positive charge, in nucleus. Mass ~1 amu.
• NEUTRONS — no charge, in nucleus. Mass ~1 amu.
• ELECTRONS — negative charge, orbit nucleus. Mass ~1/1836 amu.

IMPORTANT NUMBERS:
• ATOMIC NUMBER (Z) = number of protons (defines element).
• MASS NUMBER (A) = protons + neutrons.
• For neutral atom: electrons = protons.

ISOTOPES:
• Same Z, different A (different neutron count).
• Similar chemistry, may differ in radioactivity/stability.
• Example: ¹²C, ¹³C, ¹⁴C.

IONS:
• CATIONS (+) — lost electrons.
• ANIONS (−) — gained electrons.

ELECTRON SHELLS (Y11 BOHR MODEL):
• Shell 1: max 2 electrons.
• Shell 2: max 8.
• Shell 3: max 8 (simple) or 18 (with d subshell).
• Shell 4: up to 32.

WRITING CONFIGURATIONS (shell notation):
• Carbon (Z=6): 2, 4.
• Sodium (Z=11): 2, 8, 1.
• Chlorine (Z=17): 2, 8, 7.

SUBSHELL NOTATION (s, p, d, f):
• 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p…
• Order follows diagonal rule (Aufbau principle).

CAPACITIES:
• s = 2, p = 6, d = 10, f = 14.

EXAMPLES:
• Carbon: 1s² 2s² 2p².
• Calcium: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s².
• Iron: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶.

VALENCE ELECTRONS:
• Electrons in the OUTERMOST shell.
• Determine chemical behaviour.

NOBLE GASES:
• Full outer shell → unreactive.
• Goal of other elements: achieve noble gas configuration.

MASS SPECTROMETRY:
• Measures mass-to-charge ratio of ions.
• Used to find isotopic abundance.
• Average atomic mass = Σ(fractional abundance × mass).

CAS TIP:
• Use the periodic table on your calculator.

COMMON ERRORS:
• Mixing up mass and atomic numbers.
• Forgetting s before p within a shell.
• Wrong capacities (s=2, not 3).

STABILITY:
• Half-full or full subshells gain extra stability.
• Explains some anomalies (e.g., Cu: [Ar] 3d¹⁰ 4s¹).

Atom quick facts

• Proton = identity: Defines element.
• Neutron = isotope: Changes mass.
• Electron = chemistry: Behaviour driver.

Quick reference

| Subshell | Max electrons |
|---|---|
| s | 2 |
| p | 6 |
| d | 10 |
| Order | 1s 2s 2p 3s 3p 4s 3d |
| Example (Na) | 1s² 2s² 2p⁶ 3s¹ |

Worked example (Medium)

Write the electron configuration of a sulfur atom (Z=16) in shell and subshell notation.

1. Shell. Fill 2 + 8 + 6 = 16. → 2, 8, 6.
2. Subshell. 1s² 2s² 2p⁶ 3s² 3p⁴.
3. Valence. 3s² 3p⁴ → 6 valence electrons.

Answer: Shell: 2,8,6. Subshell: 1s² 2s² 2p⁶ 3s² 3p⁴. Valence = 6.

Worked example (Hard)

Naturally occurring chlorine consists of 75.77% ³⁵Cl (mass 34.97 amu) and 24.23% ³⁷Cl (mass 36.97 amu). Calculate the relative atomic mass of chlorine.

1. Formula. Relative atomic mass = $\Sigma$(fractional abundance × isotopic mass).
2. Calculate. (0.7577 × 34.97) + (0.2423 × 36.97) = 26.496 + 8.957 = 35.45 amu.
3. Check. This matches the periodic table value for Cl (35.45), confirming the calculation.

Answer: Relative atomic mass of Cl ≈ 35.45 amu.

Common mistakes

• Confusing atomic number with mass number Atomic number (Z) = protons only (defines element). Mass number (A) = protons + neutrons. Students often swap these. Fix: Z is always the SMALLER number; A is always the LARGER number. Z = identity, A = mass.
• Writing electron configurations in wrong order Students sometimes fill 3d before 4s, but the Aufbau order is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. The 4s fills BEFORE 3d. Fix: Follow the diagonal rule: draw the subshells and use diagonal arrows to determine filling order.
• Forgetting that ions have different electron counts A Na⁺ ion has 10 electrons (not 11). Students write neutral atom configurations for ions. Fix: For cations, REMOVE electrons from the outermost shell. For anions, ADD electrons to the outermost shell.

Mass spectrometry — weighing individual atoms

Mass spectrometry is the technique that allows chemists to determine the isotopic composition of elements and the molecular mass of compounds. It's central to VCE Chemistry and widely used in forensic science, environmental monitoring, and pharmaceutical research.

The process involves four stages:

• Ionisation: the sample is vaporised and bombarded with high-energy electrons, knocking electrons off atoms/molecules to create positive ions. This is necessary because the instrument uses electric and magnetic fields, which only affect charged particles.

• Acceleration: the ions are accelerated through an electric field, giving them kinetic energy. All ions of the same charge receive the same energy.

• Deflection: ions pass through a magnetic field that curves their path. Lighter ions and more highly charged ions are deflected MORE. Heavier ions are deflected less. This separates ions by their mass-to-charge ratio ($\frac{m}{z}$).

• Detection: a detector records the number of ions at each $\frac{m}{z}$ value, producing a mass spectrum — a bar graph showing relative abundance vs mass.

From the mass spectrum, you can:
- Identify which isotopes are present and their relative abundances.
- Calculate the relative atomic mass using the weighted average formula.
- For molecules, identify fragmentation patterns that reveal molecular structure.

The mass spectrum of chlorine, for example, shows peaks at m/z = 35 and 37, with heights in approximately a 3:1 ratio — corresponding to ³⁵Cl and ³⁷Cl isotopes. This data directly feeds into the relative atomic mass calculation of 35.45 amu.

Practice questions

1. The MASS NUMBER of an atom equals:

   1. Number of electrons.
   2. Number of protons only.
   3. Protons + neutrons.
   4. Protons + electrons.

   (1 marks)

2. Why do isotopes of an element behave similarly in chemistry?

   (3 marks)

Summary

• Explain the subatomic particles.
• Write electron configurations in shells and subshells.
• Distinguish isotopes.