A Level Chemistry: Atomic Structure

Introduction

• Chemical properties depend on atomic structure, especially electron arrangement around the nucleus.
• Focus of the lecture includes:
  • Fundamental particles
  • Mass number and isotopes
  • Electron configuration
  • Ionization
  • Mass spectrometer and substance analysis

Fundamental Particles

• Understanding of atomic models has evolved:
  • Billiard Ball Model: Atoms are solid lumps.
  • Plum Pudding Model: Atoms contain electrons diffused in a positive mass.
  • Nuclear Model: Atoms have a small nucleus with electrons orbiting.
  • Bohr Model: Electrons are arranged in specific energy levels.

Key Points on Particles

• Nucleus contains protons (+1 charge) and neutrons (0 charge), held together by the strong nuclear force.
• Relative mass:
  • Protons/Neutrons = 1
  • Electrons = 1/1840 of a proton.
• Atomic number (Z) = number of protons; Mass number (A) = protons + neutrons.

Isotopes

• Isotopes have the same number of protons but different neutrons.
• Reactivity is determined by electron configuration rather than isotopes' mass.

Example Calculations

• Fluorine (Z=9, A=19):
  • Protons = 9, Electrons = 9, Neutrons = 10.
• Sodium ion (Na+):
  • Protons = 11, Electrons = 10, Neutrons = 12.

Electron Configuration

• Electrons arranged in shells/energy levels.
  • Carbon: 6 electrons (2 in first shell, 4 in second) = 2, 4.
  • Lithium: 3 electrons (2 in first, 1 in second) = 2, 1.

Energy Levels and Sublevels

• Shells can hold:
  • 1st Shell: 2 electrons
  • 2nd Shell: 8 electrons
  • 3rd Shell: 18 electrons
• Sublevels: S, P, D, F, each with specific shapes and energy levels.

Filling Rules

1. Fill lower energy levels first.
2. Fill orbitals singly before pairing.
3. Max of 2 electrons per orbital.

Examples of Electron Configuration

• Carbon: 1s² 2s² 2p²
• Oxygen: 1s² 2s² 2p⁴
• Iron: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶
• Transition metals: 4s fills before 3d.

Ionization Energy

• Energy required to remove one mole of electrons from a mole of gaseous atoms.
• Increases with removal of electrons due to increased positive charge on ion.
• Trends:
  • Across periods: Generally increases.
  • Down groups: Generally decreases due to increased atomic radius and shielding effect.

Mass Spectrometry

• Measures mass of atoms/molecules and identifies elements.
• Steps in Time-of-Flight Mass Spectrometer:
  1. Ionization: Creates positive ions.
  2. Acceleration: Ions gain kinetic energy.
  3. Drift: Lighter ions travel faster.
  4. Detection: Ions hit detector, generating current; abundance determined.

Types of Ionization

1. Electron Gun: Electrons knock off to create ions.
2. Electrospray Ionization: Dissolved sample gains a proton.

Interpreting Mass Spectra

• Peaks represent isotopes and molecular ion peaks indicate molecular mass.
• Fragmentation occurs when molecules break into smaller pieces.

Average Atomic Mass Calculation

• Average mass calculated using isotope abundances.

• Formula:

  [ \text{Average Mass} = \frac{(abundance_1 \times mass_1) + (abundance_2 \times mass_2)}{total\ abundance} ]

Example of Average Mass

• Chlorine with 75% Cl-35 and 25% Cl-37:
  [ \text{Average} = \frac{(75 \times 35) + (25 \times 37)}{100} = 35.5 ]

Summary

• Understanding atomic structure is key for explaining chemical properties.
• Ionization energy trends relate to atomic structure and electron configuration.
• Mass spectrometry provides vital data on isotopes and molecular masses.