CC3 Atomic structure mind map

Models

• Early ideas: Dalton spheres → Thomson electron / plum pudding
• Alpha scattering: Mostly passes through; rare strong deflections
• Nuclear model: Mostly empty; tiny dense positive nucleus
• Scale: Atom ≈ 10⁻¹⁰ m; nucleus much smaller; drawings not to scale

Particles

• Charges / mass: Proton +1 / 1; neutron 0 / 1; electron −1 / tiny mass
• Neutral atoms: Electrons = protons; atomic number Z = protons
• Mass number A: Protons + neutrons; ²³₁₁Na: 11 p, 12 n, 11 e
• Calculate: Neutrons = A − Z; neutral electrons = Z
• Ions: Lose electrons: positive; gain: negative; protons unchanged
• Example: 13 protons + 14 neutrons → aluminium-27

Isotopes

• Definition: Same element / protons; different neutrons / mass numbers
• Chemistry: Same electron arrangement → very similar reactions
• Differences: Mass affects physical properties; not all are radioactive
• Distinction: Isotopes differ in neutrons; ions differ in electrons

Relative mass

• Aᵣ: Weighted mean relative to ¹²C standard; no unit
• Percent method: Σ(isotope mass × percentage) ÷ 100
• Chlorine: 75% ³⁵Cl + 25% ³⁷Cl → Aᵣ = 35.5
• Ratio method: Weighted total ÷ total abundance; 3:1 → divide by 4
• Check: Between isotope masses; nearer more abundant isotope

Connections

• Particles → Isotopes: Neutron number distinguishes isotopes of one element.
• Isotopes → Relative mass: Isotope abundances determine the weighted mean.