Atomic Structure and Ions

Overview

This lecture covers atomic structure, calculation of relative masses, isotopes, important ions and their formulae, and how to write ionic equations.

Atoms are made up of protons (nucleus, +1 charge), neutrons (nucleus, 0 charge), and electrons (orbitals, -1 charge, very small mass).
Atomic number (Z) is the number of protons; mass number (A) is protons plus neutrons.
Number of neutrons = mass number (A) - atomic number (Z).

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Isotopes have similar chemical properties but may have different physical properties due to different masses.

Relative atomic mass (R.A.M.) is the weighted mean mass of atoms compared to one twelfth of carbon-12.
R.A.M. is calculated using isotopic mass and percentage (or relative) abundance.
Relative isotopic mass is the mass of an isotope compared to one twelfth of carbon-12.
Relative molecular mass is the average mass of a molecule compared to one twelfth of carbon-12.

Use the formula: R.A.M. = (isotopic mass × abundance) / total abundance or 100.
Example: R.A.M. of Mg = [(78.7 × 24) + (10.13 × 25) + (11.17 × 26)] / 100 = 24.3.
Abundance problems can be solved using simultaneous equations.

Different isotopes produce distinct mass spectra patterns (e.g., Cl and Br diatomic molecules).
The probability of combinations affects peak abundance in the spectra.

Common ions have fixed charges depending on their group (e.g., group 1 = +1, group 2 = +2).
To form neutral compounds, balance total positive and negative charges.
Use brackets for compound ions appearing more than once (e.g., Ca(OH)₂).

Ionic equations show only ions involved in the reaction, omitting spectator ions.
Spectator ions are those not changing state or oxidation number.
Example: Pb²⁺(aq) + 2Cl⁻(aq) → PbCl₂(s) after canceling spectator ions.

Proton — subatomic particle, +1 charge, mass 1, located in nucleus.
Neutron — subatomic particle, 0 charge, mass 1, located in nucleus.
Electron — subatomic particle, -1 charge, 1/1800 mass, in orbitals.
Isotope — atoms with the same number of protons, different neutrons.
Relative atomic mass (R.A.M.) — weighted mean atom mass compared to one twelfth of carbon-12.
Relative isotopic mass — mass of one isotope compared to one twelfth of carbon-12.
Relative molecular mass — average mass of a molecule compared to one twelfth of carbon-12.
Spectator ion — an ion not involved in the main chemical change.