Chemistry counts particles you can't see by weighing them. This unit builds that bridge (the mole), shows how to read what atoms are made of from mass spectra and photoelectron spectra, and explains the periodic table's trends with one idea: Coulomb's law between the nucleus and its electrons.
You can't count particles directly, so chemists use the mole: 6.022 × 10²³ particles. The average mass of one particle in amu equals the molar mass in grams per mole, so n = m/M connects mass to number of particles.
A mass spectrum of one element shows its isotopes (by mass) and their relative abundance (by peak height). The average atomic mass is the abundance-weighted average of the isotope masses.
A pure compound always has the same ratio of element masses (law of definite proportions). The empirical formula is the lowest whole-number ratio of atoms, found by converting mass percentages to moles.
Pure substances contain one type of particle; mixtures contain two or more in proportions that can vary. Elemental analysis shows the relative numbers of atoms and can reveal whether a sample is pure.
An atom has a positive nucleus of protons and neutrons and negative electrons in shells and subshells, written as a ground-state electron configuration using the Aufbau principle. Coulomb's law explains how strongly the nucleus holds each electron.
Photoelectron spectroscopy (PES) measures how much energy it takes to remove electrons from each subshell. Each peak is a subshell; its position is the binding energy and its height is (ideally) proportional to the number of electrons in it.
Ionization energy, atomic and ionic radius, electron affinity and electronegativity follow periodic trends that come from Coulomb's law, the shell model, shielding and effective nuclear charge. Use them to predict values you don't have.
Whether two elements bond depends on their valence electrons and nuclei. Elements in the same group form analogous compounds, and an element's typical ionic charge is predicted by its valence electrons and place on the periodic table.