Chemistry

Periodic table basics, equation balancing steps, reaction types, and the formulas that come up most in intro chemistry.

Elements

Periodic table basics

Concept	Meaning
Atomic number	Number of protons in an atom's nucleus - defines the element
Atomic mass	Average mass of an atom's protons + neutrons, weighted by isotope abundance
Group (column)	Elements with the same number of valence electrons and similar chemical behavior
Period (row)	Elements with the same number of electron shells
Valence electrons	Electrons in the outermost shell - determine how an atom bonds
Metals / Nonmetals / Metalloids	Metals (left side) lose electrons easily; nonmetals (right side) gain them; metalloids sit on the border and share traits of both

Group 1 (alkali metals) and Group 17 (halogens) are the most reactive; Group 18 (noble gases) is essentially inert because its outer shell is full.

Reactions

Equations

1. Count atoms on each side: List how many of each element appear in the reactants and products before touching any numbers.
2. Balance one element at a time: Add coefficients (never change subscripts) starting with the element that appears in the fewest compounds.
3. Balance hydrogen and oxygen last: They usually appear in the most compounds, so save them for the end after other elements are balanced.
4. Double-check the totals: Recount every atom on both sides - the equation is only balanced when every element matches exactly.

Reference

Concept	Formula
Moles	`n = mass / molar mass`
Molarity	`M = moles of solute / liters of solution`
Ideal gas law	`PV = nRT`
pH	`pH = -log[H⁺]`
Density	`D = mass / volume`