Chemistry - StudyReady

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

Common reaction types

Type	Pattern	Example
Synthesis	`A + B → AB`	`2H₂ + O₂ → 2H₂O`
Decomposition	`AB → A + B`	`2H₂O₂ → 2H₂O + O₂`
Single replacement	`A + BC → AC + B`	`Zn + 2HCl → ZnCl₂ + H₂`
Double replacement	`AB + CD → AD + CB`	`AgNO₃ + NaCl → AgCl + NaNO₃`
Combustion	`Fuel + O₂ → CO₂ + H₂O`	`CH₄ + 2O₂ → CO₂ + 2H₂O`

Equations

How to balance a chemical equation

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

Key formulas

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`

Common Mistakes

Where students go wrong in chemistry

Patterns we see again and again in student work. Each one is fixable once you know to look for it.

Balancing equations by changing subscripts: You balance an equation by adjusting the coefficients in front of each formula, never the subscripts within a formula. Changing H₂O to H₂O₂ makes it a different substance. If you find yourself wanting to change a subscript, the coefficients are wrong somewhere else.
Skipping unit conversions in mole problems: Almost every stoichiometry error comes from a missing step: grams were used where moles were needed, or the molar mass was applied to the wrong substance. Write out the conversion chain (grams → moles → mole ratio → moles → grams) every time, even when it feels obvious.
Losing track of significant figures: Answers with too many digits lose marks in most courses. The rule of thumb: for multiplication and division, match the fewest significant figures in the inputs; for addition and subtraction, match the fewest decimal places. Round at the end, not partway through.
Confusing ionic and covalent bonding: Metal plus nonmetal is usually ionic (electrons transferred); nonmetal plus nonmetal is usually covalent (electrons shared). The distinction determines naming rules, properties, and how you draw the structure, so it is worth checking first on every compound.
Memorizing the periodic table instead of its patterns: You will not be asked to recite it. You will be asked why fluorine is more reactive than iodine, or why atomic radius shrinks across a period. Learn the trends and the reasons behind them (effective nuclear charge, shielding, number of shells), and the specific facts follow.