Key Points

In any chemical reaction, mass is conserved, meaning it is neither created nor destroyed. The total mass of the reactants always equals the total mass of the products.

A chemical compound always contains the same elements combined in a fixed ratio by mass, regardless of its source. For example, water ($H_2O$) always has a hydrogen to oxygen mass ratio of 1:8.

John Dalton proposed that all matter is made of tiny, indivisible particles called atoms. He stated that atoms of a given element are identical and that they rearrange, but are not created or destroyed, in chemical reactions.

An atom is the smallest particle of an element. A molecule is an electrically neutral group of two or more atoms chemically bonded together that can exist independently and retains the properties of the substance.

A covalent bond is formed when two atoms share one or more pairs of electrons to achieve a stable electron configuration. This type of bond is common between non-metal atoms.

Bonds can be single (one shared pair, e.g., $H-H$ in $H_2$), double (two shared pairs, e.g., $O=O$ in $O_2$), or triple (three shared pairs, e.g., in $N_2$).

An ionic bond forms from the electrostatic attraction between oppositely charged ions, which are created by the complete transfer of one or more electrons from a metal to a non-metal.

Cations are positively charged ions formed when an atom loses electrons (e.g., sodium ion, $Na^+$). Anions are negatively charged ions formed when an atom gains electrons (e.g., chloride ion, $Cl^-$).

The chemical formula of a compound is determined using the 'criss-cross' method with the valencies of the elements or charges of the ions. For example, for calcium ($Ca^{2+}$) and chloride ($Cl^-$), the formula is $CaCl_2$.

Polyatomic ions are charged groups of covalently bonded atoms, such as sulfate ($SO_4^{2-}$) or ammonium ($NH_4^+$). Use parentheses in formulas if more than one is present, for example, $Ca(NO_3)_2$.

To name binary covalent compounds, use prefixes (mono-, di-, tri-, etc.) to indicate the number of atoms. The second element's name ends in '-ide'. For example, $CO_2$ is carbon dioxide.

To name ionic compounds, state the cation (metal) name first, followed by the anion (non-metal) name. The anion's name typically ends in '-ide'. For example, $NaCl$ is sodium chloride.

Ionic compounds generally have high melting points, are soluble in water, and conduct electricity only when molten or dissolved in water, as their ions are then free to move.

Covalent compounds typically have low melting and boiling points, are often insoluble in water, and do not conduct electricity because they do not contain free-moving charged particles.

The molecular mass of a covalent substance is the sum of the atomic masses of all atoms in one molecule. For water ($H_2O$), the molecular mass is $(2 \times 1 \text{ u}) + (1 \times 16 \text{ u}) = 18 \text{ u}$.

Formula unit mass is used for ionic compounds and is the sum of the atomic masses of all atoms in a formula unit. For sodium chloride ($NaCl$), it is $(1 \times 23 \text{ u}) + (1 \times 35.5 \text{ u}) = 58.5 \text{ u}$.