Chemical Formulas and Symbols

A chemical formula is a shorthand way to show the elements in a compound and their proportions. The most important tool here is the subscript, the small number written below the line. A subscript tells you how many atoms of that specific element are in one molecule. If there is no number, it is understood to be $1$. For example, in $CO_2$, there is $1$ Carbon atom and $2$ Oxygen atoms. When we see parentheses, like in $Mg(OH)_2$, the subscript outside applies to everything inside the brackets. This means we have $1$ Magnesium, $2$ Oxygens, and $2$ Hydrogens.

While subscripts tell us what is inside a molecule, coefficients tell us how many molecules we have. A coefficient is the large number placed in front of a formula. To find the total number of atoms, you multiply the coefficient by the subscript of each element. Think of it like a grocery order: if one 'pack' ($H_2O$) has $2$ Hydrogens and $1$ Oxygen, then $3H_2O$ means you have $3$ packs, totaling $6$ Hydrogens and $3$ Oxygens. This is crucial for balancing equations later!

Chemistry doesn't just happen in a vacuum; we need to know the physical state of our ingredients. We use state symbols in parentheses after the formula: (s) for solid, (l) for liquid, and (g) for gas. The most unique symbol is (aq), which stands for aqueous. This means the substance is dissolved in water. For example, $NaCl(s)$ is dry table salt, but $NaCl(aq)$ is salt water. These symbols help scientists predict how fast a reaction will happen or if a precipitate will form.