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2.4 Chemical Formulas - compounds are composed of elements whose ratios are small, whole number ratios. These formulas can be depicted as molecular formulas, structural formulas, and empirical formulas.

Molecular formulas - contain the actual number of elements found in the compound and written with subscripted numbers after an element's symbol to denote the number of atoms of that element . . . . typically, the number is a 1, 2, 3 or 4 (the "1" is not written, but implied). For example, NaOH has 1 Na atom, 1 O atom and 1 H atom. Na₂O₂ has 2 Na atoms and 2 O atoms.

In Chapter 4, you will write balanced chemical equations where you must adhere to the Law of Conservation of Matter. Every element present at the beginning of a reaction must be present at the end of a reaction AND in the same amount. When electricity is passed through water, the decomposition products are hydrogen and oxygen . . .

H₂O  electricity  H₂ + O₂

To "balance the reaction", add a . . . a 2 or larger number in front of a chemical formula.

Currently, H₂O, H₂ and O₂ have coefficients of 1 (which is "understood", but not written). in front of a compound. Adding a 2 in front of H₂O and in front of H₂ balances this reaction.

2 H₂O  electricity  2 H₂ + O₂
4 H atoms  +  2 O atoms  electricity  4 H atoms  +  2 O atoms      

If a molecular formula contains a polyatomic ion (i.e. NH₄⁺, NO₃^-, SO₄^2-, PO₄^3-, etc.) that is present more than one time, it is encased in parentheses with a subscripted number after the closing parentheses. For example, (NH₄)₂SO₄ is how chemists write NH₄NH₄SO₄.

Some compounds contain attached water molecules as part of their structure. These hydrates don't "look" wet, but they lose water mass when heated. The material left after heating is called the anhydrous salt. CuSO₄ is the anhydrous salt that remains after the hydrate (CuSO₄•5H₂O) is heated.

Structural formulas - provide information on how atoms are attached. Consider methane gas (CH₄), its molecular formula gives no information about how the four Hydrogen atoms and one Carbon atom are attached. The structural formula shows that the Carbon is the central atom and it is bonded to the Hydrogen atoms. There are three ways to show the structure of a molecule: line-bond, ball-and-stick, and space-filling. The lines in the line-bond model represent a bond (2 electrons) . . . ball-and-stick models show geometric arrangements . . . . space-filling models show the geometry, atomic sizes and volume of the molecule.

Empirical formulas - the simplest whole-number ratio of the number of atoms. The molecular formula of hydrogen peroxide is H₂O₂ . . . the empirical formula is HO. Empirical formulas are important because mathematical calculations give us this ratio . . . calculations you will perform in homework and lab assignments. A step-by-step guide to calculating empirical formulas will be introduced in Section 3.2. Also in Section 3.2, you will discover that the molecular formula (i.e. the actual formula) can be determined from the empirical formula, empirical mass and molar mass.