Unit 1: Topic 3 - Elemental Composition of Pure Substances

Question 1

What is a molecule?

Answer 1

A molecule is the smallest possible particle of a given pure substance that maintains its chemical
properties. Molecules are composed of atoms connected by covalent bonds in a consistent pattern
(any two molecules of the same compound always have the same structure). Molecules are
represented by chemical formulas, which show the number of each type of atom but not how they are
connected.

For example, water is composed of molecules of H2O. Each molecule has two hydrogens and one
oxygen. Here is a picture of the water’s structure.

The bonds aren’t evident in this visualization, but the two hydrogen molecules in white are covalently bonded to the central red oxygen.

Question 2

What is a formula unit?

Answer 2

Formula unit is the term for the smallest group of atoms or ions in a pure substance that isn’t bonded
covalently. Ionic compounds are composed of formula units, not molecules since they are connected
by ionic bonds instead of covalent ones.

The name “formula unit” comes from the fact that it is defined as the smallest group of ions from the
chemical formula of that compound. For example, one formula unit of NaCl is composed of one Na+
ion and one Cl- ion, not two or three of each. Here is an image of a NaCl formula for reference.

Note how the ions are not shown as being connected in the same way as atoms in a molecule are; this is to differentiate ionic bonding from covalent and formula units from molecules

Question 3

What law describes the composition of pure compounds?

Answer 3

The Law of Definite Proportions states that a given pure compound always contains the same elements in the exact same proportions by mass. This law is leveraged in identifying compounds based on the mass percents of the elements they contain.

For instance, a molecule of carbon monoxide (CO) will always have one carbon atom and one oxygen atom. Carbon dioxide (CO2), however, will always have one carbon atom and two oxygen atoms, thus making it a different compound with different properties.

Question 4

What is a molecular formula?

Answer 4

The molecular formula of a compound is the total number of each atom in a molecule of that substance. It is a type of chemical formula and is written as a series of atomic symbols with subscripts to indicate the name and number of atoms of each element.

For example, H2O is the molecular formula of water (the “2” should be a subscript), as each water molecule includes two hydrogen atoms and one oxygen.

Question 5

What is an empirical formula, and how does it differ from a molecular formula?

Answer 5

The empirical formula of a compound is the smallest possible whole number ratio of each type of atom in a molecule. An empirical formula can be determined just by comparing the mass percent of each element in a sample of a pure substance. While empirical formulas are written in the same format as molecule formulas, they are not always the same formula for the same compound.

In many cases, especially in smaller molecules, the molecular formula and empirical formula are the same (Ex. CO2 is both the empirical and molecular formula of carbon dioxide since the 2:1 carbon-to-oxygen ratio cannot be reduced further). However, they can be different, like in glucose (a simple sugar). Glucose’s chemical formula is C6H12O6, which means its empirical formula is CH2O.