Atomic and Molecular Masses — Definition

Definition

Imagine you want to know how heavy a single tiny atom is. Atoms are incredibly small, so their masses are also incredibly tiny. To make these masses manageable and comparable, chemists developed the concept of 'atomic mass'. Instead of using grams or kilograms, which would result in extremely small decimal numbers, they introduced a special unit called the 'atomic mass unit' (amu), also denoted as 'u' (unified atomic mass unit).

Initially, scientists tried to assign a mass of 1 to the lightest atom, hydrogen. Later, oxygen was used as a reference. However, the international standard now is based on the carbon-12 isotope. One atomic mass unit (1 amu or 1 u) is defined as exactly one-twelfth (1/12) the mass of a single atom of carbon-12.

This means if a carbon-12 atom weighs 12 amu, then 1 amu is the mass of one 'unit' of that carbon atom. For example, an atom of hydrogen-1 has an atomic mass of approximately 1.008 u, meaning it's slightly heavier than 1/12th of a carbon-12 atom.

Most elements exist as a mixture of isotopes, which are atoms of the same element with different numbers of neutrons and thus different masses. For instance, chlorine exists as two main isotopes: chlorine-35 and chlorine-37.

When we talk about the 'atomic mass' of an element as listed in the periodic table, we are usually referring to the 'average atomic mass'. This is a weighted average of the atomic masses of all its naturally occurring isotopes, taking into account their relative abundances.

So, the average atomic mass of chlorine (approximately 35.45 u) is closer to 35 because chlorine-35 is more abundant than chlorine-37.

Now, what about molecules? A molecule is formed when two or more atoms bond together. The 'molecular mass' is simply the sum of the atomic masses of all the atoms present in that molecule. For example, a water molecule ($H_2O$) has two hydrogen atoms and one oxygen atom. If the atomic mass of hydrogen is 1.008 u and oxygen is 15.999 u, then the molecular mass of water would be $(2 \times 1.008,u) + (1 \times 15.999,u) = 18.015,u$.

For compounds that are ionic, like sodium chloride (table salt, $NaCl$), they don't form discrete molecules but rather a crystal lattice of ions. In such cases, we use the term 'formula mass' instead of molecular mass.

Formula mass is calculated in the same way: by summing the atomic masses of the atoms in the empirical formula (the simplest whole-number ratio of ions). For $NaCl$, it would be the atomic mass of Na plus the atomic mass of Cl.

These concepts are crucial because they allow us to relate the mass of a substance to the number of atoms or molecules it contains, which is fundamental for understanding chemical reactions and quantities.