HS Chemistry - Essentials

Atoms & Molecules

Overview of The Page

This page will cover:

• What are atoms?
• What are molecules?

An atom is the smallest particle of an element that still retains that element's properties. Atoms are the essential particles that make up everything. The Periodic Table lists the 118 known elements, with their atomic numbers, masses, and other information about them.

Atoms are made up of protons, neutrons, and electrons. The protons and neutrons reside in the center of the atom, the nucleus $which is why they are also sometimes called **nucleons**$, while the electrons orbit the atom in their shells. Protons have a positive charge of +1, neutrons have a neutral charge $no charge$, and electrons have a negative charge of -1. This helps determine a few properties of the atoms:

• Atomic Number/Proton Number: The number of protons in the nucleus

• The atom's charge: The number of electrons subtracted from the number of protons. In a neutral atom, this number is 0.

• Atomic mass: The number of nucleons $protons \+ neutrons$ in the nucleus

Atomic mass is measured in atomic mass units $amu$. 1 amu is defined as 1/12th the mass of a Carbon-12 atom $a Carbon atom with 6 protons and 6 neutrons$. When calculating atomic mass:

• Each proton/neutron has a mass of 1 amu.

• Electrons have a mass of 0 amu.

The atomic mass of the element often appears as a decimal on the Periodic Table. However, this does not indicate that the atom contains a fraction of a proton or neutron. As covered further in Isotopes and Ions, the atomic mass shown on the Periodic Table is the average from all the isotopes of that element.

However, despite there being 118 different elements. there are many more than 118 different substances in the world. A molecule is the smallest particle of a compound that still retains that substance's properties. Molecules are made of many different atoms, and molecules can range in size from a few atoms to thousands of atoms. However, the individual atoms themselves do not contain the substance's properties; it is the molecule that does.

As an example, consider a piece of plastic. If you were to cut it down until you reached the smallest piece of plastic possible, you would have a molecule of plastic. This molecule of plastic is made of Carbon atoms, Hydrogen atoms, and probably other atoms as well, but these individual atoms are not pieces of plastic. The atoms make up the molecule of plastic, but the molecule is the smallest piece of plastic you can have on hand.

However, atoms do not just bond with each other to form molecules. As covered in Unit 3, there are multiple ways in which atoms bond with each other.

One of these different ways in which atoms bond is ionic bonding. In ionic bonding, there are no separate distinct molecules, as all the ions are attracted to each other. Therefore, ionic compounds do not consist of molecules, but of formula units, where a formula unit is the smallest group of ions that still retains the properties of the ionic compound.

As an example, consider a piece of table salt $NaCl$. It consists of multiple Na⁺ and Cl^- ions. If you were to cut it down until you reached the smallest piece of table salt possible, you would have a formula unit of table salt $NaCl$. It wouldn't be right to call it a molecule, as this formula unit of table salt was bonded to multiple other formula units of table salt $before it was cut down$. However, it is the smallest piece of table salt you can have on hand, and thus a single group of NaCl ions is the formula unit of NaCl.