An atom is made of 5 protons, 6 neutrons, and 7 electrons. Another atom is made of 6 protons, 5 neutrons, and 3 electrons. What can you say about these two atoms?

Explanation: Atoms are the building blocks of all matter. They consist of three sub-atomic particles: protons, neutrons and electrons. Protons and neutrons are found in the nucleus of an atom. Electrons are found in energy levels around the nucleus as shown in the diagram representing a carbon atom with 6 protons, 6 neutrons and 6 electrons.

Sub-atomic particles

Particle Whereabouts in atom Relative mass Charge

Proton Nucleus 1 + 1

Neutron Nucleus 1 0

Electron Outside the nucleus 1/1840 - 1

Diagram of carbon atom - nucleus with electrons orbiting

Diagram of carbon atom - nucleus with electrons orbiting

In chemistry we are particularly interested in electrons. As you will see later, this is because chemical reactions involve the rearrangement of electrons. Nuclei of atoms (protons and neutrons) usually remain unchanged (except in radioactive decay).

Electrons are arranged in atoms according to their energies. This is called the electronic structure or electronic configuration of the atom. A crude but still useful model says the electrons can be in different energy levels. Electrons in a particular energy level all have the same energy as one another. The lowest energy level can accommodate up to 2 electrons. The second level can accommodate up to 8 electrons. The third level can accommodate up to 18 electrons. The diagram shows the situation for a sulfur atom.

Electronic configuration of sulfur including energy level diagram