Atoms to Molecules

What is your brain made of?

Everything you can see, touch, smell, feel, and taste is made of atoms. Atoms are the basic building-block of all matter (including you and me, and everyone else you’ll ever meet), so if we want to know about what Earth is made of, then we have to know a few things about these incredibly small objects.


Everyday experience should convince you that matter is found in myriad forms, yet all the matter you have ever seen is made of atoms, or atoms stuck together in configurations of dizzying complexity. A chemical element is a substance that cannot be made into a simpler form by ordinary chemical means. The smallest unit of a chemical element is an atom , and all atoms of a particular element are identical.

Parts of an Atom

There are two parts to an atom ( Figure below ):

  • At the center of an atom is a nucleus made up of two types of particles called protons and neutrons.
    • Protons have a positive electrical charge. The number of protons in the nucleus determines what element the atom is.
    • Neutrons are about the size of protons but have no charge.
  • Electrons , much smaller than protons or neutrons, have a negative electrical charge, move at nearly the speed of light, and orbit the nucleus at exact distances, depending on their energy.

Diagram of an atom

Major parts of an atom. What chemical element is this? (Hint: 3 protons, 3 electrons)

Atomic Mass

Because electrons are minuscule compared with protons and neutrons, the number of protons plus neutrons gives the atom its atomic mass . All atoms of a given element always have the same number of protons, but may differ in the number of neutrons found in the nucleus.


Atoms are stable when they have a full outermost electron energy level. To fill its outermost shell, an atom will give, take, or share electrons. When an atom either gains or loses electrons, this creates an ion . Ions have either a positive or a negative electrical charge. What is the charge of an ion if the atom loses an electron? An atom with the same number of protons and electrons has no overall charge, so if an atom loses the negatively charged electron, it will have more protons, therefore, a positive charge. Ions with a positive charge are referred to as a cation.What is the charge of an ion if the atom gains an electron? If the atom gains an electron, there will be more electrons and will have a negative charge. Anions are ions that have a negative charge.


In the previous section we said that many atoms are more stable when they have a net charge: they are more stable as ions. When a cation gets close to an anion, they link up because of their different net charges — positive charges attract negative charges and vice versa. When two or more atoms link up, they create a molecule . A molecule of water is made of two atoms of hydrogen (H) and one atom of oxygen (O). The molecular mass is the sum of the masses of all the atoms in the molecule. A collection of molecules is called a compound.


  • An atom has negatively-charged electrons in orbit around its nucleus, which is composed of positively-charged protons and neutrons, which have no charge.
  • An atom that gains or loses electrons is an ion. Positively charged ions are cations, negatively charged ions are anions.