Parts of an Atom

We once thought that atoms are indivisible, but we now know that they are made of three subatomic particles: the proton, the neutron, and the electron. This activity helps to reinforce the location, mass, and charge of these particles and the meaning of two key vocabulary terms: atomic number and mass number. Students will identify each part of an atom and describe how to find their atomic and mass numbers.

• The proton and neutron are found in the nucleus of the atom. The proton has a relative mass of 1 amu and a relative charge of +1. The neutron has a relative mass of 1 amu and a neutral charge. Most of the mass of the atom is found in the small, dense nucleus because these two particles are the more massive of the three.

• The electron, by comparison, has a mass almost 2000 times less than a proton or neutron. That means that it would take around 2000 electrons to equal the mass of one proton. Because the biggest atoms we know of have only 118 electrons, the electron contributes almost nothing to the atom in terms of mass. Each does, however, have a relative charge of -1.

• The atomic number is defined as the number of protons in an atom. Because atoms are electrically neutral, the atomic number also tells us how many electrons are in a neutral atom.

• The mass number is simply the mass of the atom. Since the protons and neutrons contribute the mass to the atom, the mass number tells us the number of protons and neutrons combined.