Structure of Atoms

Elements are pure substances. Elements are composed of a single kind of atom, as opposed to compounds which contain two or more kinds of atoms (see lesson 5). Some examples of elements are: gold, silver, aluminum, iron, oxygen gas, sulfur, etc. There are over 100 known pure elements found here on Earth.

Atom means "indivisible" in Latin. Atoms are the smallest known particles which have all the properties of an element. In other words, elements can be separated as small as their individual atoms, but no farther. The next logical question becomes, "What are atoms made of?" We will answer that in a moment.

It is a fact that no human has ever laid eyes upon a single atom. This is because they are too small to be detected by vision, no matter how many times they are magnified. In fact, atoms are so tiny that they can not even be seen by electron microscopes which are the most powerful microscopes in the world. How do we know they even exist, and how do we know what they look like? The answer to these questions is that we do not know exactly what atoms look like, but, by using advanced mathematics and physics, scientists have been able to deduce their existence and structure. We represent atoms by drawing them as simple structures, as seen below in this animation of a hydrogen atom.

To answer the question "What are atoms made of?", atoms are composed of subatomic particles. There are three subatomic particles that we will learn about. The three subatomic particles are the proton, the neutron, and the electron (see animation above).

Protons are positively charged particles which are always found in the nucleus of the atom. The number of protons in an atom is called the atomic number. Protons have a mass of 1 AMU. AMU stands for Atomic Mass Unit, which is the unit by which we measure the masses of atoms and molecules. We use AMU's because atoms are far too small to be measured by metric units such as grams.

Neutrons have no charge, hence the name neutron means neutral, or no charge. The total number of protons and neutrons is called the atomic mass, or atomic mass number. Neutrons, like protons, have a mass of 1 AMU. Neutrons are actually slightly larger in mass than protons, but we assume for simplicity's sake that they also have a mass of 1 AMU. Neutrons, like protons, are found only in the nucleus of atoms.

Electrons are negatively charged particles which orbit the nucleus and have a mass of about 1/2000th that of a proton. For purposes of simplicity, we assign electrons a mass value of 0 AMU's.