All matter—anything that has mass and takes up space—is made up of tiny particles called atoms. The term ‘atom’ originated from a Greek word meaning “indivisible” or unable to be divided further. For a long time scientists believed that atoms were the smallest particle of any kind of matter. However, in the late 19th century, it was discovered that atoms themselves were composed of even smaller particles.
Atoms are microscopic – they are too small to be seen, even with the most powerful microscopes – but understanding their composition and structure has come about through experimentation and indirect observation. For example, there are roughly one million atoms within the thickness of a single human hair that cannot be seen without the aid of special technology. Each individual atom is round, like a little ball; it is always in motion, and seems to have edges that are fuzzy, with no distinct beginning or ending points.
To understand atoms on a simple level it is best to observe some form of matter, like the lead inside a pencil. We are then likely to see the following:
Carbon atoms in the pencil’s lead are vibrating in place. Inside each atom there are three distinct particles: protons and neutrons, which are stable and clustered together, and electrons that are always orbiting in motion around them. Proton particles, being located in the dense nucleus, determine what element an atom will be and tend to be heavier than the other particles. The number of protons within the nucleus defines the atomic number, which is unique to each chemical element. If a proton is added or removed from an element, it becomes a different element altogether. The atomic number for any element can be found by looking at a periodic table, indicated by the whole number beside the symbol for that element. For example, being made up of carbon, the lead has six protons, just like every other carbon atom. All protons have a positive +1 charge. This means that they wouldn’t be able to stick together in the nucleus if it weren’t for the neutrons.
Neutrons also share the same space in the nucleus at the centre of the atom and are about the same mass as the protons. They are neutral so they do not have any charge. It seems that the neutrons are needed to hold the protons together in the nucleus by a force termed the nuclear binding force, one of the strongest forces that exist in nature. The nucleus itself takes up a very small portion of the atom. To give a practical idea of its proportion, if the atom was the size of a football field, “the nucleus would be the size of a household fly on the 55 yard line”! If an atom is neutral, it will have the same number of electrons as protons. Thus the carbon atom in the pencil’s lead has six electrons to match the protons.
The third kind of subatomic particles, electrons, are the smallest in size and add virtually nothing to the mass of the atom. Each proton and neutron contained in the nucleus is about 1,837 times more massive than an electron. However, the space occupied by the electrons is much larger than the space taken up by the nucleus. Electrons are responsible for the atom’s fuzzy edge because they are in constant motion in the space around the nucleus. This prevents any distinct outer surface on the atom. Electrons have a negative -1 charge. They are held in orbit around the nucleus by the electromagnetic force (their negative charges are attracted to the positively charged nucleus). However, electrons can be lost, gained, or shared to create bonds with other atoms in chemical reactions. Thus the chemical properties of an element are determined by how its electrons are arranged because electrons are involved in all chemical reactions.
Having said all that, it should be added that scientists now know that protons and neutrons contain even smaller particles called quarks. They have identified six known quarks (Up and Down, Charm and Strange, Top and Bottom) which join together in groups of three to create either positively charged protons or neutral neutrons, depending on the specific combination.