In a physical science classroom, matter is anything that has mass and takes up space. All matter is made up of tiny particles called atoms, which are classified in a chart called the periodic table of the elements. Every element has a unique atom. Sometimes, atoms combine to make new substances. These combined atoms are called molecules.
Because of the size of atoms, their existence was a matter of conjecture for a very long time. In the early twentieth century, Dutch scientist Neils Bohr proposed a model for the structure of atoms that, though too simple for advanced purposes, is still a workable model today for simple questions about atomic structure.
Parts of an Atom
An atom has three different kinds of particles: protons, neutrons and electrons. Protons and neutrons are found in the center, or nucleus, of an atom. Both of these particles have significant mass. Protons have a positive electrical charge, and neutrons, as the name suggests, have a neutral electrical charge. Outside the nucleus is where electrons can be found. Electrons have a negative electrical charge and a negligible amount of mass.
The Bohr Model
In his model, Bohr demonstrated that electrons travel around the outside of the nucleus in paths called orbits. Rather than a random arrangement, Bohr postulated that electrons had differing levels of energy, which determined how far from the nucleus they would be; the greater the energy, the further from the nucleus.
Atoms have never literally been seen by humans. Our eyes use visible light to see, and an atom is much smaller than the wavelength of visible light that is reflected from objects that we can see. An apparatus called an electron microscope, which uses reflected electrons to create an adequate image, has been used since the 1930s.
Because each element has a unique atomic structure, no other element has exactly the same number of protons, neutrons and electrons. Gold's atomic number is 79, which corresponds to the number of protons in the nucleus of an atom of gold. There are 117 neutrons in the nucleus. Gold's 79 electrons exist at six different energy levels. From lowest to highest energy level, the numbers of electrons are 2, 8, 18, 32, 18 and 1.