Copper is a metal, which means that among its properties is that of being a good conductor of heat and electricity. Although elements can vary widely in their properties, each element has a very similar structure built using different combinations of the same three subatomic particles: positively charged protons, uncharged neutrons, and negatively charged electrons. A copper atom is simply a combination of these particles comprising exactly 29 protons. To understand the structure of a copper atom, you must first understand the properties of each of the three subatomic particles. At the center of every atom is a nucleus made up of protons and neutrons. Protons and neutrons have a very similar mass. The mass of these particles is measured in atomic mass units, which makes them very easy to express; Protons and neutrons each have a relative mass of about 1 atomic mass unit. The difference between these two particles is in their charge. Protons have a relative charge of +1, while neutrons have a relative charge of 0. Electrons therefore surround the nucleus at different energy levels. Electrons have a relative charge of -1, but their relative mass is much smaller than that of protons or neutrons. They have a relative mass of about 1/1836 atomic mass unit. The number of protons in an atom defines its element; only atoms with 29 protons are copper atoms, and all atoms with 29 protons are copper atoms. However, there can be isotopes of an element, which are atoms with a different number of neutrons. Copper has two stable isotopes: 63Cu and 65Cu. The superscript before the element in an isotope is the isotope's mass number, which indicates the combined number of protons and neutrons in the atom. A 63Cu atom has a... half-paper... However, due to the preference for half- or full-whole sublevels, an electron from the 4s sublevel is instead placed in the 3d sublevel, giving the configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s1. Here, the 3d subshell is completely full while the 4s subshell is half full, making this configuration more stable. Finally, atoms can be paramagnetic or diamagnetic. Paramagnetic atoms are attracted by a magnetic field, while diamagnetic atoms are repelled by a magnetic field. Atoms are paramagnetic if they have orbitals that contain only one electron, called unpaired electrons. Atoms are diamagnetic if no orbital is occupied by a single electron, meaning that all of their electrons are paired. Because copper's 4s orbital contains only one electron, copper is paramagnetic, meaning it is attracted to magnetic fields.