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The simplest particle of an element that retains all of the properties of that element is an atom. The properties of different kinds of atoms determine the structure and properties of the matter they compose. Atoms are so small that their structure cannot be directly observed. However, scientists have developed models that describe the structure of the atom. One model is shown in Figure 2-2.

The Nucleus

The central region, or nucleus, makes up the bulk of the mass of the atom and consists of two kinds of subatomic particles, a proton and a neutron. The proton is positively charged and the neutron has no charge. The number of protons in an atom is called the atomic number of the element. In the periodic table of elements, the atomic number generally appears directly above the chemical symbol, as shown in Figure 2-1. The atomic number of fluorine is 9, which indicates that each atom of the element fluorine has nine protons. The mass number of an atom is equal to the total number of protons and neutrons of the atom. The mass number of fluorine is 19, which indicates that each atom of fluorine has 10 neutrons.


In an atom, the number of positively charged protons is balanced by an equal number of small, negatively charged particles called electrons. The net electrical charge of an atom is zero. Electrons are high-energy particles that have very little mass. They move about the nucleus at very high speeds and are located in orbitals. An orbital is a three-dimensional region around a nucleus that indicates the probable location of an electron. Electrons in orbitals that are farther away from the nucleus have greater energy than electrons that are in orbitals closer to the nucleus. When all orbitals are combined, there is a cloud of electrons surrounding the nucleus, as shown in Figure 2-2.

Orbitals correspond to specific energy levels. Each energy level corresponds to a group of orbitals that can hold only a certain, total number of electrons. For example, the orbital that corresponds to the first energy level can hold only two electrons. The first energy level is the highest energy level for the elements hydrogen and helium. There are four orbitals in the second energy level, and that energy level can hold up to eight total electrons, with a maximum of two electrons in each orbital.


All atoms of an element have the same number of protons. However, all atoms of an element do not necessarily have the same number of neutrons. Atoms of the same element that have a different number of neutrons are called isotopes. Additional neutrons change the mass of the element. Most elements are made up of a mixture of isotopes, as shown in Figure 2-2. The average atomic mass of an element takes into account the relative amounts of each isotope in the element, and this average is the mass found in the periodic table.

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