Introduction to Elements

An element is a pure substance that cannot be separated into simpler substances. This means an element is a substance made entirely of one type of atom. For example, every atom in a piece of copper is the same. Scientists have identified 90 elements that occur naturally in our universe. Some elements you might recognize include oxygen, iron, and calcium. There are also over 20 man-made elements, such as einsteinium. Man-made elements are radioactive and unstable and decay over time into lighter elements.

Most living and nonliving things are made up of a combination of elements. Elements chemically combine in a variety of ways to form compounds. A compound is made of two or more different elements bonded together in an exact ratio according to their masses. For example, water is a compound made of hydrogen and oxygen with a mass ratio of 1 to 8. However, if the mass ratio of hydrogen to oxygen is 1 to 16, the compound formed would be hydrogen peroxide. While water and hydrogen peroxide are both made of hydrogen and oxygen, they are different compounds with distinct properties.

Although atoms are amazingly small, scientists have discovered they are made up of even smaller particles - protons, neutrons, and electrons. The nucleus of the atom contains neutrons and positively charged protons. Neutrons have no charge, but are thought to help to keep the nucleus together. The number of protons in the nucleus of an atom is the atomic number of that atom. All atoms of an element have the same atomic number. For example, all atoms of the oxygen element have 8 protons in their nucleus. Therefore, oxygen has an atomic number of 8. The atomic number represents the most important property of an element.

Each element has a limited number of naturally occurring isotopes. Isotopes are different forms of the same atom. An isotope of an element has the same number of protons but different numbers of neutrons. Because atomic mass is determined by the number of protons and neutrons in an atom, isotopes of the same element have different masses. For example, while the most common isotope of oxygen has 8 neutrons, other isotopes of oxygen have 9 or 10 neutrons. While some isotopes are stable, others are radioactive and unstable. They decay to form other isotopes by releasing radiation in the form of particles or electromagnetic waves.

Scientists express the mass of an atom in atomic mass units (amu). The mass of a proton, the positively charged particle in the nucleus, is about 1 amu. The mass of a neutron, the particle in the nucleus with no charge, although a bit more massive than a proton is still considered to be 1amu. Each isotope of an element is identified by its mass number. The mass number is the sum of the number of protons and neutrons in an atom. The mass of an electron is so small that it is not used in calculating atomic mass. Because most elements contain a mixture of two or more isotopes, the atomic mass of an element is the weighted average of the masses of all the naturally occurring isotopes of that element.

The specific way the electrons are arranged in an atom is called the electron configuration. Electrons play an important role in how elements interact with each other and form compounds. Electrons are negatively charged particles that surround the nucleus in the form of a cloud. The atomic number of an atom, which is equivalent to its number of protons, also represents the number of electrons in that atom. The electrons are distributed among orbital shells or energy levels (1, 2, 3 and so on) that are different distances from the nucleus. The larger the number of the energy level, the farther it is from the nucleus. Electrons that are in the highest or outmost energy level are called valence electrons. The valence electrons are the ones that are lost, gained or shared during chemical bonding. In the electron configuration for oxygen which has a total of 8 electrons, the first orbital or energy level closest to the nucleus is filled with two electrons. The second energy level can hold up to eight electrons. It begins to fill with the remaining six electrons when the first level is full.

The outermost orbital shell, called the valence shell, is most often involved in chemical bonding. Oxygens valence shell has six electrons but it can hold up to eight electrons. Therefore, oxygen will often combine with other elements that allow it to share or lose two electrons. Atoms with four or more electrons in the valence shell like to gain electrons to fill the shell. Atoms with three or less electrons in their valence shell like to lose electrons to reveal the full inner shell. Atoms with full valence shells will not combine with other elements.