Most of the elements important in biology need eight electrons in their outermost shell in order to be stable, and this rule of thumb is known as the octet rule. In general, atoms are most stable, least reactive, when their outermost electron shell is full. This outermost shell is known as the valence shell, and the electrons found in it are called valence electrons. The number of electrons in the outermost shell of a particular atom determines its reactivity, or tendency to form chemical bonds with other atoms.
The shell closest to the nucleus, 1n, can hold two electrons, while the next shell, 2n, can hold eight, and the third shell, 3n, can hold up to eighteen. Thus, the electron shells of an atom are populated from the inside out, with electrons filling up the low-energy shells closer to the nucleus before they move into the higher-energy shells further out. Image credit: modified from OpenStax BiologyĪtoms, like other things governed by the laws of physics, tend to take on the lowest-energy, most stable configuration they can.
Energy must be added to move an electron outward to a higher energy level, and energy is released when an electron falls down from a higher energy level to a closer-in one. For instance, if an electron absorbs energy from a photon, it may become excited and move to a higher-energy shell conversely, when an excited electron drops back down to a lower-energy shell, it will release energy, often in the form of heat.īohr model of an atom, showing energy levels as concentric circles surrounding the nucleus. In order to move between shells, an electron must absorb or release an amount of energy corresponding exactly to the difference in energy between the shells. By convention, each shell is assigned a number and the symbol n-for example, the electron shell closest to the nucleus is called 1n. Each electron shell has a different energy level, with those shells closest to the nucleus being lower in energy than those farther from the nucleus. The Bohr model shows the atom as a central nucleus containing protons and neutrons, with the electrons in circular electron shells at specific distances from the nucleus, similar to planets orbiting around the sun.
An element can donate, accept, or share electrons with other elements to fill its outer shell and satisfy the octet rule.Īn early model of the atom was developed in 1913 by the Danish scientist Niels Bohr (1885–1962). Because the outermost shells of the elements with low atomic numbers (up to calcium, with atomic number 20) can hold eight electrons, this is referred to as the octet rule. To achieve greater stability, atoms will tend to completely fill their outer shells and will bond with other elements to accomplish this goal by sharing electrons, accepting electrons from another atom, or donating electrons to another atom. Because of these vacancies in the outermost shells, we see the formation of chemical bonds, or interactions between two or more of the same or different elements that result in the formation of molecules. Not all elements have enough electrons to fill their outermost shells, but an atom is at its most stable when all of the electron positions in the outermost shell are filled.