III.

Electrons in Atoms

An atom consists of neutrons and protons packed into a dense nucleus with electrons orbiting around the nucleus. The neutrons have no electrical charge, while each proton carries a positive charge that is equal and opposite to the negative charge of the electron. Each chemical element is defined by the number of protons in the nucleus of its atoms; this number is the element’s atomic number. The electrons are equal in number to the protons in the atom, balancing the electrical charge of the nucleus. In other words, the atom’s net charge is zero, and the atom is said to be neutral.

A.

Electron Orbitals

Scientists cannot simultaneously measure both the exact location of an electron and its precise speed and direction, so they cannot measure the path a specific electron takes as it orbits the nucleus. The law of physics governing this phenomenon is called the uncertainty principle. Scientists can, however, determine the area an electron will probably occupy, and the probability of finding the electron at some place inside this area. A map of this area and its probabilities forms a cloudlike pattern known as an orbital. Each orbital can contain two electrons, but these electrons can not have identical properties, so they must spin in opposite directions. Orbitals are grouped into shells, like the layers of an onion, around the nucleus. Each shell can contain a limited number of orbitals, which means that each shell can contain a limited number of electrons. Each shell corresponds to a certain level of energy, and all the electrons in the shell have this same level of energy. As the shells get farther from the nucleus, they can contain more electrons, and the electrons in the shells have higher energy. See also Chemistry: Electron Cloud.

B.

Electrons and Light Emission

When an atom’s energy is at its minimum, it is said to be in a ground state. In this ground state, the atom’s electrons occupy the innermost available shells, those closest to the nucleus. When atoms are excited by heat, by an electric current, or by light or some other form of radiation, the atoms’ electrons can acquire energy and jump from an inner to an outer shell, leaving a vacancy in the inner shell. The atom seeks to shed this surplus energy, leading the electron in the outer orbit to fall back down to an inner vacancy. As it falls, the electron releases energy in the form of a photon, a tiny flash of light. The color of the light depends on the amount of energy emitted.

When an electron moves to a different shell, it does not gradually go from one shell to another, but instead jumps directly to the other shell. These jumps are like steps on a staircase (and are different from a smooth incline, or hill). The electron also absorbs or emits the energy to make jumps in steps. It cannot gradually build up or lose energy, but must instantly absorb the exact amount of energy needed to make a certain jump, or instantly emit the exact amount needed to fall to a lower shell. Each element has a different pattern of allowed jumps within its electronic structure, so the element’s atoms can only absorb or emit a distinct set of energies, or spectrum of colors. In this way, a scientist can tell which elements are present in a sample by looking at the colors absorbed or emitted when the sample is excited by heat, electricity, or light. See also Spectroscopy