![]() In francium, the heaviest element of the group, the outer-shell electron is in the seventh-shell orbital, significantly further out on average from the nucleus than those electrons filling all the shells below it in energy. In hydrogen, that s orbital is in the lowest possible energy state of any atom, the first-shell orbital (and represented by hydrogen's position in the first period of the table). For instance, the outer-shell (or "valence") electrons of the first group, headed by hydrogen all have one electron in an s orbital. Progressing through a group from lightest element to heaviest element, the outer-shell electrons (those most readily accessible for participation in chemical reactions) are all in the same type of orbital, with a similar shape, but with increasingly higher energy and average distance from the nucleus. The chemical properties of an atom are largely determined by the arrangement of the electrons in its outermost ("valence") shell (although other factors, such as atomic radius, atomic mass, and increased accessibility of additional electronic states also contribute to the chemistry of the elements as atomic size increases). This leads directly to the structure of the periodic table. The order in which the states are filled is as follows: The fact that the 3d state is higher in energy than the 4s state but lower than the 4 p is the reason for the existence of the transition metals. In the ground state of an atom, the states are "filled" in order of increasing energy i.e., the first electron goes into the lowest energy state, the second into the next lowest, and so on. It can be shown that the total capacity of a shell is 2n^2. ![]() States with the same value of l are more closely related, and said to lie within the same electron subshell.įor instance, the n = 1 shell only possesses an s subshell and can only take 2 electrons, the n = 2 shell possesses an s and a p subshell and can take 8 electrons overall, the n = 3 shell possesses s, p and d subshells and has a maximum of 18 electrons, and so on. ![]() States with the same value of n are related, and said to lie within the same electron shell. It is denoted s, and can only take the values 1/2 or -1/2 (sometimes referred to as "up" and "down"). The spin quantum number is an intrinsic property of the electron and independent of the other numbers. For instance, if the electron is in an n=2,\ l=1 state, m can be either -1, 0, or 1. The magnetic quantum number is denoted m, and can take any integer value in the range -l \le m \le l.For instance, if the electron is in an n=2 state, l can be either 0 or 1. The azimuthal quantum number is denoted l, and can take any integer value in the range 0 \le l \le n-1.The principal quantum number is denoted n, and can take any integer value greater than or equal to 1.Three of these are properties of the atomic orbital in which it sits (a more thorough explanation is given at that article). The state of an electron in an atom is given by four quantum numbers. As a result, atomic electron configurations are more commonly discussed.Įlectron configuration in atoms Summary of the quantum numbers This is relatively simple for hydrogen, much more difficult for other atoms, and extremely difficult for molecules. If one wishes to deduce the configuration, one must know the orbitals. The electron configuration of a system is determined by its orbitals and by the number of electrons present. If left in equilibrium, it will always have this configuration, though the electrons may be temporarily " excited" to other configurations. If a state with lower energy is available, the electron will, given time, switch to that state (and emit its excess energy as a photon).Īs a result, any system has only one stable electron configuration. An electron is not stable if it is not in the state with the lowest possible energy.Once a state is occupied by an electron, the next electron must occupy a different state. Electrons are fermions and are thus subject to the Pauli exclusion principle, which states that no two fermions can occupy the same quantum state at once.Each state generally has a different energy than any other state. The possible states are determined by electron orbitals. In a confined space, such as an atom or molecule, the energy and other properties of an electron are quantized, or restricted to certain possible states.The notion of electron configuration is predicated on three facts:
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