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.*Note: there is currently no known element whose electrons occupy the g subshell.The notion of electron configuration is predicated on three facts:
Total Number of Electrons in the n th shell (2n 2) or 2(total number of orbitals in the shell) Let's summarize the number of electrons held in Shells and Subshells of n= 1 to n=5.
The s subshell has 1 orbital that can hold up to 2 electrons, the p subshell has 3 orbitals that can hold up to 6 electrons, the d subshell has 5 orbitals that hold up to 10 electrons, and the f subshell has 7 orbitals with 14 electrons. So, this tells us that each subshell has double the electrons per orbital. In both cases, n=2, l=0, m l=0, but the orange electron has an m s= +1/2 and the blue electron has an m s=-1/2. To correct this, we represent one electron as pointing up and the other is pointing down. In the first example, the 2 electrons would have the same m s quantum number, and therefore, will be incorrect, since no two electrons are exactly alike. One electron is spin up (m s = +1/2) and the other would spin down (m s = -1/2). A single orbital can hold a maximum of two electrons, which must have opposing spins otherwise they would have the same four quantum numbers, which is forbidden. The first three (n, l, and m l) may be the same, but the fourth quantum number must be different. The Pauli exclusion principle states that no two electrons can have the same four quantum numbers.
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