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Term symbols usually specify the total for all electrons in an atom, but are sometimes used to describe electrons in a given subshell or set of subshells, for example to describe each open subshell in an atom having more than one. The ground state term symbol for neutral atoms is described, in most cases, by Hund's rules.
This notation is used to specify electron configurations and to create the term symbol for the electron states in a multi-electron atom. When writing a term symbol, the above scheme for a single electron's orbital quantum number is applied to the total orbital angular momentum associated to an electron state. [4]
In 1928 Eugene Wigner and E.E. Witmer proposed rules to determine the possible term symbols for diatomic molecular states formed by the combination of a pair of atomic states with given atomic term symbols. [4] [5] [6] For example, two like atoms in identical 3 S states can form a diatomic molecule in 1 Σ g +, 3 Σ u +, or 5 Σ g + states.
The lightest atom that requires the second rule to determine the ground state term is titanium (Ti, Z = 22) with electron configuration 1s 2 2s 2 2p 6 3s 2 3p 6 3d 2 4s 2. In this case the open shell is 3d 2 and the allowed terms include three singlets ( 1 S, 1 D, and 1 G) and two triplets ( 3 P and 3 F).
The N atom and the O + ion are isoelectronic because each has five valence electrons, or more accurately an electronic configuration of [He] 2s 2 2p 3. Similarly, the cations K +, Ca 2+, and Sc 3+ and the anions Cl −, S 2−, and P 3− are all isoelectronic with the Ar atom. CO, CN −, N 2, and NO +
The shapes of the five orbitals occupied in nitrogen. The two colours show the phase or sign of the wave function in each region. From left to right: 1s, 2s (cutaway to show internal structure), 2p x, 2p y, 2p z. A nitrogen atom has seven electrons. In the ground state, they are arranged in the electron configuration 1s 2 2s 2 2p 1 x 2p 1 y 2p 1 z.
This page shows the electron configurations of the neutral gaseous atoms in their ground states. For each atom the subshells are given first in concise form, then with all subshells written out, followed by the number of electrons per shell. For phosphorus (element 15) as an example, the concise form is [Ne] 3s 2 3p 3.
In each term of an electron configuration, n is the positive integer that precedes each orbital letter (helium's electron configuration is 1s 2, therefore n = 1, and the orbital contains two electrons). An atom's nth electron shell can accommodate 2n 2 electrons. For example, the first shell can accommodate two electrons, the second shell eight ...