ATOMS: Electronic Configuration of Atoms

What Are Electronic Configurations?

The distribution of electrons in their atomic orbitals is called an electronic configuration. The electronic configurations of atoms add in sequence each of the atomic subshells containing electrons. This notation expresses the number of electrons they contain written in superscript. For example, magnesium has an electron configuration of 1s²2s²2p⁶3s². When the atomic number is very large, the electronic configuration can be abbreviated by condensing the standard notation. In these cases, the sequence of occupied subshells is replaced by its noble gas equivalent. The gas symbol is added in square brackets, followed by the standard notation for the levels that are not fully occupied. For example, to abbreviate the electronic configuration of sodium we can use neon or helium: for the former, we would have [Ne]3s¹ since the electronic configuration of neon is [He]2s²2p⁶. If we use helium, the configuration is abbreviated as 1s². Electron configurations have several uses: They can be used to find the valence of elements. Since elements with similar electron configurations often have similar properties, the configurations are useful for predicting the characteristics of the elements. Interpretation of atomic spectra. The notation used for the distribution of electrons in the atomic orbitals of atoms came into use when the atomic model was disclosed by Ernest Rutherford and Niels Bohr’s presentation of the model of the atom in 1913.

Writing Electron Configurations

Representing the Shells

Each shell can hold a maximum number of electrons. This number is dependent on the principal quantum number, denoted by the letter ‘n’. The quantum number is found by applying the equation 2n2, with ‘n’ as the shell number. The types of shells and the maximum number of atoms per shell are shown below.

Shell and ‘n’ value	Max. Electrons in the Electron Configuration
K shell, n=1	2*12 = 2
L shell, n=2	2*22 = 8
M shell, n=3	2*32 = 18
N shell, n=4	2*42 = 32

Representation of Subshells

The azimuthal quantum number (denoted “l”) is central to the subshells because it defines how the electrons are distributed in the subshells. The quantum number depends on the value of the principal number (n). For example, if n takes a value of n=4, this means that there are four different possible subshells. In case n=4, there would be subshells l=0, l=1, l=2 and l=3. These are denoted as subshells s, p, d and f, respectively. To find how many electrons can be accommodated inside the subshells, the formula 2*(2l + 1) is used. Thus, one can have up to 2 in the s-subshell, a maximum of 6 in the p-subshell, up to 10 in the d-subshell and a maximum of 14 electrons in the f-subshells. All subshells up to n=4 are tabulated below.

Principle Quantum Number Value	Value of Azimuthal Quantum Number	Resulting Subshell in the Electron Configuration
n=1	l=0	1s
n=2	l=0	2s
	l=1	2p
n=3	l=0	3s
	l=1	3p
	l=2	3d
n=4	l=0	4s
	l=1	4p
	l=2	4d
	l=3	4f

Filling Atomic Orbitals

Three main rules will determine how orbitals are occupied by electrons.

1. Aufbau’s principle: this principle states that electrons will occupy orbitals from lowest energy to highest energy first.
2. The Pauli exclusion principle: dictates that the maximum number of electrons in each orbital cannot exceed two. In addition, each electron must have oppositely directed spins.
3. Hund’s rule: which states that in each orbital within a subspace, electrons will first occupy a single electron before they are occupied by a second electron in an orbital.

Electronic Configuration of Copper

Copper ( II ) has an abbreviated electron configuration of [Ar] 3d¹⁰4s¹. However, this configuration is not at all similar to the argon configuration. When compared to other ions of groups IA and IIA, this element has strong Lewis acid characteristics. Applying the AufBau Principle, Cu ( Z = 29 ) has an electronic configuration of [Ar]3d¹⁰4s¹ instead of [Ar]3d⁹4s².

Electronic Configuration of Chromium

Chromium has 24 electrons around its neutral nucleus. Cr has an abbreviated notation of [Ar] 3d⁵ 4s¹ instead of [Ar] 3d⁴4s². Such unexpected electronic configurations occur with silver and gold. Indeed, these elements are in the same group as copper and therefore have similar properties. For the latter, the electronic configuration is 1s²2s²2p⁶3s²3p⁶4s¹3d⁵.