Electron Configurations of Atoms

The electron configuration of an atom shows the number of electrons in each sublevel in each energy level of the ground-state atom. To determine the electron configuration of a particular atom, start at the nucleus and add electrons one by one until the number of electrons equals the number of protons in the nucleus. Each added electron is assigned to the lowest-energy sublevel, the 3s, 3p, 4s, 3d, and so on. This order is difficult to remember and often hard to determine from energy-level diagrams.

A more convenient way to remember the order is to use the picture below. The principal energy levels are listed in columns, starting at the left with the 1s level. To use this figure, read along the diagonal lines in the direction of the arrow.

The arrow shows a second way to remembering the order in which sublevels fill.

An atom of hydrogen (atomic number 1) has one proton and one electron. The single electron is assigned to the 1s sublevel, the lowest-energy sublevel in the lowest-energy level. Therefore, the electron configuration of hydrogen is written:

Boron (atomic number 5) has five electrons. Four electrons fill both the 1s and 2s orbitals. The fifth electron is added to a 2p orbital, the sublevel next higher in energy. The electron configuration of boron is:

Table below shows the electron configurations of the elements with atomic numbers 1 to 18. The electron configurations of elements with higher atomic number can be wrtitten by following the orbital-filling.

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