Because the helium nucleus is different from the hydrogen nucleus, neither of the helium electrons will have exactly the same energy as the single hydrogen electron, even though all are in the 1s sublevel. Two electrons completely fill the first energy level. Therefore, the electron configuration of hydrogen is written:įor helium (atomic number 2), which has two electrons, the electron configuration is: The single electron is assigned to the 1s sublevel, the lowest-energy sublevel in the lowest-energy level. The order is summarized under the diagram.įIGURE 5.9 The arrow shows a second way of remembering the order in which sublevels fill.Īn atom of hydrogen (atomic number 1) has one proton and one electron. To use this figure, read along the diagonal lines in the direction of the arrow. The principal energy levels are listed in columns, starting at the left with the 1s level. This order is difficult to remember and often hard to determine from energy-level diagrams such as Figure 5.8Ī more convenient way to remember the order is to use Figure 5.9. The first sublevel filled will be the 1s sublevel, then the 2s sublevel, the 2p sublevel, the 3s, 3p, 4s, 3d, and so on. Each added electron is assigned to the lowest-energy sublevel available. 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.

The electron configuration of an atom shows the number of electrons in each sublevel in each energy level of the ground-state atom.

Ground state electron configuration