Electronic configuration

The electrons of an atom are distributed over very specific atomic orbitals. The electronic configuration describes this electronic distribution around the nucleus. Orbitals are complex shapes that are determined using quantum mechanics. The same atom can have several electronic configurations, and therefore, several energy states. The lowest energy configuration is called the ground state. All other configurations correspond to "excited states".

The logic of this animation follows Hund's rule. Note that some elements such as gold are exceptions to this rule. 

Abbreviated writing of the electronic configuration: writing the electronic configuration of an atom quickly becomes tedious. Let's take the example of the zinc atom (Zn). Its configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ . It is customary to abbreviate this notation using the noble gas that precedes zinc in the periodic table, namely Argon (Ar). Thus, the electronic configuration of Zn can be more compactly written [Ar] 4s² 3d¹⁰, which corresponds to the configuration of argon supplemented by 2 electrons in the s orbital of level 4 (4s²) then 10 electrons in the d orbitals of level 3 (3d¹⁰).

Thanks to the IAEA (International Atomic Energy Agency) for its valuable source of information that is The IAEA's NUCLEUS information resource portal. Its API (Livechart Data Download API) was of great help in developing this educational animation.

• To illustrate the quantization of energy levels
• To define the concepts of spin and orbitals
• To make the link between the conventional writing of the electronic configuration, and the representation of the atom (Bohr model).
• To know how to find one's way in Mendeleev's table based on the electronic configuration.
• To write the electronic configuration of any element of the periodic table.

For this animation, we rely on the Aufbau principle which empirically defines the placement of electrons according to their angular momentum.

4 levels of information are considered for each electron:

• The circle on which it is placed, corresponding to its energy level,
• The subshell it occupies (colored area s, p, d, f, ...),
• The orbital (pair of slots in the subshell) occupied in the subshell,
• Its position in the orbital according to the value of its spin.

An atom, just like the elementary particles that compose it, is a quantum system whose dynamics (energy, momentum, angular momentum, ...) is measured with quantum numbers, with integer or half-integer values. The atom is thus characterized by quantum numbers:

• n, principal quantum number corresponds to the energy level of the electron in the atom (represented by circles in the animation).
• l, azimuthal quantum number corresponds to the orbital angular momentum of the electron. ℓ indicates the electronic subshell: s shell (pink, ℓ = 0), p shell (green, ℓ = 1), d shell (orange, ℓ = 2), f shell (purple, ℓ = 3), g shell (ℓ = 4), h shell (ℓ = 5)... ℓ varies from 0 to n-1.
• m (or mℓ), magnetic quantum number corresponds to the projection of the angular momentum on an axis. ml indicates the orbital number occupied in the electronic subshell.
• s (or ms), spin magnetic quantum number, represents the electron spin. s is ½ or -½ and indicates the electron's position in its orbital.

According to the Aufbau principle, the placement of electrons follows an empirical rule that predicts the order in which electrons fill the electronic subshells of the neutral atom in its ground state (not excited). Electrons fill the electronic shells of lowest energy by filling the s, p, d, f subshells in order (in the animation, this corresponds to completing each step of the staircase in order).

1s → 2s → 2p → 3s → 3p → 3d → 4s → 4p → 4d → 4f → ...

Hund's rule distributes electrons in the electronic subshell so as to maximize the sum of the spins of the electrons, while respecting Pauli's exclusion principle (2 electrons cannot share the same quantum numbers).

Thus on each orbital, only two electrons of opposite spin can be found. Each orbital of a subshell is first occupied by a single electron all having the same spin, until the appearance of pairs of electrons in each orbital.

The construction of the atom using the “Particles” buttons respects this empirical rule. However, 20 elements have an electronic configuration that does not respect the Aufbau principle (Cr, Ni, Cu, Nb, Mo, Ru, Rh, Pd, Ag, La, Ce, Gd, Pt, Au, Ac, Th, Pa, U, Cm, Lr). In this case, a "Displacement" button appears to display the real electronic configuration of the atom. The exceptions are indicated in red on the staircase.

The elements are arranged in the periodic table according to their atomic number and electronic configuration:

• Elements in the same row of the table have the same number of electronic shells (n).
• Each line of the table is divided into blocks according to the electronic subshells of its elements: it is the orbital number l of the highest energy that defines the s, p, d, f blocks of the table.

Ignoring exceptions, all elements in the same column of the table have the same valence shell.

The position of helium in the p block rather than the s block is also an exception. Its position refers to its chemical property as a noble gas and not to its electronic configuration (He should be placed next to H in the s block). All noble gases occupy the last column of the periodic table.