We are not going to place one of the electrons in the 3s orbital just for the sake of keeping a maximum number of unpaired electrons. Notice that placing the electron unpaired in the 3s orbital is also incorrect because, it is important to mention, that Hund’s rule applies to electrons in the same energy level. Let’s show the application of Hund’s rule in explaining the electron configuration of carbon: There is a stronger repulsive interaction between two electrons in the same orbital compared to when they occupy separate orbitals of equal energy. Hund’s rule is another demonstration of the same principle which is the tendency to adopt the lowest energy state possible. We can also formulate it as the lowest energy configuration for an atom is the one having the maximum number of unpaired electrons within the same energy sublevel. This is the Hund’s rule, which states that electrons will fill all the degenerate orbitals (equal in energy) with parallel spins (both arrows up or down) first before pairing up in one orbital. And it turns out that the electron goes to the next ( empty) p orbital rather than fitting in with the other electron. Now, the next election, which indicates carbon, is going to have the option of pairing up with the one in the p orbital or going to the next empty p orbital. To understand Hund’s tule, let’s first write the electron configuration of boron: For example, notice how the electrons in each p orbital of oxygen, fluorine, and neon are with opposite spins: The principle applies to all the orbitals. Therefore, the m s must be different (+1/2 or -1/2) so we don’t violate Pauli’s exclusion principle. and m l = –1, 0, or +1 (doesn’t matter which one because for the same orbital it will be identical). So, if the electrons are in the same orbital, they must have the same n, l, m l values, and therefore, the only one that can be different is the m s which is the spin of the electron shown by the position of the arrow.įor example, for the electrons in a 2 p orbital, n = 2, l = 1. This is according to the Pauli exclusion principle which states that no two electrons in an atom can have the same four quantum numbers. Notice that the arrow representing the second electron is pointing down, opposite to the first arrow which indicates that they must have opposite spins. Therefore, He has two electrons and they both go to the 1s level because each orbital, regardless of if it is s, p, d, or f, can only accommodate a maximum of two electrons, and they wouldn’t go to the 2s level without filling the 1s:Īnd this keeps going by adding one more electron to the next lowest energy orbital for every subsequent element in the periodic table. Remember, the number of electrons is equal to the atomic number as the number of electrons must be equal to the number of electrons in a neutral atom. Next, we have helium, He – the second element in the periodic table. The electron configuration of He with orbital diagrams can be shown as: For example, hydrogen has one electron and of course, we are going to put it in the 1s orbital it has the lowest energy. Let’s look at a few examples of following the Aufbau principle. The general order for filling the electrons based on the energy levels and orbitals can be shown as in the picture below: Notice that in the periodic table, it is opposite to what we show on the energy level diagram since the periods indicate the principal quantum number ( n), which is the energy level, and they increase as we go down the periodic table. So, for a given value of n:Į ( s orbital) < E ( p orbital) < E ( d orbital) < E ( f orbital) Here is a chart on the energy levels for up n = 4 which include the d sublevel:Ĭonsider also that within the same principal level, orbitals with a lower value of l have lower energy ( E) and therefore, are filled first. Remember, the main energy level is given by the principal quantum number, n, and it increases down the periodic table. Aufbau’s PrincipleĪufbau (German aufbauen, “to build up”) principle tells us that electrons fill the orbitals in the order of increasing their energy level. Aufbau’s Principle, Hund’s Rule, and Pauli’s Exclusion Principle are rules for writing electron configurations, so go over the main principles of electron configurations if you need to before we start discussing them.
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