Quantum NumbersGeneral rules of Electron configuration

The electron configuration of one atomic types (neutral or ionic) enables us to understand the shape and also energy the its electrons. Many general rules are taken into factor to consider when assigning the "location" that the electron to its prospective energy state, but these assignments space arbitrary and it is constantly uncertain as to which electron is gift described. Knowing the electron configuration of a types gives united state a far better understanding that its bonding ability, magnetism and also other cg-tower.comistry properties.

You are watching: What three principles guide the electron configuration of an atom


The electron configuration is the standard notation supplied to describe the electronic structure of an atom. Under the orbit approximation, we let every electron occupy an orbital, which have the right to be fixed by a single wavefunction. In doing so, we obtain three quantum number (n,l,ml), which space the very same as the ones acquired from addressing the Schrodinger"s equation for Bohr"s hydrogen atom. Hence, many of the rules that we use to describe the electron"s attend to in the hydrogen atom can likewise be used in systems involving multiple electrons. When assigning electron to orbitals, we have to follow a set of 3 rules: the Aufbau Principle, the Pauli-Exclusion Principle, and Hund"s Rule.

The wavefunction is the equipment to the Schrödinger equation. By solving the Schrödinger equation for the hydrogen atom, we achieve three quantum numbers, specific the principal quantum number (n), the orbital angular momentum quantum number (l), and the magnetic quantum number (ml). Over there is a fourth quantum number, called the spin magnetic quantum number (ms), i beg your pardon is not derived from solving the Schrödinger equation. Together, these 4 quantum numbers can be supplied to explain the place of one electron in Bohr"s hydrogen atom. This numbers deserve to be thought of as an electron"s "address" in the atom.


To assist describe the proper notation because that electron configuration, the is best to carry out so v example. Because that this example, us will use the iodine atom. There room two means in which electron configuration can be written:

I: 1s22s22p63s23p64s23d104p65s24d105p5


I: 5s24d105p5

In both that these types of notations, the order of the power levels must be created by enhanced energy, showing the variety of electrons in each subshell together an exponent. In the quick notation, you location brackets around the preceding noble gas facet followed by the valence covering electron configuration. The periodic table shows that kyrpton (Kr) is the ahead noble gas listed before iodine. The noble gas configuration encompases the power states lower than the valence covering electrons. Therefore, in this case =1s22s22p63s23p64s23d104p6.

Principal Quantum Number (n)

The primary quantum number n shows the shell or power level in i m sorry the electron is found. The value of n deserve to be collection between 1 come n, whereby n is the worth of the outermost covering containing an electron. This quantum number have the right to only it is in positive, non-zero, and also integer values. The is, n=1,2,3,4,..

For example, one Iodine atom has actually its outmost electrons in the 5p orbital. Therefore, the principle quantum number for Iodine is 5.

Orbital Angular momentum Quantum Number (l)

The orbital angular inert quantum number, l, suggests the subshell that the electron. Girlfriend can additionally tell the form of the atomic orbital v this quantum number. One s subshell synchronizes to l=0, a p subshell = 1, a d subshell = 2, a f subshell = 3, and so forth. This quantum number deserve to only it is in positive and integer values, although it have the right to take top top a zero value. In general, for every worth of n, there space n values of l. Furthermore, the worth of l arrays from 0 come n-1. Because that example, if n=3, l=0,1,2.

So in regards come the example used above, the l values of Iodine for n = 5 are l = 0, 1, 2, 3, 4.

Magnetic Quantum Number (ml)

The magnetic quantum number, ml, represents the orbitals of a provided subshell. For a given l, ml can variety from -l to +l. A ns subshell (l=1), because that instance, deserve to have three orbitals equivalent to ml = -1, 0, +1. In other words, it defines the px, py and also pzorbitals the the p subshell. (However, the ml number don"t necessarily exchange mail to a offered orbital. The reality that there space three orbitals merely is indicative of the 3 orbitals that a ns subshell.) In general, for a offered l, there space 2l+1 possible values for ml; and in a n primary shell, there are n2 orbitals uncovered in that energy level.

Continuing on from out instance from above, the ml values of Iodine space ml = -4, -3, -2, -1, 0 1, 2, 3, 4. These arbitrarily exchange mail to the 5s, 5px, 5py, 5pz, 4dx2-y2, 4dz2, 4dxy, 4dxz, and also 4dyz orbitals.

Spin Magnetic Quantum Number (ms)

The rotate magnetic quantum number deserve to only have a worth of either +1/2 or -1/2. The value of 1/2 is the turn quantum number, s, which describes the electron"s spin. Due to the rotate of the electron, it generates a magnetic field. In general, one electron with a ms=+1/2 is dubbed ­­an alpha electron, and also one with a ­ms=-1/2 is called a beta electron. No 2 paired electrons can have the same spin value.

Out that these four quantum numbers, however, Bohr postulated that only the principal quantum number, n, identify the power of the electron. Therefore, the 3s orbit (l=0) has the same power as the 3p (l=1) and 3d (l=2) orbitals, regardless of a distinction in l values. This postulate, however, stop true only for Bohr"s hydrogen atom or various other hydrogen-like atoms.

When dealing with multi-electron systems, us must consider the electron-electron interactions. Hence, the previously described postulate breaks under in the the energy of the electron is now established by both the major quantum number, n, and the orbital angular momentum quantum number, l. Although the Schrodinger equation for many-electron atom is extremely daunting to resolve mathematically, we have the right to still describe their electronic structures via electron configurations.

General rule of Electron Configuration

There are a set of basic rules the are supplied to number out the electron configuration of an atomic species: Aufbau Principle, Hund"s Rule and the Pauli-Exclusion Principle. Before continuing, it"s essential to know that each orbital have the right to be populated by two electrons of opposite spin (which will be further discussed later). The complying with table reflects the possible number of electrons that deserve to occupy every orbital in a offered subshell.

subshell number the orbitals total variety of possible electron in every orbital
s 1 2
p 3 (px, py, pz) 6
d 5 (dx2-y2, dz2, dxy, dxz, dyz) 10
f 7 (fz3, fxz2, fxyz, fx(x2-3y2), fyz2, fz(x2-y2), fy(3x2-y2)


Using our example, iodine, again, we check out on the regular table the its atom number is 53 (meaning it includes 53 electron in the neutral state). Its complete electron configuration is 1s22s22p63s23p64s23d104p65s24d105p5. If you counting up every one of these electrons, you will see that it adds as much as 53 electrons. Notification that each subshell have the right to only save on computer the max lot of electrons as shown in the table above.

Aufbau Principle

The word "Aufbau" is German for "building up". The Aufbau Principle, additionally called the building-up principle, says that electron"s accounting orbitals in stimulate of boosting energy. The bespeak of occupation is as follows:



Hund"s dominance states that when electrons accounting degenerate orbitals (i.e. Same n and also l quantum numbers), they must first occupy the empty orbitals before double occupying them. Furthermore, the most stable configuration results when the spins space parallel (i.e. Every alpha electron or every beta electrons). Nitrogen, for example, has 3 electron occupying the 2p orbital. Follow to Hund"s Rule, lock must an initial occupy every of the 3 degenerate ns orbitals, namely the 2px orbital, 2py orbital, and also the 2pz orbital, and with parallel spins (Figure 2). The configuration listed below is incorrect due to the fact that the third electron rectal does no occupy the north 2pz orbital. Instead, it rectal the half-filled 2px orbital. This, therefore, is a violation that Hund"s ascendancy (Figure 2).

Figure 2. A visual representation of the Aufbau Principle and also Hund"s Rule. Note that the filling of electrons in each orbital(px, py and also pz) is arbitrarily as long as the electrons space singly filled prior to having two electrons occupy the very same orbital.(a)This diagram represents the correct filling the electrons for the nitrogen atom. (b) This diagramrepresents the incorrectfilling the the electrons for the nitrogen atom.

Electronic construction of Cations and also Anions

The means we designate electronic configurations because that cations and anions is essentially similar to the for neutral atoms in your ground state. That is, us follow the three important rules: Aufbau Principle, Pauli-exclusion Principle, and Hund"s Rule. The electronic configuration the cations is assigned by remove electrons an initial in the outermost p orbital, followed by the s orbital and also finally the d orbitals (if any more electrons must be removed). Because that instance, the floor state digital configuration that calcium (Z=20) is 1s22s22p63s23p64s2. The calcium ion (Ca2+), however, has actually two electron less. Hence, the electron configuration for Ca2+ is 1s22s22p63s23p6. Because we have to take away two electrons, we first remove electron from the outermost covering (n=4). In this case, every the 4p subshells room empty; hence, we begin by removing from the s orbital, i m sorry is the 4s orbital. The electron configuration for Ca2+ is the very same as the for Argon, which has 18 electrons. Hence, we deserve to say the both room isoelectronic.

The digital configuration of anions is assigned by including electrons according to Aufbau Principle. We include electrons to fill the outermost orbital that is occupied, and then add much more electrons to the next higher orbital. The neutral atom chlorine (Z=17), because that instance has actually 17 electrons. Therefore, its ground state electronic configuration can be written as 1s22s22p63s23p5. The chloride ion (Cl-), top top the other hand, has an additional electron because that a total of 18 electrons. Adhering to Aufbau Principle, the electron rectal the partly filled 3p subshell first, do the 3p orbital fully filled. The digital configuration because that Cl- can, therefore, it is in designated as 1s22s22p63s23p6. Again, the electron configuration for the chloride ion is the same as the for Ca2+ and Argon. Hence, they room all isoelectronic to every other.


1. Which of the princples explained over tells united state that electron that space paired cannot have actually the exact same spin value?

2. Discover the values of n, l, ml, and also ms because that the following:

a. Mg

b. Ga

c. Co

3. What is a possible mix for the quantum number of the 5d orbital? Give an example of an aspect which has the 5d orbital as it"s many outer orbital.

4. I m sorry of the adhering to cannot exist (there may be much more than one answer):

a. N = 4; l = 4; ml = -2; ms = +1/2

b. N = 3; l = 2; ml = 1; ms = 1

c. N = 4; l = 3; ml = 0; multiple sclerosis = +1/2

d. N = 1; l = 0; ml = 0; ms = +1/2

e. N = 0; l = 0; ml = 0; ms = +1/2

5. Create electron configurations for the following:

a. P

b. S2-

c. Zn3+


1. Pauli-exclusion Principle

2. A. N = 3; l = 0, 1, 2; ml = -2, -1, 0, 1, 2; ms can be one of two people +1/2 or -1/2

b. N = 4; l = 0, 1, 2, 3; ml = -3, -2, -1, 0, 1, 2, 3; ms can be one of two people +1/2 or -1/2

c. N = 3; l = 0, 1, 2; ml = -2, -1, 0, 1, 2, 3; ms have the right to be one of two people +1/2 or -1/2

3. N = 5; l = 3; ml = 0; ms = +1/2. Osmium (Os) is an example.

4. A. The value of l can not be 4, due to the fact that l ranges from (0 - n-1)

b. Ms can only be +1/2 or -1/2

c. Okay

d. Okay

e. The worth of n can not be zero.

See more: Can You Use Tanning Oil In A Tanning Bed ? Best Tanning Oil For Tanning Beds

5. A. 1s22s22p63s23p3

b. 1s22s22p63s23p6

c. 1s22s22p63s23p64s23d7


Atkins, P. W., & De Paula, J. (2006). Physical cg-tower.comistry because that the Life Sciences. new York, NY: W. H. Freeman and Company. Petrucci, R. H., Harwood, W. S., & Herring, F. G. (2002). General cg-tower.comistry: principles and contemporary Applications. top Saddle River, NJ: Prentice-Hall, Inc. Shagoury, Richard. cg-tower.comistry 1A lecture Book. 4th Ed. Custom Publishing. 2006. Print