To recognize the basics of adding electrons to atomic orbitalsTo recognize the basics of the Aufbau principle

The electron construction of an element is the setup of its electron in its atomic orbitals. By knowing the electron configuration of one element, we can predict and explain a great deal of its bsci-ch.orgistry.

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The Aufbau Principle

We build the routine table by complying with the aufbau rule (from German, meaning “building up”). An initial we recognize the variety of electrons in the atom; then we include electrons one at a time to the lowest-energy orbital obtainable without violating the Pauli principle. We use the orbital power diagram of number (PageIndex1), recognizing the each orbital have the right to hold two electrons, one through spin up ↑, corresponding to ms = +½, i beg your pardon is arbitrarily composed first, and also one v spin under ↓, corresponding to ms = −½. A filled orbital is shown by ↑↓, in i beg your pardon the electron spins are claimed to it is in paired. Here is a sbsci-ch.orgatic orbital diagram because that a hydrogen atom in its ground state:

Figure (PageIndex1): One electron in.

From the orbit diagram, we can write the electron configuration in one abbreviated form in which the inhabited orbitals are determined by their primary quantum number n and their value of l (s, p, d, or f), through the variety of electrons in the subshell suggested by a superscript. Because that hydrogen, therefore, the solitary electron is inserted in the 1s orbital, i m sorry is the orbital shortest in energy (Figure (PageIndex1)), and the electron configuration is created as 1s1 and also read together “one-s-one.”

A neutral helium atom, with an atomic number of 2 (Z = 2), has actually two electrons. We location one electron in the orbital the is shortest in energy, the 1s orbital. Indigenous the Pauli exemption principle, we recognize that an orbital have the right to contain two electrons v opposite spin, for this reason we location the 2nd electron in the very same orbital together the very first but pointing down, so the the electrons are paired. The orbit diagram for the helium atom is therefore

written together 1s2, whereby the superscript 2 implies the pairing that spins. Otherwise, our configuration would certainly violate the Pauli principle.

The next aspect is lithium, through Z = 3 and three electrons in the neutral atom. We understand that the 1s orbital can hold 2 of the electrons with their spins paired. Figure 6.29 tells us that the next lowest power orbital is 2s, so the orbit diagram because that lithium is

This electron construction is composed as 1s22s1.

The next element is beryllium, v Z = 4 and four electrons. Us fill both the 1s and 2s orbitals to achieve a 1s22s2 electron configuration:

When we reach boron, with Z = 5 and five electrons, us must ar the fifth electron in one of the 2p orbitals. Due to the fact that all 3 2p orbitals are degenerate, it doesn’t matter which one we select. The electron construction of boron is 1s22s22p1:

At carbon, through Z = 6 and six electrons, us are faced with a choice. Have to the sixth electron be placed in the very same 2p orbit that already has an electron, or have to it go in one of the north 2p orbitals? If it goes in an empty 2p orbital, will certainly the 6th electron have actually its spin aligned v or it is in opposite come the rotate of the fifth? In short, i m sorry of the following three orbit diagrams is correct for carbon, remembering that the 2p orbitals space degenerate?

Because that electron-electron interactions, it is an ext favorable energetically for an electron to be in an unoccupied orbital than in one that is already occupied; hence we have the right to eliminate choice a. Similarly, experiment have presented that an option b is slightly higher in energy (less stable) than selection c since electrons in degenerate orbitals prefer to heat up v their spins parallel; thus, we deserve to eliminate selection b. Choice c illustrates Hund’s dominion (named after the German physicist Friedrich H. Hund, 1896–1997), i m sorry today says that the lowest-energy electron configuration for an atom is the one that has the maximum number of electrons with parallel spins in degenerate orbitals. Through Hund’s rule, the electron configuration of carbon, i beg your pardon is 1s22s22p2, is taken to correspond to the orbital diagram displayed in c. Experimentally, that is found that the ground state of a neutral carbon atom does without doubt contain 2 unpaired electrons.

When we acquire to nitrogen (Z = 7, with seven electrons), Hund’s preeminence tells united state that the lowest-energy arrangement is

with 3 unpaired electrons. The electron construction of nitrogen is thus 1s22s22p3.

At oxygen, v Z = 8 and eight electrons, we have actually no choice. One electron must be combine with another in among the 2p orbitals, which offers us two unpaired electrons and also a 1s22s22p4 electron configuration. Because all the 2p orbitals space degenerate, the doesn’t matter which one has the pair of electrons.

Similarly, fluorine has the electron configuration 1s22s22p5:

When us reach neon, with Z = 10, we have actually filled the 2p subshell, offering a 1s22s22p6 electron configuration:

Notice the for neon, together for helium, all the orbitals v the 2p level are fully filled. This fact is an extremely important in dictating both the bsci-ch.orgical reactivity and the bonding of helium and neon, together you will certainly see.

Valence Electrons

As we proceed through the regular table in this way, composing the electron construction of larger and also larger atoms, it becomes tedious to keep copying the configurations of the filled within subshells. In practice, bsci-ch.orgists simplify the notation by making use of a bracketed noble gas prize to represent the construction of the noble gas native the preceding row because all the orbitals in a noble gas space filled. For example, represents the 1s22s22p6 electron construction of neon (Z = 10), therefore the electron configuration of sodium, v Z = 11, i beg your pardon is 1s22s22p63s1, is composed as 3s1:

NeonZ = 101s22s22p6
SodiumZ = 111s22s22p63s1 = 3s1

Because electron in filled inside orbitals space closer come the nucleus and an ext tightly bound to it, they room rarely involved in bsci-ch.orgistry reactions. This way that the bsci-ch.orgistry of one atom depends mostly on the electrons in that is outermost shell, i m sorry are dubbed the valence electrons. The streamlined notation allows us to watch the valence-electron configuration an ext easily. Using this notation to compare the electron configuration of sodium and lithium, we have:

Sodium1s22s22p63s1 = 3s1
Lithium1s22s1 = 2s1

It is readily obvious that both sodium and also lithium have one s electron in your valence shell. We would because of this predict the sodium and also lithium have actually very comparable bsci-ch.orgistry, i beg your pardon is certainly the case.

As we proceed to construct the eight facets of duration 3, the 3s and also 3p orbitals space filled, one electron in ~ a time. This heat concludes through the noble gas argon, which has the electron configuration 3s23p6, equivalent to a filled valence shell.

Example (PageIndex1): electronic Configuration the Phoshorus

Draw an orbital diagram and also use the to have the electron construction of phosphorus, Z = 15. What is that valence electron configuration?

Given: atomic number

Asked for: orbital diagram and valence electron configuration for phosphorus


Locate the nearest noble gas coming before phosphorus in the periodic table. Then subtract its number of electrons native those in phosphorus to attain the variety of valence electron in phosphorus.Referring to number Figure (PageIndex1), draw an orbit diagram to represent those valence orbitals. Complying with Hund’s rule, place the valence electrons in the obtainable orbitals, beginning with the orbital the is lowest in energy. Write the electron configuration from her orbital diagram.Ignore the inner orbitals (those that correspond to the electron construction of the nearest noble gas) and also write the valence electron construction for phosphorus.


A due to the fact that phosphorus is in the third row of the periodic table, we understand that it has a closed covering with 10 electrons. We begin by subtracting 10 electrons from the 15 in phosphorus.

B The added five electrons are inserted in the next obtainable orbitals, which figure (PageIndex1) tells us space the 3s and also 3p orbitals:

Hund’s preeminence tells united state that the continuing to be three electrons will occupy the degenerate 3p orbitals separately but with your spins aligned:

The electron configuration is 3s23p3.

C We acquire the valence electron configuration by ignoring the inside orbitals, which for phosphorus method that we neglect the closeup of the door shell. This gives a valence-electron construction of 3s23p3.

Exercise (PageIndex1)

Draw an orbit diagram and also use it to derive the electron configuration of chlorine, Z = 17. What is that valence electron configuration?


3s23p5; 3s23p5

The basic order in i beg your pardon orbitals space filled is shown in number (PageIndex2). Subshells matching to each value of n space written native left to best on succeeding horizontal lines, where each heat represents a row in the regular table. The stimulate in i beg your pardon the orbitals space filled is indicated by the diagonal lines running from the upper appropriate to the reduced left. Accordingly, the 4s orbit is filled prior to the 3d orbital because of shielding and penetration effects. Consequently, the electron configuration of potassium, which starts the fourth period, is 4s1, and also the configuration of calcium is 4s2. Five 3d orbitals are filled through the following 10 elements, the shift metals, complied with by 3 4p orbitals. Notice that the critical member the this heat is the noble gas krypton (Z = 36), 4s23d104p6 = , which has actually filled 4s, 3d, and 4p orbitals. The 5th row the the regular table is essentially the same as the fourth, other than that the 5s, 4d, and also 5p orbitals room filled sequentially.

Figure (PageIndex2): Predicting the order in which Orbitals room Filled in Multielectron Atoms. If you write the subshells because that each worth of the major quantum number on successive lines, the observed order in i beg your pardon they are filled is suggested by a collection of diagonal lines running from the upper best to the reduced left.

The sixth row that the routine table will be various from the preceding two because the 4f orbitals, which have the right to hold 14 electrons, room filled in between the 6s and the 5d orbitals. The facets that save on computer 4f orbitals in their valence shell are the lanthanides. When the 6p orbitals are finally filled, we have reached the next (and critical known) noble gas, radon (Z = 86), 6s24f145d106p6 = . In the last row, the 5f orbitals space filled in between the 7s and the 6d orbitals, which gives the 14 actinide elements. Due to the fact that the big number of protons makes their nuclei unstable, all the actinides space radioactive.

Example (PageIndex2): Electron construction of Mercury

Write the electron construction of mercury (Z = 80), showing all the inner orbitals.

Given: atomic number

Asked for: complete electron configuration


Using the orbit diagram in number (PageIndex1) and also the periodic table as a guide, to fill the orbitals till all 80 electrons have actually been placed.


By placing the electron in orbitals adhering to the order presented in figure (PageIndex2) and also using the routine table together a guide, we obtain

1s2row 12 electrons
2s22p6row 28 electrons
3s23p6row 38 electrons
4s23d104p6row 418 electrons
5s24d105p6row 518 electrons
row 1–554 electrons

After filling the first five rows, us still have actually 80 − 54 = 26 much more electrons come accommodate. According to figure (PageIndex2), we should fill the 6s (2 electrons), 4f (14 electrons), and 5d (10 electrons) orbitals. The an outcome is mercury’s electron configuration:

1s22s22p63s23p64s23d104p65s24d105p66s24f145d10 = Hg = 6s24f145d10

with a filled 5d subshell, a 6s24f145d10 valence shell configuration, and a full of 80 electrons. (You should always check to be certain that the total variety of electrons equates to the atom number.)

Exercise (PageIndex2): Electron configuration of Flerovium

Although facet 114 is not stable sufficient to happen in nature, 2 isotopes of aspect 114 were created for the very first time in a nuclear reactor in 1999 by a team that Russian and American scientists. The facet is called after the Flerov activities of nuclear Reactions of the share Institute because that Nuclear study in Dubna, Russia, wherein the aspect was found in 1998. The name of the laboratory, in turn, honors the Russian physicist Georgy Flyorov. Write the complete electron configuration for aspect 114.



The electron configuration of the aspects are presented in figure (PageIndex2), i beg your pardon lists the orbitals in the order in i m sorry they space filled. In several cases, the ground state electron configurations are various from those guess by number (PageIndex1). Several of these anomalies happen as the 3d orbitals space filled. For example, the it was observed ground state electron configuration of chromium is 4s13d5 quite than the predicted 4s23d4. Similarly, the observed electron construction of copper is 4s13d10 rather of s23d9. The yes, really electron configuration might be rationalized in terms of an included stability associated with a half-filled (ns1, np3, nd5, nf7) or to fill (ns2, np6, nd10, nf14) subshell. Given the little differences between higher energy levels, this included stability is sufficient to transition an electron native one orbital to another. In heavier elements, various other more complex effects can likewise be important, leading to some the the added anomalies indicated in number (PageIndex2). For example, cerium has an electron configuration of 6s24f15d1, which is difficult to rationalize in basic terms. In many cases, however, these noticeable anomalies perform not have important bsci-ch.orgical consequences.

Additional stability is linked with half-filled or to fill subshells.

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Based ~ above the Pauli principle and also a understanding of orbit energies obtained using hydrogen-like orbitals, it is possible to construct the routine table by pour it until it is full up the obtainable orbitals beginning with the lowest-energy orbitals (the aufbau principle), which gives rise to a certain arrangement the electrons because that each aspect (its electron configuration). Hund’s rule says that the lowest-energy plan of electron is the one that places them in degenerate orbitals through their spins parallel. Because that bsci-ch.orgical purposes, the most necessary electrons room those in the outermost major shell, the valence electrons.

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