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The arrangement of electrons in the orbitals of an atom is called the electron configuration    of the atom. We describe an electron configuration with a symbol that contains three pieces of information ( [link] ):

  1. The number of the principal quantum shell, n ,
  2. The letter that designates the orbital type (the subshell, l ), and
  3. A superscript number that designates the number of electrons in that particular subshell.

For example, the notation 2 p 4 (read "two–p–four") indicates four electrons in a p subshell ( l = 1) with a principal quantum number ( n ) of 2. The notation 3 d 8 (read "three–d–eight") indicates eight electrons in the d subshell (i.e., l = 2) of the principal shell for which n = 3.

A light blue hemisphere is labeled H. At a location about midway between the center and outer edge of the hemisphere, a small yellow-orange sphere is shown that is labeled with a negative sign. To the right of this diagram is the electron configuration 1 s superscript 1. The superscript is shown in a small yellow-orange circle. This superscript is labeled, “Number of electrons in subshell,” and the s is labeled, “Subshell.”
The diagram of an electron configuration specifies the subshell ( n and l value, with letter symbol) and superscript number of electrons.

The aufbau principle

To determine the electron configuration for any particular atom, we can “build” the structures in the order of atomic numbers. Beginning with hydrogen, and continuing across the periods of the periodic table, we add one proton at a time to the nucleus and one electron to the proper subshell until we have described the electron configurations of all the elements. This procedure is called the Aufbau principle    , from the German word Aufbau (“to build up”). Each added electron occupies the subshell of lowest energy available (in the order shown in [link] ), subject to the limitations imposed by the allowed quantum numbers according to the Pauli exclusion principle. Electrons enter higher-energy subshells only after lower-energy subshells have been filled to capacity. [link] illustrates the traditional way to remember the filling order for atomic orbitals. Since the arrangement of the periodic table is based on the electron configurations, [link] provides an alternative method for determining the electron configuration. The filling order simply begins at hydrogen and includes each subshell as you proceed in increasing Z order. For example, after filling the 3 p block up to Ar, we see the orbital will be 4s (K, Ca), followed by the 3 d orbitals.

This figure includes a chart used to order the filling of electrons into atoms. At the top is a blue circle labeled “1 s.” In a row beneath this circle are 6 additional blue circles labeled “2 s” through “7 s.” A column to the right begins just right of 2 s and contains pink circles labeled 2 p through 7 p. A column to the right begins just right of 3 p and contains yellow circles labeled 3 d through 6 d. No circles are placed to the right of the 7 s and 7 p circles. A final column on the right begins right of 4 d. It includes grey circles labeled, “4 f” and, “5 f.” No circles are placed right of 6 d. Through these circles, arrows are included in the figure pointing down and to the left. The first arrow begins in the upper right and passes through 1 s. The second arrow begins just below and passes through 2 s. The third arrow passes through 2 p and 3 s. The fourth arrow passes through 3 p and 4 s. This pattern of parallel arrows pointing downward to the left continues through all circles completing the pattern 1 s 2 s 2 p 3 s 3 p 4 s 3 d 4 p 5 s 4 d 5 p 6 s 4 f 5 d 6 p 7 s 5 f 6 d 7 p.
The arrow leads through each subshell in the appropriate filling order for electron configurations. This chart is straightforward to construct. Simply make a column for all the s orbitals with each n shell on a separate row. Repeat for p , d , and f . Be sure to only include orbitals allowed by the quantum numbers (no 1 p or 2 d , and so forth). Finally, draw diagonal lines from top to bottom as shown.
In this figure, a periodic table is shown that is entitled, “Electron Configuration Table.” Beneath the table, a square for the element hydrogen is shown enlarged to provide detail. The element symbol, H, is placed in the upper left corner. In the upper right is the number of electrons, 1. The lower central portion of the element square contains the subshell, 1 s. Helium and elements in groups 1 and 2 are shaded blue. In this region, the rows are labeled 1 s through 7 s moving down the table. Groups 3 through 12 are shaded orange, and the rows are labeled 3 d through 6 d moving down the table. Groups 13 through 18, except helium, are shaded pink and are labeled 2 p through 6 p moving down the table. The lanthanide and actinide series across the bottom of the table are shaded grey and are labeled 4 f and 5 f respectively.
This periodic table shows the electron configuration for each subshell. By “building up” from hydrogen, this table can be used to determine the electron configuration for any atom on the periodic table.

We will now construct the ground-state electron configuration and orbital diagram for a selection of atoms in the first and second periods of the periodic table. Orbital diagrams are pictorial representations of the electron configuration, showing the individual orbitals and the pairing arrangement of electrons. We start with a single hydrogen atom (atomic number 1), which consists of one proton and one electron. Referring to [link] or [link] , we would expect to find the electron in the 1 s orbital. By convention, the m s = + 1 2 value is usually filled first. The electron configuration and the orbital diagram are:

Questions & Answers

definition of isomerism
Madam Reply
Compounds that have the same molecular formula, but different structural formula. Sent from Egypt
Abdelkarim
who is the father of chemistry
Naomi Reply
what is hybridization and bonding
Simon Reply
please who is the father of chemistry
Naomi
Antoine Lavoisier
Abdelkarim
I recommend reading on Google.
Abdelkarim
Also, god has created everything
Abdelkarim
(Allah, the creator)
Abdelkarim
how to determine the empirical formula
Vickie Reply
what is neutralisation reaction
Ugbaje Reply
It is a reaction where an acid (aq) reacts with an alkali (aq) to form salt AND water.
Abdelkarim
What is the ratio of the average kinetic energy of a SO 2 molecule to that of an O 2 molecule in a mixture of two gases? What is the ratio of the root mean square speeds, u rms , of the two gases?
xMah_Bx Reply
what is methyl orange
Wisdom Reply
its an indicator
Abigail
It is an organic molecule that reacts with acid/base medium and shows change in colour (due to formed products). It can be used to test for alcohols which are prohibited to drink in Islam as it is bad for brain and liver and immunity.
Abdelkarim
it is an indicator used to determine the end point in an acid-base titration
xMah_Bx
explain more about this topic
Ayomide Reply
Which topic please?
Abdelkarim
covalent bond explanation
EPHETA Reply
God has lictured these rules that the electrons move with them and the space time curve. So we prwy to God in Islam or we suffer. Please learn about Islam and science and mention God and thank him.
Abdelkarim
yes, it is very true. God is the one who inspires science. Then we, as his children, have the privilege to learn about what he has created. I'm still a novis at chemistry. I still have a lot to learn.
Eric
Beautiful, however, what I learned from the Quran is that god has created human and has a mission like managing the ecology, building, learning, mentioning god (saying glory to Almighty for instance few times and periodic). And the moat important prayer lile prophet Muhammed the last meassenger.
Abdelkarim
Also God forgives all sins except assossiating any one with him like sons and daughters or stone sculpture. Beautifully, if some one stops from saying this God can switch their sins to virtueness. And God all merciful doesnt get bored forgiving people who ask for forgiveness.
Abdelkarim
Also, there is a versw in the Quran that sates: { corruption has appeared in the land and the sea he will taste them from what their hands have gained} this reminds me of covid from stupid politicians who will be throne to hell.
Abdelkarim
nature of bond in N2 molecule is
Ahmad Reply
Triple covalent bond, and that is why it is hardly reactive because in order to react you must break three strong covalent bonds.
Abdelkarim
what is electromagnetic energy
Onyekwu Reply
in a school of 120 students, 41studied mathematics, 48studied chemistry and 42 studied physics, 16 studied both chemistry and mathematics, 14 studied mathematics and physics, 18 studied chemistry and physics and 9 studied all the three subjects. how many of them studied exactly one subject?
Kafayat Reply
Does Chromium oxidize?
Jag Reply
yes
Alaa
@Alaa It doesn't.
Jag
yes. it's a very strong oxidizing agent
xMah_Bx
what is compound
Queen Reply
what are the types of hydrocarbon
Dolapo Reply
homologous series is under what
Dolapo
Don't get your Question.
Jag
OK Aliphatic and Aromatic.
Jag
Practice Key Terms 7

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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