How to Write the Electron Configuration of an Atom

The electron configuration of an atom is a series of numbers representing the electron orbitals. Electron orbitals are regions of space of different shapes surrounding the nucleus of an atom in which electrons are arranged in an orderly manner. Through electron configuration you can quickly determine how many electron orbitals are in an atom, and how many electrons are in each orbital. Once you understand the basic principles of electron configuration, you will be able to write your own electron configuration and be able to do chemistry tests with confidence.

Table of Contents

Determine the number of electrons using the periodic table

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Find the atomic number of the atom. Each atom has a specific number of electrons associated with it. Locate the element on the periodic table. The atomic number is a positive integer starting at 1 (for hydrogen) and increasing by 1 unit for each atom thereafter. The atomic number is the number of protons of the atom – so it is also the number of electrons of the atom in the ground state.

Determine the charge of an atom. An electrically neutral atom has the correct number of electrons as shown on the periodic table. However, an atom with a charge will have more or less electrons based on the magnitude of its charge. If you work with charged atoms, add or subtract the corresponding number of electrons: add one electron for each negative charge and subtract one for each positive charge.

For example, a sodium atom with a charge of +1 will have one electron removed from its base atomic number 11. Therefore, the sodium atom will have a total of 10 electrons.

Memorize the basic orbital list. When an atom gains electrons, the electrons are arranged into orbitals in a specific order. When electrons fill orbitals, the number of electrons in each orbital is even. We have the following orbitals:

The s orbital (any number with the letter “s” behind it in the electron configuration) has only one orbital, and according to the Pauli Exclusion Principle , each orbital can only hold up to 2 electrons, so each s orbital can hold only 2. electrons.
The p orbital has 3 orbitals, so it can hold up to 6 electrons.
The d orbital has 5 orbitals, so it can hold up to 10 electrons.
The f orbital has 7 orbitals, so it can hold up to 14 electrons.
Memorize the order of the names of the orbitals according to the following easy-to-remember sentences: ^{[1] X Research source}
S on Strike Use F near the Match of the Wise .

For atoms with more electrons, the orbitals continue to be written alphabetically after the letter k, omitting the letters that were used.

Learn electron configuration. The electron configuration is written so that it clearly shows the number of electrons in the atom, as well as the number of electrons in each orbital. Each orbital is written in a certain order, with the number of electrons in each orbital written to the right of the orbital name. The final electron configuration is a string consisting of the orbitals’ names and the number of electrons written on the top right of them.

The following example is a simple electron configuration: 1s ² 2s ² 2p ⁶ . This configuration shows that there are two electrons in the 1s orbital, two electrons in the 2s orbital, and six electrons in the 2p orbital. 2 + 2 + 6 = 10 electrons (total). This electron configuration is that of an electrically neutral neon atom (neon’s atomic number is 10).

Memorize the order of orbitals. Note that the orbitals are numbered according to the electron shell, but energetically ordered. For example, the saturated 4s ² orbital is lower in energy (or more stable than) the saturated or unsaturated 3d ¹⁰ orbital, so the subclass 4s is written first. Once you know the order of the orbitals, you can arrange the electrons in them according to the number of electrons in that atom. The order of placement of electrons in orbitals is as follows: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, 8s.

The electron configuration of an atom with each electron-filled orbital is written as follows: 1s ² 2s ² 2p ⁶ 3s ² 3p ⁶ 4s ² 3d ¹⁰ 4p 6 ^5s² 4d ¹⁰ 5p 6 ^6s² 4f ¹⁴ 5d ¹⁰ 6p ⁶ 7s ² 5f ¹⁴ 6d ¹⁰ 7p ⁶
Note that if all the shells are filled, the electron configuration above is that of Og (Oganesson), 118, which is the atom with the highest number on the periodic table – containing all the electron shells known today for with an electrically neutral atom.

Arrange electrons into orbitals according to the number of electrons in the atom. For example, if you wanted to write the electron configuration of an electrically neutral calcium atom, the first thing to do would be to find its atomic number on the periodic table. The atomic number of calcium is 20, so we will write the configuration of the atom with 20 electrons in the order above.

Arrange electrons into orbitals in the above order until there are 20 electrons. The 1s orbital gets two electrons, 2s gets two, 2p gets six, 3s gets two, 3p gets six, and the 4s gets two (2 + 2 + 6 +2 +6 + 2 = 20). Hence the electron configuration of calcium is: 1s ² 2s ² 2p ⁶ 3s ² 3p ⁶ 4s ² .
Note: The energy level changes as the electron shell increases. For example, when you write to the 4th energy level, the subclass 4s is written first, then 3d. After writing the fourth energy level, you will move on to the fifth level and start the layered arrangement again. This happens only after the 3rd energy level.

Use the periodic table as a visual shortcut. Perhaps you noticed that the shape of the periodic table corresponds to the order of the orbitals in the electron configuration. For example, the atoms in the second column from the left always end at “s ² “, the atoms on the far right of the middle always end at “d ¹⁰ “, etc. Use the periodic table to write configuration – the arrangement of electrons in the orbitals will correspond to the position shown on the periodic table. See the section below:

The two leftmost columns are atoms with electron configurations ending in s orbitals, the right part of the periodic table are atoms with electron configurations ending in p orbitals, the middle part are atoms ending in orbitals d, and the lower part is the atoms ending in the f orbital.
For example, when writing the electron configuration of the element chlorine, make the following argument: This atom is in the third row (or “period”) of the periodic table. It is also located in the fifth column of the p-orbital block on the periodic table. Hence the electron configuration will end up being …3p ⁵ .
Careful! Subclass d and f orbitals on the periodic table corresponding to energy levels other than their periods. For example, the first row of the d orbital block corresponds to the 3d orbital even though it is period 4, while the first row of the f orbital corresponds to the 4f orbital even though it is period 6.

Learn to write the reduced electron configuration. The atoms along the right edge of the periodic table are called noble gases . These elements are very chemically inert. To simplify writing long electron configurations, write in square brackets the chemical symbol for the nearest noble gas that has fewer electrons than that atom, then continue writing the electron configurations of the next orbitals . See the section below:

To understand this concept, write down the electron configuration of an example. Suppose we need to write the reduced electron configuration of zinc (atomic number 30) through the noble gas configuration. The full electron configuration of zinc is: 1s ² 2s ² 2p ⁶ 3s ² 3p ⁶ 4s ² 3d ¹⁰ . Note, however, that 1s ² 2s ² 2p ⁶ 3s ² 3p ⁶ is the configuration of the noble gas agon. Just replace this part of zinc’s electron symbol with the chemical symbol for agon in square brackets ([Ar]).
Hence the reduced electron configuration of zinc is [Ar]4s ² 3d ¹⁰ .

Using ADOMAH periodic table

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Learn ADOMAH periodic table. This method of writing electron configurations does not require memorization. However, this method requires a rearranged periodic table, because in the regular periodic table, from the fourth row, the number of periods does not correspond to the electron shell. Find an ADOMAH Periodic Table, which is a special chemical periodic table designed by scientist Valery Tsimmerman. You can find this periodic table on the internet. ^{[2] X Research Source}

On the ADOMAH Periodic Table, the horizontal rows are groups of elements such as halogens, inert gases, alkali metals, alkaline earth metals etc. The vertical columns correspond to the electron shell and are called “stairs” (diagonals connecting the s, p, d and f blocks) corresponding to the period.
Helium is arranged next to hydrogen because both have a unique 1s orbital. The period blocks (s,p,d and f) are shown on the right side and the number of electron shells is shown at the base. The names of the elements are written in rectangular boxes numbered from 1 to 120. These numbers are ordinary atomic numbers, representing the total number of electrons in an electrically neutral atom.

Find the element on ADOMAH’s periodic table. To write the electron configuration of an element, determine its symbol on the ADOMAH Periodic Table and cross out all elements with higher atomic numbers. For example, if you want to write the electron configuration of eribium (68), cross out the elements 69 to 120.

Notice the numbers 1 through 8 at the base of the periodic table. This is the number of electron shells or columns. Do not notice columns with only elements crossed out. For eribi, the remaining columns are 1, 2, 3, 4, 5 and 6.

Count the number of orbitals to the position of the atom to be configured. Look at the block symbol shown to the right of the periodic table (s, p, d and f) and look at the number of columns shown at the base of the table, regardless of the diagonal between the blocks, divide the column into block-columns and write They are in order from bottom to top. Ignore columns-blocks containing only crossed out elements. Write down the columns-blocks starting with the column number and then the block symbol, like this: 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 6s (in the case of eribi).

Note: The above electron configuration of Er is written in order of increasing number of electron shells. This configuration can also be written in the order of arrangement of electrons in orbitals. Follow the stairs from top to bottom instead of columns when writing columns-blocks: 1s ² 2s ² 2p ⁶ 3s ² 3p ⁶ 4s ² 3d ^{10 4p}⁶ 5s ² 4d ¹⁰ 5p ⁶ 6s ² 4f ¹² .

Count the number of electrons for each orbital. Count the number of uncrossed electrons in each block-column, line up one electron for each element, and write the number of electrons next to the mass symbol for each block-column, like this: 1s ² 2s ² 2p 6 ^3s² 3p ⁶ 3d ¹⁰ 4s ² 4p ⁶ 4d ¹⁰ 4f ¹² 5s ² 5p ⁶ 6s ² . In this example, this is the electron configuration of eribium.

Identify unusual electron configurations. There are eighteen common exceptions to the electron configuration of atoms in the lowest energy state, aka the ground state. As a general rule, they only deviate in the last two to three electron positions. In this case, the actual electron configuration causes the electrons to have a lower energy state than that atom’s standard configuration. The unusual atoms are:

Cr (…, 3d5, 4s1); Cu (…, 3d10, 4s1); Nb (…, 4d4, 5s1); Mo (…, 4d5, 5s1); Ru (…, 4d7, 5s1); Rh (…, 4d8, 5s1); Pd (…, 4d10, 5s0); Ag (…, 4d10, 5s1); La (…, 5d1, 6s2); Ce (…, 4f1, 5d1, 6s2); GD (…, 4f7, 5d1, 6s2); Au (…, 5d10, 6s1); Ac (…, 6d1, 7s2); Th (…, 6d2, 7s2); Pa (…, 5f2, 6d1, 7s2); U (…, 5f3, 6d1, 7s2); Np (…, 5f4, 6d1, 7s2) and Cm (…, 5f7, 6d1, 7s2).

Advice

When the atom is ionic, it means that the number of protons is not equal to the number of electrons. The charge of the atom will then be shown in the upper right corner (usually) of that element’s symbol. Thus an antimony atom with a charge of +2 will have an electron configuration of 1s ² 2s ² 2p ⁶ 3s 2 3p ⁶^4s² 3d ¹⁰ 4p ⁶ 5s ² 4d ¹⁰ 5p ¹ . Note that 5p ³ is changed to 5p ¹ . Be careful that the configuration of an electrically neutral atom ends in any orbital other than s and p . When taking away electrons, you can only take electrons in valence orbitals (s and p orbitals). Thus if a configuration ends at 4s ² 3d ⁷ , and that atom has a charge of +2, the configuration changes to 4s ⁰ 3d ⁷ . We see that 3d ⁷is unchanged , but only the electron in the s orbital has been removed.
Every atom has a tendency to return to the stable state, and the most stable electron configuration will have enough s and p orbitals (s2 and p6). The noble gases have this electron configuration, which is why they rarely participate in reactions and are on the right side of the periodic table. Hence if a configuration ends at 3p ⁴ , it only needs two more electrons to become stable (giving away six electrons, including the electron of the s orbital, would require more energy, so giving away four electrons would easier). If a configuration ends at 4d ³ , it only needs to give away three electrons to reach the stable state. Similarly, new subshells that accept half the electrons (s1, p3, d5..) are more stable, for example p4 or p2, but s2 and p6 will be even more stable.
You can also use the valence electron configuration to write the electron configuration of an element, the final s and p orbitals. So the valence electron configuration of the antimony atom is 5s ² 5p ³ .
Ions are not like that because they are much more stable. Skip the above two steps of this article and follow the same procedure, depending on where you start and how many or few electrons you have.
To find the atomic number from its electron configuration, add up all the numbers that follow the letters (s, p, d, and f). This method is only correct if it is a neutral atom, if it is an ion you cannot use this method. Instead you have to add or subtract the number of electrons received or given away.
The number following the letter must be written in the upper right corner, you must not write it the wrong way when taking the test.
There are two different ways to write electron configurations. You can write in ascending order of electron shell, or in orbital arrangement of electrons, as shown for the eribium atom.
There are cases where the electron needs to be “pushed up”. That’s when the orbital is only missing one electron to get half or all of the electrons, then you have to take an electron from the nearest s or p orbital to move it into the orbital in need of that electron.
We cannot say the “energy level stability” of the new subshell accepts half of the electrons. That’s an oversimplification. The reason for the stability of the energy level of the new subshell receiving “half the number of electrons” is that each orbital has only one single electron, so the electron-electron repulsion is minimized.

Steps

Determine the number of electrons using the periodic table

Using ADOMAH periodic table

Advice

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