# Organization of Electrons in Atoms (2023)

### Learning Objectives

1. Learn how electrons are organized in atoms.
2. Represent the organization of electrons by an electron configuration.

Now that you know that electrons have quantum numbers, how are they arranged in atoms? The key to understanding electronic arrangement is summarized in the Pauli exclusion principle: no two electrons in an atom can have the same set of four quantum numbers. This dramatically limits the number of electrons that can exist in a shell or a subshell.

Electrons are typically organized around an atom by starting at the lowest possible quantum numbers first, which are the shells-subshells with lower energies. Consider H, an atom with a single electron only. Under normal conditions, the single electron would go into the n = 1 shell, which has only a single s subshell with one orbital (because m can equal only 0). The convention is to label the shell-subshell combination with the number of the shell and the letter that represents the subshell. Thus, the electron goes in the 1s shell-subshell combination. It is usually not necessary to specify the m or ms quantum numbers, but for the H atom, the electron has m = 0 (the only possible value) and an ms of either +1/2 or −1/2.

The He atom has two electrons. The second electron can also go into the 1s shell-subshell combination but only if its spin quantum number is different from the first electron’s spin quantum number. Thus, the sets of quantum numbers for the two electrons are {1, 0, 0, +1/2} and {1, 0, 0, −1/2}. Notice that the overall set is different for the two electrons, as required by the Pauli exclusion principle.

The next atom is Li, with three electrons. However, now the Pauli exclusion principle implies that we cannot put that electron in the 1s shell-subshell because no matter how we try, this third electron would have the same set of four quantum numbers as one of the first two electrons. So this third electron must be assigned to a different shell-subshell combination. However, the n = 1 shell doesn’t have another subshell; it is restricted to having just ℓ = 0, or an s subshell. Therefore, this third electron has to be assigned to the n = 2 shell, which has an s (ℓ = 0) subshell and a p (ℓ = 1) subshell. Again, we usually start with the lowest quantum number, so this third electron is assigned to the 2s shell-subshell combination of quantum numbers.

The Pauli exclusion principle has the net effect of limiting the number of electrons that can be assigned a shell-subshell combination of quantum numbers. For example, in any s subshell, no matter what the shell number, there can be a maximum of only two electrons. Once the s subshell is filled up, any additional electrons must go to another subshell in the shell (if it exists) or to higher-numbered shell. A similar analysis shows that a p subshell can hold a maximum of six electrons. A d subshell can hold a maximum of 10 electrons, while an f subshell can have a maximum of 14 electrons. By limiting subshells to these maxima, we can distribute the available electrons to their shells and subshells.

### Example 4

How would the six electrons for C be assigned to the n and ℓ quantum numbers?

Solution

The first two electrons go into the 1s shell-subshell combination. Two additional electrons can go into the 2s shell-subshell, but now this subshell is filled with the maximum number of electrons. The n = 2 shell also has a p subshell, so the remaining two electrons can go into the 2p subshell. The 2p subshell is not completely filled because it can hold a maximum of six electrons.

Test Yourself

How would the 11 electrons for Na be assigned to the n and ℓ quantum numbers?

two 1s electrons, two 2s electrons, six 2p electrons, and one 3s electron

Now that we see how electrons are partitioned among the shells and subshells, we need a more concise way of communicating this partitioning. Chemists use an to represent the organization of electrons in shells and subshells in an atom. An electron configuration simply lists the shell and subshell labels, with a right superscript giving the number of electrons in that subshell. The shells and subshells are listed in the order of filling.

For example, an H atom has a single electron in the 1s subshell. Its electron configuration is

H: 1s1

He has two electrons in the 1s subshell. Its electron configuration is

He: 1s2

The three electrons for Li are arranged in the 1s subshell (two electrons) and the 2s subshell (one electron). The electron configuration of Li is

Li: 1s22s1

Be has four electrons, two in the 1s subshell and two in the 2s subshell. Its electron configuration is

Be: 1s22s2

Now that the 2s subshell is filled, electrons in larger atoms must go into the 2p subshell, which can hold a maximum of six electrons. The next six elements progressively fill up the 2p subshell:

B: 1s22s22p1
C: 1s22s22p2
N: 1s22s22p3
O: 1s22s22p4
F: 1s22s22p5
Ne: 1s22s22p6

Now that the 2p subshell is filled (all possible subshells in the n = 2 shell), the next electron for the next-larger atom must go into the n = 3 shell, s subshell.

### Example 5

What is the electron configuration for Na, which has 11 electrons?

Solution

The first two electrons occupy the 1s subshell. The next two occupy the 2s subshell, while the next six electrons occupy the 2p subshell. This gives us 10 electrons so far, with 1 electron left. This last electron goes into the n = 3 shell, s subshell. Thus, the electron configuration of Na is 1s22s22p63s1.

Test Yourself

What is the electron configuration for Mg, which has 12 electrons?

1s22s22p63s2

(Video) CHE 100 - Organization of Electrons in Atoms (5.1)

For larger atoms, the electron arrangement becomes more complicated. This is because after the 3p subshell is filled, filling the 4s subshell first actually leads to a lesser overall energy than filling the 3d subshell. Recall that while the principal quantum number largely dictates the energy of an electron, the angular momentum quantum number also has an impact on energy; by the time we get to the 3d and 4s subshells, we see overlap in the filling of the shells. Thus, after the 3p subshell is completely filled (which occurs for Ar), the next electron for K occupies the 4s subshell, not the 3d subshell:

K: 1s22s22p63s23p64s1, not 1s22s22p63s23p63d1

For larger and larger atoms, the order of filling the shells and subshells seems to become even more complicated. There are some useful ways to remember the order, like that shown in Figure 8.7 “Electron Shell Filling Order”. If you follow the arrows in order, they pass through the subshells in the order that they are filled with electrons in larger atoms. Initially, the order is the same as the expected shell-subshell order, but for larger atoms, there is some shifting around of the principal quantum numbers. However, Figure 8.7 “Electron Shell Filling Order” gives a valid ordering of filling subshells with electrons for most atoms.

Figure 8.7 Electron Shell Filling Order Starting with the top arrow, follow each arrow. The subshells you reach along each arrow give the ordering of filling of subshells in larger atoms. The n = 5 and higher shells have more subshells, but only those subshells that are needed to accommodate the electrons of the known elements are given.

### Example 6

What is the predicted electron configuration for Sn, which has 50 electrons?

Solution

We will follow the chart in Figure 8.7 “Electron Shell Filling Order” until we can accommodate 50 electrons in the subshells in the proper order:

Sn: 1s22s22p63s23p64s23d104p65s24d105p2

Verify by adding the superscripts, which indicate the number of electrons: 2 +2 +6 +2 +6 +2 +10 +6 +2 +10 +2 = 50, so we have placed all 50 electrons in subshells in the proper order.

Test Yourself

What is the electron configuration for Ba, which has 56 electrons?

1s22s22p63s23p64s23d104p65s24d105p66s2

As the previous example demonstrated, electron configurations can get fairly long. An abbreviated electron configurationuses one of the elements from the last column of the periodic table, which contains what are called the noble gases, to represent the core of electrons up to that element. Then the remaining electrons are listed explicitly. For example, the abbreviated electron configuration for Li, which has three electrons, would be

Li: [He]2s1

where [He] represents the two-electron core that is equivalent to He’s electron configuration. The square brackets represent the electron configuration of a noble gas. This is not much of an abbreviation. However, consider the abbreviated electron configuration for W, which has 74 electrons:

W: [Xe]6s24f145d4

(Video) Quantum Numbers, Atomic Orbitals, and Electron Configurations

This is a significant simplification over an explicit listing of all 74 electrons. So for larger elements, the abbreviated electron configuration can be a very useful shorthand.

### Example 7

What is the abbreviated electron configuration for P, which has 15 electrons?

Solution

With 15 electrons, the electron configuration of P is

P: 1s22s22p63s23p3

The first immediate noble gas is Ne, which has an electron configuration of 1s22s22p6. Using the electron configuration of Ne to represent the first 10 electrons, the abbreviated electron configuration of P is

P: [Ne]3s23p3

Test Yourself

What is the abbreviated electron configuration for Rb, which has 37 electrons?

[Kr]5s1

There are some exceptions to the rigorous filling of subshells by electrons. In many cases, an electron goes from a higher-numbered shell to a lower-numbered but later-filled subshell to fill the later-filled subshell. One example is Ag. With 47 electrons, its electron configuration is predicted to be

Ag: [Kr]5s24d9

However, experiments have shown that the electron configuration is actually

Ag: [Kr]5s14d10

This, then, qualifies as an exception to our expectations. At this point, you do not need to memorize the exceptions; but if you come across one, understand that it is an exception to the normal rules of filling subshells with electrons, which can happen.

## Electron Configuration Energy Diagrams

We have just seen that electrons fill orbitals in shells and subshells in a regular pattern, but why does it follow this pattern? There are three principles which should be followed to properly fill electron orbital energy diagrams:

1. The Aufbau principle
2. The Pauli exclusion principle
3. Hund’s rule

The overall pattern of the electron shell filling order emerges from the Aufbau principle (German for “building up”): electrons fill the lowest energy orbitals first. Increasing the principle quantum number, n, increases orbital energy levels, as the electron density becomes more spread out away from the nucleus. In many-electron atoms (all atoms except hydrogen), the energy levels of subshells varies due to electron-electron repulsions. The trend that emerges is that energy levels increase with value of the angular momentum quantum number, l, for orbitals sharing the same principle quantum number, n. This is demonstrated in Figure 8.8, where each line represents an orbital, and each set of lines at the same energy represents a subshell of orbitals.

As previously discussed, the Pauli exclusion principle states that we can only fill each orbital with a maximum of two electrons of opposite spin. But how should we fill multiple orbitals of the same energy level within a subshell (eg. The three orbitals in the 2p subshell)? Orbitals of the same energy level are known as degenerate orbitals, and we fill them using Hund’s rule: place one electron into each degenerate orbital first, before pairing them in the same orbital.

Let’s examine a few examples to demonstrate the use of the three principles.

Boron is atomic number 5, and therefore has 5 electrons. First fill the lowest energy 1s orbital with two electrons of opposite spin, then the 2s orbital with 2 electrons of opposite spin and finally place the last electron into any of the three degenerate 2p orbitals (Figure 8.9). Figure 8.9. Boron electron configuration energy diagram

(Video) The Electronic Structure Of The Atom

Moving across the periodic table, we follow Hund’s rule and add an additional electron to each degenerate 2p orbital for each subsequent element (Figure 8.10). At oxygen we can finally pair up and fill one of the degenerate 2p orbitals. Figure 8.10. Electron configuration energy diagrams for carbon, nitrogen and oxygen.

### Key Takeaways

• The Pauli exclusion principle limits the number of electrons in the subshells and shells.
• Electrons in larger atoms fill shells and subshells in a regular pattern that we can follow.
• Electron configurations are a shorthand method of indicating what subshells electrons occupy in atoms.
• Abbreviated electron configurations are a simpler way of representing electron configurations for larger atoms.
• Exceptions to the strict filling of subshells with electrons occur.
• Electron configurations are assigned from lowest to highest energy following the Aufbau principle
• One electron is placed in each degenerate orbital before pairing electrons following Hund’s rule.
• Electron configuration energy diagrams follow three principles: the Aufbau principle, the Pauli exclusion principle and Hund’s rule.

### Exercises

1. Give two possible sets of four quantum numbers for the electron in an H atom.

2. Give the possible sets of four quantum numbers for the electrons in a Li atom.

3. How many subshells are completely filled with electrons for Na? How many subshells are unfilled?

4. How many subshells are completely filled with electrons for Mg? How many subshells are unfilled?

5. What is the maximum number of electrons in the entire n = 2 shell?

6. What is the maximum number of electrons in the entire n = 4 shell?

7. Write the complete electron configuration for each atom.

a) Si, 14 electrons

b) Sc, 21 electrons

8. Write the complete electron configuration for each atom.

a) Br, 35 electrons

b) Be, 4 electrons

9. Write the complete electron configuration for each atom.

a) Cd, 48 electrons

b) Mg, 12 electrons

10. Write the complete electron configuration for each atom.

a) Cs, 55 electrons

(Video) Electron Configuration - Basic introduction

b) Ar, 18 electrons

11. Write the abbreviated electron configuration for each atom in Exercise 7.

12. Write the abbreviated electron configuration for each atom in Exercise 8.

13. Write the abbreviated electron configuration for each atom in Exercise 9.

14. Write the abbreviated electron configuration for each atom in Exercise 10.

15. Draw electron configuration energy diagrams for potassium, and bromine.

1.

{1, 0, 0, 1/2} and [1, 0, 0, −1/2}

3.

Three subshells (1s, 2s, 2p) are completely filled, and one shell (3s) is partially filled.

5.

8 electrons

7.

a) 1s22s22p63s23p2

b) 1s22s22p63s23p64s23d1

9.

a) 1s22s22p63s23p64s23d104p65s24d10

b) 1s22s22p63s2

11.

a) [Ne]3s23p2

b) [Ar]4s23d1

13.

a) [Kr]5s24d10

b) [Ne]3s2

15. (Video) Introduction to electron organization in atoms 1. 

## FAQs

### How are electrons in an atom organized? ›

The electrons in an atom are arranged in shells that surround the nucleus, with each successive shell being farther from the nucleus. Electron shells consist of one or more subshells, and subshells consist of one or more atomic orbitals.

How is the atom organized? ›

An atom is a complex arrangement of negatively charged electrons arranged in defined shells about a positively charged nucleus. This nucleus contains most of the atom's mass and is composed of protons and neutrons (except for common hydrogen which has only one proton).

How are the electrons arranged for each element? ›

Moving through the elements in the periodic table, each atom has one more electron than the last because the number of electrons is the same as the atomic number. Electrons occupy the shells in order, starting with the shell that is nearest the nucleus.

Which is the correct order to take when filling in electrons? ›

In order as: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p… 1s will be filled first, with the maximum of 2 electrons. 2s will be filled next, with the maximum of 2 electrons. 2p will be filled next, with the maximum of 6 electrons.

How are protons and electrons arranged? ›

Protons and neutrons are in the center of the atom, and they make up the nucleus. Protons have a positive charge. Neutrons have a neutral charge (no charge). Electrons have a negative charge, and are far away from the nucleus.

How are the 3 parts of an atom arranged? ›

In accordance with the Standard Model of particle physics, protons and neutrons make up the nucleus of the atom, while electrons orbit it in a "cloud". The electrons in an atom are attracted to the protons in the nucleus by the electromagnetic force.

How are electrons arranged in a shell? ›

Each shell can contain only a fixed number of electrons: the first shell can hold up to two electrons, the second shell can hold up to eight (2 + 6) electrons, the third shell can hold up to 18 (2 + 6 + 10) and so on. The general formula is that the nth shell can in principle hold up to 2(n2) electrons.

Which gets filled with electrons first? ›

The Aufbau principle states that electrons fill lower-energy atomic orbitals before filling higher-energy ones (Aufbau is German for "building-up").

Which electrons leave first? ›

The electrons lost first will come from the highest energy level, furthest from the influence of the nucleus. So the 4s orbital must have a higher energy than the 3d orbitals.

Where will the electrons in an atom arrange first? ›

Electrons are typically organized around an atom by starting at the lowest possible quantum numbers first, which are the shells-subshells with lower energies. Consider H, an atom with a single electron only.

### What is the correct order of protons neutrons and electrons? ›

Proton—positive; electron—negative; neutron—no charge. The charge on the proton and electron are exactly the same size but opposite. The same number of protons and electrons exactly cancel one another in a neutral atom.

Why are electrons arranged the way they are? ›

The main idea behind electron arrangements is that electrons can only exist at certain energy levels. By understanding the energy levels of electrons in an atom, we can predict properties and understand behavior of the atom. As shown in the figure below, there are multiple energy levels where electrons can be found.

What are the 4 components of an atom? ›

The atomic mass or weight is measured in atomic mass units. Protons and neutrons make up the nucleus of an atom and the electrons orbit. Electrons contribute only a tiny part to the mass of the atomic structure, however, they play an important role in the chemical reactions that create molecules.

How are protons arranged in an atom? ›

Inside an atom are three types of sub-atomic particles: protons, neutrons and electrons. The proton is a particle that carries a positive electric charge and is arranged in the center of the atom. The center of the atom is called the nucleus. The second type of particle in the nucleus is called the neutron.

How do I remember electron configuration? ›

The more electrons an element has, the more orbitals it will have to fill. This is a memory aid that everyone that has ever taken general chemistry has seen. Follow the diagonals sequential, from tail to head. Remember that s gets two electrons, p gets six electrons, d gets 10 electrons, and f gets 14 electrons.

How do electrons move from shell to shell? ›

By convention, each shell is assigned a number and the symbol n—for example, the electron shell closest to the nucleus is called 1n. In order to move between shells, an electron must absorb or release an amount of energy corresponding exactly to the difference in energy between the shells.

How are electrons arranged in an atom quizlet? ›

When electrons occupy orbitals of equal energy, one electron enters each orbital until all the orbitals contain one electron with parallel spins. An atomic orbital may describe at most two electrons. Electrons enter orbitals of lowest energy first. The most stable arrangement of electrons around the nucleus of an atom.

How are electrons arranged in an atom Quizizz? ›

“Electrons occupy orbitals of the same energy in a way that makes the number of electrons with the same spin direction as large as possible”. “An atomic orbital may describe at most two electrons”. A quantum mechanical property of electrons and may be thought as clockwise and anti-clockwise.

How are electrons arranged in an atom Brainpop? ›

Electrons are arranged in energy levels. Lower energy levels are close to the nucleus and higher energy levels are farther away from the nucleus.

What are the 3 rules that define how electrons are arranged? ›

Definition: The arrangement of electrons in an atom, which is prescribed by three rules – the Aufbau principle, the Pauli exclusion principle, and Hund's rule.

### How are quarks arranged in protons and neutrons? ›

Protons contain two up quarks and one down quark. Neutrons contain one up quark and two down quarks. The nucleus is held together by the "strong nuclear force," which is one of four fundamental fources (gravity and electromagnetism are two others).

How do you sort atoms by size? ›

In the periodic table, atomic radii decrease from left to right across a row and increase from top to bottom down a column. Because of these two trends, the largest atoms are found in the lower left corner of the periodic table, and the smallest are found in the upper right corner (Figure 3.2. 4).

Which of the following best describes what an electron configuration is? ›

Which of the following best describes what an electron configuration is? An electron configuration is the system used to name each principle energy level and the sublevels in which electrons are found within an atom.

How do you explain electrons to a child? ›

Inside an atom, the very small unit found inside an element, there are even tinier particles that move like planets orbiting around the sun. These particles move at such an incredible speed that you cannot even see them under a microscope. We call them electrons, and they are particles charged up with tons of energy.

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