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Lesson Explainer: Electronic Configurations Science

In this explainer, we will learn how to write electronic configurations for atoms or ions and identify an atom from its electronic configuration.

The energy levels of an atom cannot all hold the same number of electrons. The first level can hold a maximum of two electrons, while the second and third can hold up to eight and eighteen electrons each. The electrons of an atom tend to occupy all possible states in the lowest energy levels before they start to fill higher levels.

Table 1: The maximum number of electrons for the four lowest energy levels of an atom.

Level SymbolLevel Order (𝑛)Maximum Number of Electrons
K12
L28
M316
N432

Definition: Energy Levels

Energy levels are distances from the nucleus that describe the location of electrons.

Example 1: Determining How Many Electrons Are Needed to Fill Energy Levels K and L

In total, how many electrons are needed for both the K and L energy levels to be full?

Answer

The energy levels of an atom are all different and have different abilities to contain electrons. The innermost energy level has the label K and can only hold a maximum of two electrons. The second-innermost energy level has the label L and can contain up to eight electrons. So, we need 10 electrons to fill energy levels K and L. The answer to this question is 10 electrons.

The nitrogen element has an atomic number of seven. So, each nitrogen atom has a total of seven negatively charged electrons. The first two of these electrons fill energy level K as it has the lowest energy state of all levels. The other five electrons are in energy level L as it has the next lowest energy state of all levels.

Figure 1: The distribution of electrons in a nitrogen atom.

Definition: Electronic Configuration

Electronic configuration is the distribution of electrons in energy levels.

Example 2: Determining the Distribution of Five Electrons among the Energy Levels of an Atom

An atom contains 5 electrons. How many of these electrons occupy the L energy level?

Answer

The electrons of an atom tend to occupy all possible states in the lowest energy levels before they start to fill higher energy levels. They will fill the lowest energy first and the higher ones after that. The atom in question has a total of five electrons. So, two of these electrons will be in the lowest energy level, and the remaining three will be in energy level L. The answer to this question is, therefore, 3 electrons.

The sodium atom has an altogether different electronic configuration because it has an atomic number of eleven. It has eleven electrons, and this is too many electrons for energy levels K and L alone. Ten of its electrons can fill the two lowest energy levels K and L. Its last remaining electron must stay in energy level M.

Figure 2: The electronic configuration of a sodium atom.

Example 3: Identifying the Electronic Configuration of an Oxygen Atom

An atom of oxygen has 8 protons in its nucleus. Which energy level diagram shows the correct electronic configuration for oxygen?

Answer

All atoms have no overall electric charge because they have the same number of protons and electrons. An oxygen atom must have eight electrons as it has eight protons. Two of these electrons must fill the innermost energy level with label K. The remaining six electrons must be in the second-innermost energy level with label L. We can use this information to determine that A is the answer.

Elements tend to be chemically active if they have fewer than eight electrons in their outer energy level. Elements like oxygen and fluorine react with many other substances because they do not have eight outer-level electrons. An oxygen atom is reactive because it has six outer-level electrons. A fluorine atom is also reactive because it has seven.

Figure 3: The electronic configuration of a fluorine atom.

Most noble gas atoms are essentially inert because they have eight outer-shell electrons. Neon is a noble gas as it has eight electrons in its outer energy level. Neon gas is chemically stable and cannot react with other substances unless the reaction conditions are incredibly unusual.

Figure 4: The electronic configuration of the neon noble gas atom.

Example 4: Stating the Number of Outer-Shell Electrons in Inert Gas Atoms Like Neon

In general, how many electrons are in the outer level of an atom of an inert gas such as neon?

  1. 4
  2. 10
  3. 12
  4. 6
  5. 8

Answer

Most noble gas atoms are stable and essentially chemically inert because they have eight outer-shell electrons. Neon is a noble gas and does not usually react with other substances because its atoms have eight outer-shell electrons. The answer to this question is the number eight. So, E must be the correct answer.

Argon is another noble gas that has a total of eighteen electrons. Ten of these electrons fill the two lowest energy levels K and L. The remaining eight electrons stay in energy level M. Argon atoms are chemically stable and essentially inert. They do not tend to react with any other substances because they have eight outer-shell electrons.

Figure 5: The electronic configuration of the argon noble gas atom.

Example 5: Identifying the Least Reactive Atom from Electronic-Configuration Diagrams

Which of the following energy level diagrams corresponds to the least reactive atom?

Answer

Atoms tend to be stable and chemically inert if they have eight outer-shell electrons. They tend to be chemically active if they have fewer than eight electrons in their outer shell. Choices A, B, D, and E all have fewer than eight outer-shell electrons. So, they all describe chemically active atoms. Choice C has eight electrons and must, therefore, represent a stable and chemically inert atom. We can use this information to determine that C is the answer to this question.

Helium is the only noble gas that does not have eight outer-shell electrons. Helium atoms are chemically inert but only have two outer electrons rather than the expected eight. Helium atoms are light, small, and incredibly stable. They do not usually react with any other substances.

Key Points

  • Electrons tend to fill energy levels in order of increasing energy states.
  • The electronic configuration is the distribution of electrons in energy levels.
  • Atoms are ordinarily active if they do not have eight outer-level electrons.
  • The noble gas atoms are inert because they have eight outer-level electrons.

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