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THINK ABOUT IT

Recall that the chemistry of the elements is closely related to the number of valence electrons in their atoms. The valence electrons are found in the outermost electron shell of an atom.

What does the periodic table indicate about the arrangements of electrons?

To answer this question, you will explore

EXPLORING THE TOPIC

Subshells in Atoms

Electrons are arranged into shells numbered n = 1, 2, 3, and so on. The number of electron shells in an atom is the same as the number of the period where the element is located on the periodic table. Each shell has a maximum number of electrons. For instance, the n = 2 shell cannot have more than 8 electrons.

Scientific evidence has led chemists to propose that electron shells are further divided into electron subshells. Imagine magnifying the basic atomic model and finding that each shell is composed of subshells. Notice that the number of subshells that a shell has is equal to n.

THE s, p, d, AND f SUBSHELLS

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The subshells have special names. They are called the s, p, d, and f subshells. Just like the basic shells, each subshell has a maximum capacity of electrons. The s subshells can have a maximum of 2 electrons, p subshells can have a maximum of 6 electrons, and d subshells can have a maximum of 10 electrons. Finally, f subshells can have a maximum of 14 electrons.

Notice that the name of each subshell is labeled using both the basic shell number and the subshell letter (1s, 2s, 2p, and so on).

Electron Configurations

It can be time-consuming to draw subshell models of the atoms to show the arrangements of the electrons, especially for atoms with large atomic numbers. Chemists have developed a shorthand notation called an electron configuration to keep track of the electrons in an atom. The electron configurations for the first ten elements are shown here.

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Each subshell is written using the shell number and the subshell letter. In addition, the number of electrons in each subshell is indicated with a superscript number.

Notice that the superscript numbers add up to the total number of electrons for that atom.

The sequence in which electrons fill up the subshells is 1s, 2s, 2p, 3s, 3p. After the element argon, the pattern changes slightly.

Connecting the Periodic Table to Electron Arrangements

An outline of the periodic table appears here with color-coding to show the subshell for the outermost electron of each element. For example, any element located in the green area will have its outermost electron(s) in a p subshell.

As you proceed across the periodic table from one element to the next, one additional proton and one additional electron are added, along with one or more neutrons. Each additional electron goes into a specific subshell. If an atom is located in the orange areas of the table, the last electron is placed into an s subshell. If an atom is located in the blue area of the table, the last electron is placed into a d subshell. And so on.

The elements in each block have related properties. The elements in the s-block are reactive metals. The elements in the d-block tend to form colorful compounds that are used as pigments. The elements in the p-block tend to form colorless compounds.

DECODING THE TABLE

To write out an electron configuration for a specific element, you can simply “read” from the periodic table, moving across from left to right and then down to the next row. For example, the sequence of subshells for argon, Ar, is 1s, 2s, 2p, 3s, 3p. The electron configuration for argon is 1s²2s²2p⁶3s²3p⁶.

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Everything runs smoothly until you reach the fourth row of the periodic table. After argon, you might expect the next electron to be in the 3d subshell. However, this does not happen. The next element is potassium, K. Like the other elements in Group 1A, potassium has one electron in the s subshell. So, you place an electron in 4s before 3d. The electron configuration for potassium is 1s²2s²2p⁶3s²3p⁶4s¹. The electron configuration for arsenic is 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p³.

You may have noticed that you only have to look at the ending of each electron configuration to figure out the identity of the element associated with it. The ending provides you with the exact spot on the periodic table where you can find the element.

Noble Gas Shorthand

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Depending on the element, the electron configuration can be lengthy to write. Plus, each element just repeats the electron configuration of the previous element but adds one more electron.

Rather than repeat the same thing every time, chemists have devised a quicker way to write out electron configurations. They use the noble gas at the end of each period as a placeholder to symbolize all of the filled subshells before that place on the table. Using this “shorthand” method, the electron configuration of cobalt is [Ar]4s²3d⁷.

Shorthand notation allows you to make some interesting comparisons. Notice that the noble gas shorthand notation emphasizes the valence electrons. Using this method, it is easy to see that each element in Group 2A has two valence electrons, both located in an s subshell.

LESSON SUMMARY

What does the periodic table indicate about the arrangements of electrons?

Electrons in atoms are arranged into basic shells labeled n = 1, 2, 3, and so on. These shells are divided into subshells. The number of subshells in each shell is equal to n. The subshells are referred to as s, p, d, and f subshells. The s, p, d, and f subshells can have a maximum of 2, 6, 10, and 14 electrons, respectively. Chemists use electron configurations to specify the arrangements of electrons in subshells. The periodic table provides the information needed to write electron configurations.

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