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# 6.8: Blocks of the Periodic Table

Some people like classical music, others like jazz or country. Music styles change from one period of time to the next, and from one region to another. Each type of music has its own language that describes it. Classical music has a certain structure, style, and content. There are different expressions of classical music—symphony, concerto, sonata. Jazz has a different set of characteristics from classical and different styles of performance. Each type of music can be described and compared to other types on the basis of certain common qualities like notes, chords, and melodic styles.

The elements in the periodic table could be considered to be similar to types of music. Each set of elements has a unique set of properties, with different sets of elements having some common characteristics in terms of electron arrangements. We can see patterns of electronic structure and reactivity in the periodic table that allow us to better understand the behavior of individual elements.

## Periods and Blocks

There are seven horizontal rows of the periodic table, called periods. The length of each period is determined by the number of electrons that are capable of occupying the sublevels that fill during that period, as seen in the table below.

Period Number of Elements in Period Sublevels in Order of Fill
Table $$\PageIndex{1}$$: Period Length and Sublevels in the Periodic Table
1 2 $$1s$$
2 8 $$2s$$ $$2p$$
3 8 $$3s$$ $$3p$$
4 18 $$4s$$ $$3d$$ $$4p$$
5 18 $$5s$$ $$4d$$ $$5p$$
6 32 $$6s$$ $$4f$$ $$5d$$ $$6p$$
7 32 $$7s$$ $$5f$$ $$6d$$ $$7s$$

Recall that the four different sublevels each consist of a different number of orbitals. The $$s$$ sublevel has one orbital, the $$p$$ sublevel has three orbitals, the $$d$$ sublevel has five orbitals, and the $$f$$ sublevel has seven orbitals. In the first period, only the $$1s$$ sublevel is being filled. Since all orbitals can hold two electrons, the entire first period consists of just two elements. In the second period, the $$2s$$ sublevel, with two electrons, and the $$2p$$ sublevel, with six electrons, are being filled. Consequently, the second period contains eight elements. The third period is similar to the second, filling the $$3s$$ and $$3p$$ sublevels. Notice that the $$3d$$ sublevel does not actually fill until after the $$4s$$ sublevel. This results in the fourth period containing 18 elements due to the additional 10 electrons that are contributed by the $$d$$ sublevel. The fifth period is similar to the fourth. After the $$6s$$ sublevel fills, the $$4f$$ sublevel with its 14 electrons fills. This is followed by the $$5d$$ and the $$6p$$. The total number of elements in the sixth period is 32. The later elements in the seventh period are still being created. So while there is a possible number of 32 elements in the period, the current number is slightly less.

The period to which a given element belongs can be easily determined by its electron configuration. For example, consider the element nickel $$\left( \ce{Ni} \right)$$. Its electron configuration is $$\left[ \ce{Ar} \right] \: 3d^8 \: 4s^2$$. The highest occupied principal energy level is the fourth, indicated by the 4 in the $$4s^2$$ portion of the configuration. Therefore, nickel can be found in the fourth period of the periodic table.

Based on electron configurations, the periodic table can be divided into blocks denoting which sublevel is in the process of being filled. The $$s$$, $$p$$, $$d$$, and $$f$$ blocks are illustrated below.

The figure also illustrates how the $$d$$ sublevel is always one principal level behind the period in which that sublevel occurs. In other words, the $$3d$$ sublevel fills during the fourth period. The $$f$$ sublevel is always two levels behind. The $$4f$$ sublevel belongs to the sixth period.

## Summary

• The horizontal rows of the periodic table are called periods.
• The length of a period depends on how many electrons are needed to occupy the sublevels that fill the period.
• Blocks indicate which sublevel is being filled.

## Contributors and Attributions

• CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.

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