Question Video: Recalling the Second Organizing Principle of the Periodic Table

Which atomic feature do elements in the same period of the periodic table contain an equal number of?

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Video Transcript

Which atomic feature do elements in the same period of the periodic table contain an equal number of?

A period is a row on the periodic table. Let’s examine a few elements from period two. The number in the top of each box indicates the number of protons in an atom of that element. We can see that lithium, carbon, oxygen, and neon all have a different number of protons. The bottom number in each box represents the average atomic mass. We can see that lithium, carbon, oxygen, and neon all have different average atomic masses. In a neutral atom, the number of protons will equal the number of electrons. This means that neutrally charged lithium, carbon, oxygen, and neon will not contain the same number of electrons.

We have examined all of the information that we can gather directly from the periodic table. Now let’s take a look at the electron arrangement for a neutral atom of each of these elements. These are the Bohr models for a neutral atom of each of these elements. In the Bohr model, each circle is an electron shell and each dot represents an electron. More specifically, the electrons on the outermost shell are called the valence electrons.

We already established that each of these neutral atoms contains a different number of electrons. We can now see that they all have a different number of valence electrons as well. All of these atoms do have one similarity: their number of electron shells. We can see that each of these atoms in row two or period two has a first and second electron shell. The number of electron shells an atom contains will be the same for each element in the same period of the periodic table.

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