Video Transcript
Which of the following best describes a period in the periodic table? (A) A collection of elements in a single column. (B) A collection of elements in a single diagonal. (C) A collection of elements in a single row. (D) All elements with a specific number of neutrons in their nucleus. (E) All nonmetals.
A period is a collection of elements with the same highest occupied electron shell. Electron shells can be represented by circles in a Bohr model of the atom. In this model, the center represents the nucleus, with the first circle around the center representing the first shell. The second circle represents the second shell, and so on. The first shell can hold a maximum of two electrons, with the second and third shells being able to hold up to eight electrons each.
Let’s take a look at the Bohr models of these six elements. We’ll draw the model for oxygen together. Oxygen’s atomic number is eight. The atomic number represents the number of protons in the atom. In a neutral atom, the number of protons and the number of electrons are the same. This means that we should place eight electrons in the Bohr model of this oxygen atom.
We will use dots to represent the electrons. The first shell can hold a maximum of two electrons. This means that we must place six additional electrons in the second shell so that the model has a total of eight electrons. Here are the Bohr models for fluorine, neon, sulfur, chlorine, and argon. Notice that while oxygen, fluorine, and neon have different numbers of electrons and different numbers of electrons in the second shell, they all only contain shells one and two, with the highest occupied shell being shell two. Sulfur, chlorine, and argon also have different numbers of electrons. But they all have the same highest occupied shell, shell number three.
We should be able to see that elements which are in the same row of the periodic table have the same highest occupied shell. This means that elements in the same row can be considered a period.