Video Transcript
Which of the following defines the octet rule? (A) A rule that refers to the tendency of an atom to have empty valence shells. (B) A rule that refers to the tendency of an atom to have eight electrons in the valence shell. (C) A rule that refers to the tendency of electrons to orbit around an atom in energy shells. (D) A rule that refers to the tendency of electrons to reside in the nucleus of an atom.
Given that an octet is a group of eight things, we could immediately guess that answer choice (B) is the correct answer, as this answer choice is the only one that refers to a group of eight. Answer choice (B) is in fact the correct answer to this question, but we haven’t really discussed what the octet rule really is or why atoms have a tendency to have eight electrons in the valence shell.
Since we’ve already identified the correct answer choice, let’s just remove those other answer choices before we begin our discussion. The valence shell is the outermost electron shell in an atom. Having a full valence shell is a very stable electron configuration for an atom. This is why the noble gases, like the element neon, for example, are known for being very stable. All of the noble gases have full valence shells. The valence shell for many common elements, including neon, can hold a maximum of eight electrons, hence the name the octet rule.
However, not all elements have eight electrons in the valence shell. For example, hydrogen and helium’s valence shell can only hold two electrons. Because having a full valence shell is such a stable electron configuration, all atoms have a tendency to gain, lose, or share electrons so that the valence shell is full. Fluorine is an example of an element that tends to gain electrons. Atoms of fluorine have seven electrons in their valence shell. So fluorine atoms have a tendency to gain an electron, which fills the valence shell and forms a fluoride ion.
An element that tends to lose electrons to have a full valence shell is sodium. Sodium has just one electron in its valence shell, so sodium atoms have a tendency to lose that electron to form ions with a charge of one plus. The final way that atoms can fill their valence shell is by sharing electrons to create bonds. For example, fluorine and hydrogen are both one electron away from having a full valence shell. If a bond is created between a hydrogen and fluorine atom, both atoms share their valence electrons, which fills the valence shell for both atoms.
So to wrap this all up, a full valence shell is a very stable electron configuration. Atoms of many elements can hold eight electrons in their valence shell. So atoms have a tendency to gain, lose, or share electrons so that there are eight electrons in the valence shell. The octet rule describes this tendency, and so it was best to find as answer choice (B) a rule that refers to the tendency of an atom to have eight electrons in the valence shell.
