Atoms and molecules may seem too tiny to study and understand. Despite their miniscule size, however, scientific studies have revealed much about their behavior, including how atoms combine to form molecules. Over time, these studies have led to the octet rule.
Defining the Octet Rule
The octet rule says that many elements share an octet (8) of electrons in their valence (outermost) electron shell when they form compounds. A formal definition of the octet rule, from Northwestern University, states that "Atoms will lose, gain or share electrons to achieve the electron configuration of the nearest noble gas (8 valence electrons except for He with 2)." Remember that "He" represents helium.
Helium is stable with its two electrons so, like the other noble gases, helium doesn't usually combine with other elements. Elements closest to helium (hydrogen, lithium and beryllium) gain or lose electrons so that only two electrons remain in the outer electron shell. This caveat is sometimes listed as an exception to the octet rule, sometimes considered part of the octet rule and sometimes called the duet rule.
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Lewis Dot Diagrams
Lewis dot diagrams represent the number and relative positions of valence electrons. For example, the helium Lewis dot structure shows two valence electrons and is written as :He. The Lewis dot diagram for oxygen, which has six valence electrons, could be written as :Ö: while the beryllium Lewis dot diagram could be written as :Be: because beryllium has four valence electrons.
Lewis dot diagrams help visualize how atoms share electrons in compounds. For example, hydrogen (H) atoms only have one electron. The Lewis dot diagram .H shows one dot before the symbol H. Hydrogen gas tends to travel in pairs, however, so the hydrogen molecule Lewis dot diagram (H:H) shows the two atoms sharing electrons. The connection between the two atoms can be shown as a dash instead of dots. The chemical shorthand representing this linking of atoms looks like this: H.+.H = H:H or H-H.
How to Use the Octet Rule
The octet rule states that atoms will share or borrow electrons in order to reach the number of valence electrons of the nearest noble gas.
The cation is the element looking to lose electrons. These elements are in Groups I-IV on the periodic table. Group I can lose or share one electron, Group II will lose or share two electrons and so on.
The anion is the atom looking to gain electrons. These elements are in Groups IV-VII on the periodic table. Group IV will gain or share four electrons, Group V will gain or share three electrons, Group VI can gain or share two electrons and Group VII can gain or share one electron.
Hydrogen (Group I) has one electron, so the Lewis dot diagram shows .H with one dot before hydrogen’s symbol H. Oxygen (Group VI) has six electrons, so the Lewis dot diagram shows :Ö: with six dots spaced around oxygen’s symbol O.
Consider hydrogen (Group I) and oxygen (Group VI). The oxygen molecule with its six electrons wants two more electrons. Hydrogen has one valence electron and wants two valence electrons. When hydrogen and oxygen combine to make water, the oxygen borrows the electrons from two hydrogen atoms. In Lewis dot format, the water molecule looks like H:O:H with additional pairs of dots above and below the oxygen symbol (O) to show a total of eight electrons surrounding the O and a pair of electrons for each hydrogen (H) atom. Both oxygen and hydrogen now have complete outer valence shells.
Visualizing With the Octet Rule
The octet rule helps visualize how atoms and molecules combine by looking at how they share electrons. For example, carbon dioxide forms a stable molecule by sharing electrons between one carbon atom (Group IV) and two oxygen atoms (Group VI). The carbon and oxygen atoms combine by sharing a pair of electrons. The Lewis dot diagram shows the shared pair of electrons as doubled dots between atoms, written as :Ö::C::Ö: (or :Ö=C=Ö:). Examining the Lewis dot diagram shows that each element symbol has eight valence electrons, an octet, around each atom.
Exceptions to the Octet Rule
Besides the duet version of the octet rule, two other exceptions to the octet rule sometimes occur. One exception occurs when elements in Rows 3 and beyond exceed the eight valence electrons of the octet rule. The other exception occurs with Group III elements.
Group III elements have three valence electrons. The boron Lewis dot structure shows boron valence electrons forming a triangle .Ḃ. because the negatively charged electrons repel or push away from each other. For boron to chemically combine with hydrogen, an octet requires five hydrogen atoms. This molecule, however, is impossible due to the number and spacing of the negative charges of the electrons. A highly reactive molecule forms when boron (and other Group III elements) shares electrons with only three hydrogen atoms, forming the compound BH3, which only has six valence electrons.
TL;DR (Too Long; Didn't Read)
Some periodic tables label the groups differently. Group I is labeled Group 1, Group II is Group 2, Group III is Groups 3 through 12, Group IV is Group 13, Group V is Group 14, and so on with Group VIII labeled as Group 18.