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The Octet Rule
- Another way to understand the patterns by which atoms gain or lose electrons is to look at their energy levels.
All the noble gases have filled energy levels. So it is possible to restate the pattern for ion formation in a new
way: Atoms tend to gain or lose electrons so that they end up with completely filled energy levels. Notice that
the noble gases neon and argon each have 8 electrons in their valence energy levels. The Octet Rule (also
called the rule of 8) states that atoms bond in such a way as to have 8 electrons in their valence energy level.
("Oct-" means 8, so an octet is a group of 8.) This is just another way to say that atoms tend to be stable with
full outer energy levels. However, it is a handy rule for figuring out an elements charge or valence by looking
for the octet.
- For example, chlorine is just below fluorine in group 17. All atoms in this family have 7 valence electrons. By
the octet rule, fluorine will have an octet of electrons (8 electrons) in its valence energy level if it gains an
electron to form the F ion. This means that a chlorine atom will also gain one electron, forming the Cl ion.
- The exceptions to the octet rule are hydrogen, lithium, and beryllium. They each need only two electrons in
their valence energy levels because their nearest noble gas, helium, has two electrons.
- The situation is more complicated with transition metals. In the periodic table, these are all the metals from
scandium to zinc inclusive, and any metals directly below them. All metals tend to lose electrons to become more
stable, but it is difficult for atoms to lose more than about three electrons. This is because, every time an electron
is lost, the remaining electrons are held more tightly by the nucleus. Gold, for example, can lose at most three
electrons, to form Au . Depending on the chemical conditions, iron can lose either two or three electrons, to form
Fe or Fe . The elements boron, silicon, and carbon rarely form ions. Predicting the number of electrons transition
metals will lose is difficult. Consult the periodic table for the ion charge for these elements. The first charge given
is the most common.
- The situation is more complicated with transition metals. In the periodic table, these are all the metals from
scandium to zinc inclusive, and any metals directly below them. All metals tend to lose electrons to become more
stable, but it is difficult for atoms to lose more than about three electrons. This is because, every time an electron
is lost, the remaining electrons are held more tightly by the nucleus. Gold, for example, can lose at most three
electrons, to form Au . Depending on the chemical conditions, iron can lose either two or three electrons, to form
Fe or Fe . The elements boron, silicon, and carbon rarely form ions. Predicting the number of electrons transition
metals will lose is difficult. Consult the periodic table for the ion charge for these elements. The first charge given
is the most common.
- The exceptions to the octet rule are hydrogen, lithium, and beryllium. They each need only two electrons in
their valence energy levels because their nearest noble gas, helium, has two electrons.
- For example, chlorine is just below fluorine in group 17. All atoms in this family have 7 valence electrons. By
the octet rule, fluorine will have an octet of electrons (8 electrons) in its valence energy level if it gains an
electron to form the F ion. This means that a chlorine atom will also gain one electron, forming the Cl ion.
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