Referring to the octet rule, atoms attempt to get a noble gas electron configuration, which is eight valence electrons. Sodium has one valence electron, so giving it up would result in the same electron configuration as neon. Chlorine has seven valence electrons, so if it takes one it will have eight an octet. Chlorine has the electron configuration of argon when it gains an electron. The octet rule could have been satisfied if chlorine gave up all seven of its valence electrons and sodium took them.
In that case, both would have the electron configurations of noble gasses, with a full valence shell. However, their charges would be much higher. Atoms are more stable when they have no charge, or a small charge. Introduction In , Richard Abegg formulated what is now known as Abegg's rule , which states that the difference between the maximum positive and negative valences of an element is frequently eight.
Since there is no 1p subshell, 1s is followed immediately by 2s, and thus level 1 can only have at most two valence electrons. Hydrogen only needs one additional electron to attain this stable configuration, through either covalent sharing of electrons or by becoming the hydride ion :H — , while lithium needs to lose one by combining ionically with other elements. This leads to hydrogen and lithium both having two electrons in their valence shell—the same electronic configuration as helium—when they form molecules by bonding to other elements.
There are also a variety of molecules in which there are too few electrons to provide an octet for every atom. Boron and aluminum, from Group III or 13 , display different bonding behavior than previously discussed.
These atoms each have three valence electrons, so we would predict that these atoms want to bond covalently in order to gain 5 electrons through sharing to fulfill the octet rule. However, compounds in which boron or aluminum atoms form five bonds are never observed, so we must conclude that simple predictions based on the octet rule are not reliable for Group III.
Consider boron trifluoride BF 3. The bonding is relatively simple to model with a Lewis structure if we allow each valence level electron in the boron atom to be shared in a covalent bond with each fluorine atom. In this compound, the boron atom only has six valence shell electrons, but the octet rule is satisfied by the fluorine atoms.
Lewis structure of boron trifluoride : Each pair of dots represents a pair of electrons. When placed between two atoms, the electrons are in a bond. A bond can be drawn as a line between two atoms, which also indicates two electrons. We might conclude from this one example that boron atoms obey a sextet rule.
However, boron will form a stable ion with hydrogen, BH 4 — , in which the boron atom does have a complete octet. In addition, BF 3 will react with ammonia NH 3 , to form a stable compound, NH 3 BF 3 , for which a Lewis structure can be drawn that shows boron with a complete octet. Boron trifluoride-ammonia complex : This covalent compound NH 3 BF 3 shows that boron can have an octet of electrons in its valence level. Compounds of aluminum follow similar trends. Aluminum trichloride AlCl 3 , aluminum hydride AlH 3 , and aluminum hydroxide Al OH 3 indicate a valence of three for aluminum, with six valence electrons in the bonded molecule.
However, the stability of aluminum hydride ions AlH 4 — indicates that Al can also support an octet of valence shell electrons.
Although the octet rule can still be of some utility in understanding the chemistry of boron and aluminum, the compounds of these elements are harder to predict than for other elements. Some elements, most notably nitrogen, can form compounds that do not obey the octet rule. The Octet Rule says that an atom is in its most stable state when it has a full valence shell 8 electrons in its valence shell.
So, atoms take the electrons of other atoms to satisfy the Octet Rule. For example, carbon only has four valence electrons. It can combine with four hydrogen atoms, which each have only one electron, to fill its valence shell. This is how methane, or CH4, is created. What can atoms do to satisfy the octet rule? The same goes for the other halogens. How many electrons does chlorine have?
There are 7 valence electrons for chlorine, and it would have 1 bond and three pairs of dots. Same for bromine Br and iodine I. The noble gasses are noble. They are already born with 8 electrons, generally, so they already obey the octet rule without bonding. The figure above shows neon Ne surrounded by 4 pairs of dots, without any bonds.
The same would apply for Ar, Kr, and Xe. There are more exceptions to the octet rule for the noble gasses. Note that helium He is noble. It is element number 2 with only 2 electrons, so it cannot possibly have an octet of 8 electrons. Still, helium is noble, and elements near helium on the periodic table are stabilized with 2 electrons not an octet of 8.
Speaking of exceptions to the octet rule, apparently somebody made XeF 2 in a fancy lab. Yet it did, apparently twice. Hydrogen is very different than the other elements. Hydrogen is another of the exceptions to the octet rule. It is born with 1 electron. It takes an electron to form a bond. So hydrogen just forms a bond. It is drawn as an H with one stick or line.
No dots showing any hydrogen valence electrons. The hydrogen valence electrons act like helium, element number 2.
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