Why does boron make 3 bonds

For a fourth bond to form, another atom would have to donate two electrons to the bond. This happens in many cases, such as when an amine reacts. In other cases, an atom already bonded may donate electrons, such as in boron trichloride. Finally, borane gas itself can engage in 3-center two electron bonding which is really weird, but provides a full octet when two boron atoms share the bonding electrons in a boron-hydrogen bond. Sign up to join this community. The best answers are voted up and rise to the top.

Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Why does Boron only need 6 valence electrons unlike the standard 8? Ask Question. Asked 5 years, 6 months ago. Active 1 month ago. But the element is flanked on all sides of the table by atoms that take very different bonding strategies. On one side are metals like beryllium, which can give away all their outermost electrons to form ionic bonds.

On the other are so-called main group elements like carbon and nitrogen, which prefer sharing electrons in covalent bonds. Carbon and nitrogen in particular can share up to three of their outer electrons with another atom of the same kind, forming triple bonds. These are stronger than single or double bonds and so result in some of the most stable molecules known. Boron is more aloof than these neighbours, though.

So much so that in its stable solid state at room temperature, it barely even reacts with boiling acid. The two oxygens with the single bonds to sulfur have seven electrons around them in this structure six from the three lone pairs and one from the bond to sulfur.

That is one electron more than the number of valence electrons that oxygen would have on its own, and as such those two oxygens carry a formal charge of Remember that with formal charges, the goal is to keep the formal charges or the difference between the formal charges of each atom as small as possible. The ICl 4 - ion thus has 12 valence electrons around the central Iodine in the 5 d orbitals.

Expanded Lewis structures are also plausible depictions of molecules when experimentally determined bond lengths suggest partial double bond characters even when single bonds would already fully fill the octet of the central atom.

Despite the cases for expanded octets, as mentioned for incomplete octets, it is important to keep in mind that, in general, the octet rule applies. Mike Blaber Florida State University. Exception 1: Species with Odd Numbers of Electrons The first exception to the Octet Rule is when there are an odd number of valence electrons.

That is exactly what is done to get the correct Lewis structure for nitrogen monoxide: Figure 2. Exception 2: Incomplete Octets The second exception to the Octet Rule is when there are too few valence electrons that results in an incomplete Octet. Figure 4 If you look Figure 4, you can see that the fluorine atoms possess extra lone pairs that they can use to make additional bonds with boron, and you might think that all you have to do is make one lone pair into a bond and the structure will be correct.

If we add one double bond between boron and one of the fluorines we get the following Lewis Structure Figure 5 : Figure 5 Each fluorine has eight electrons, and the boron atom has eight as well! This suggests the possibility of a semi-ionic structure such as seen in Figure 6: Figure 6 None of these three structures is the "correct" structure in this instance. Solution 1. Draw connectivities: 3.

Add octets to outer atoms: 4. Does central electron have octet? It has 6 electrons Add a multiple bond double bond to see if central atom can achieve an octet: 6.

The central Boron now has an octet there would be three resonance Lewis structures However In this structure with a double bond the fluorine atom is sharing extra electrons with the boron. Thus, the structure of BF 3 , with single bonds, and 6 valence electrons around the central boron is the most likely structure BF 3 reacts strongly with compounds which have an unshared pair of electrons which can be used to form a bond with the boron:.

Exception 3: Expanded Valence Shells More common than incomplete octets are expanded octets where the central atom in a Lewis structure has more than eight electrons in its valence shell.

A strict adherence to the octet rule forms the following Lewis structure: Figure 12 If we look at the formal charges on this molecule, we can see that all of the oxygen atoms have seven electrons around them six from the three lone pairs and one from the bond with sulfur. If instead we made a structure for the sulfate ion with an expanded octet, it would look like this: Figure 13 Looking at the formal charges for this structure, the sulfur ion has six electrons around it one from each of its bonds.

Draw the connectivities: 3. Add octet of electrons to outer atoms: 4. Practice Problems Draw the Lewis structure for the molecule I 3 -. Draw the molecule ClF 3. The central atom for an expanded octet must have an atomic number larger than what? Draw the Lewis structure for the molecule NO 2. Which Lewis structure is more likely?

This does not mean that the octet rule is useless—quite the contrary. As with many rules, there are exceptions, or violations. There are three violations to the octet rule. Odd-electron molecules A molecule with an odd number of electrons in the valence shell of an atom. Although they are few, some stable compounds have an odd number of electrons in their valence shells. With an odd number of electrons, at least one atom in the molecule will have to violate the octet rule. The Lewis electron dot diagram for NO is as follows:.

Although the O atom has an octet of electrons, the N atom has only seven electrons in its valence shell. Although NO is a stable compound, it is very chemically reactive, as are most other odd-electron compounds. Electron-deficient molecules A molecule with less than eight electrons in the valence shell of an atom.