Question

Write Lewis structures for the following species, both of which involve coordinate covalent bonding: (a) tetrafluoroborate ion, \(\mathrm{BF}_{4}^{-}\), used in metal cleaning and in electroplating baths (b) boron trifluoride ethylamine, used in curing epoxy resins (ethylamine is \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NH}_{2}\) )

Short answer

The Lewis structure for tetrafluoroborate ion, BF4-, predicts a central boron atom bonded to four peripheral fluorine atoms, with each atom obeying the octet rule. In boron trifluoride ethylamine, the BF3 part has a central boron atom bonded to three peripheral fluorine atoms, whereas the ethylamine part has the nitrogen's lone pair participating in coordinate covalent bonding with the boron atom in BF3.

Step-by-step solution

Writing Lewis Structures for Tetrafluoroborate Ion (BF4-)

In the BF4- ion, boron (B) is the central atom surrounded by four fluorine (F) atoms. Both B and F follow the octet rule, except that B can also have less than eight electrons. Start by identifying the number of valence electrons: B has 3 and each F has 7, for a total of 31. Because the ion has a negative charge of -1, one extra electron must be added, making for a total of 32. Each of the fluorine atoms will form a single bond with boron, using up 2*4=8 electrons. The remaining 32-8=24 electrons are distributed over the fluorine atoms such that each F atom follows the octet rule.

Writing Lewis Structures for Boron Trifluoride Ethylamine (BF3 C2H5NH2)

This molecule contains two parts: boron trifluoride (BF3) and ethylamine (C2H5NH2), which bond together. The BF3 molecule will be the same as in the previous step. In ethylamine, the two carbons form a single bond, the last carbon is linked to three hydrogen atoms and the first carbon is linked to the higher priority nitrogen (N) atom. Two hydrogen atoms connect to the nitrogen atom, while the nitrogen atom's lone pair participates in coordinate covalent bonding with the boron atom in the BF3 molecule.

Verification of Lewis Structures

Check that all the atoms in the structure have full outer electron shells (2 for hydrogen, 8 for other atoms), except for boron, which can be stable with less than 8 electrons. Also, verify that the total charge of the ion/molecule is correct for the first and second parts of the problem respectively.

Key concepts

Coordinate Covalent Bonding

Coordinate covalent bonding, also known as dative bonding, occurs when a pair of electrons from one atom is donated to another atom, usually to fill its octet. Unlike a typical covalent bond where each atom supplies one electron, in coordinate covalent bonding, both electrons come from the same atom.

For example, in the reaction between boron trifluoride and ethylamine, the ethylamine has a nitrogen atom with a lone pair of electrons. The boron atom of BF₃ lacks a full octet and can accept this pair of electrons, forming a coordinate covalent bond. This type of bonding is critical in many biological processes and industrial applications where the transfer of electron pairs allows for the formation of stable complexes.

Significance in Molecular Structures

Coordinate covalent bonding can be identified in Lewis structures by looking for atoms that accept electron pairs without contributing electrons themselves. This is often seen with elements in the center of the structure that have empty orbitals and can accept electron pairs, such as boron in BF₃.

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a crucial role in chemical bonding and reactions. These electrons determine how an atom will interact with others and are often illustrated in Lewis structures, which are diagrams that show the distribution of valence electrons among atoms within a molecule.

For instance, in tetrafluoroborate ion, BF₄⁻, and boron trifluoride ethylamine, the valence electrons dictate how atoms will bond. Boron has three valence electrons, while fluoride has seven. Valence electrons can be shared (as in covalent bonds), transferred (as in ionic bonds), or donated (as in coordinate covalent bonds).

How to Count Valence Electrons

To determine the number of valence electrons in an atom, look at its group number on the periodic table. For example, boron is in group 13, meaning it has three valence electrons. In the case of negatively charged ions, like BF₄⁻, an extra electron is added to the total valence electron count to account for the negative charge.

The Octet Rule

The octet rule is a chemical rule of thumb that states that atoms tend to bond in such a way that they each have eight electrons in their valence shell, giving them the same electronic configuration as a noble gas. This arrangement provides stability to the atom. Hydrogen is an exception, as it is content with two electrons, matching the nearest noble gas helium.

However, there are exceptions to the octet rule. Elements such as boron, which are found in the lewis structures from the exercise, are stable with less than an octet. The concept of the octet rule explains the bonding in BF₃ and BF₄⁻, since fluorine atoms require one additional electron to complete their octet, which they achieve by forming bonds with the boron atom.

Exceptions to the Octet Rule

Several elements don't always follow the octet rule, including hydrogen, helium, lithium, beryllium, and boron. Additionally, third-period and heavier elements can exceed the octet by using their d-orbitals, leading to expanded valence shells, which is a concept not further elaborated in this exercise.