Question

Using Lewis symbols and Lewis structures, diagram the formation of \(\mathrm{BF}_{3}\) from \(\mathrm{B}\) and \(\mathrm{F}\) atoms, showing valence- shell electrons. (a) How many valence electrons does B have initially? (b) How many bonds F has to make in order to achieve an octet? (c) How many valence electrons surround the \(\mathrm{B}\) in the \(\mathrm{BF}_{3}\) molecule? (d) How many valence electrons surround each \(\mathrm{F}\) in the \(\mathrm{BF}_{3}\) molecule? (e) Does \(\mathrm{BF}_{3}\) obey the octet rule?

Short answer

(a) B has 3 valence electrons initially. (b) F has to make 1 bond to achieve an octet. (c) In the \(\mathrm{BF}_3\) molecule, B is surrounded by 6 valence electrons. (d) Each F in the \(\mathrm{BF}_3\) molecule is surrounded by 8 valence electrons. (e) \(\mathrm{BF}_3\) does not obey the octet rule, as B has an incomplete octet.

Step-by-step solution

(a) Determine the number of valence electrons for B

To find the number of valence electrons of B, we need to look at its position in the periodic table. Boron (B) belongs to Group 13, which means it has 3 valence electrons.

(b) Determine how many bonds F has to make to achieve an octet

Fluorine (F) belongs to Group 17 in the periodic table, meaning it has 7 valence electrons. Since each atom wants to achieve a stable octet (8 valence electrons), F needs to form 1 bond (with 1 more electron) to achieve an octet.

(c) Create the Lewis structure for \(\mathrm{BF}_3\) and count valence electrons around B

To create the Lewis structure for \(\mathrm{BF}_3\), we'll follow these steps: 1. Place the least electronegative atom (B) in the center. 2. Surround B with F atoms, connected by single bonds 3. Distribute the remaining valence electrons as lone pairs on the F atoms The Lewis structure will look like this: ``` F | B-F-B | F ``` In the \(\mathrm{BF}_3\) molecule, there are 3 single bonds (each containing 2 valence electrons) attached to B and no lone pair electrons. Thus, B is surrounded by 6 valence electrons.

(d) Count valence electrons around each F in \(\mathrm{BF}_3\)

In the \(\mathrm{BF}_3\) molecule, each F is connected to B by a single bond and has 3 lone pairs of electrons. Thus, each F has 2 (from the bond) + 6 (from the lone pairs) = 8 valence electrons, achieving an octet.

(e) Determine if \(\mathrm{BF}_3\) obeys the octet rule

The octet rule states that an atom seeks to have a total of 8 valence electrons, either through sharing (covalent bonds) or transferring (ionic bonds). In the \(\mathrm{BF}_3\) molecule, each F atom has achieved an octet with 8 valence electrons. However, the central B atom is surrounded by only 6 valence electrons. Therefore, \(\mathrm{BF}_3\) does not obey the octet rule, as B has an incomplete octet.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons found in an atom and are crucial in determining how the element interacts with others. Every atom seeks stability, which is often achieved when it has a full outer shell of electrons. This behavior can be understood through the concept of valence electrons, which are involved in bond formation.
For example, boron (B), found in Group 13 of the periodic table, has 3 valence electrons. Its chemical characteristics and reactivity are dictated by these electrons.

• It can form covalent bonds by sharing its valence electrons with other atoms.
• In a compound like \(\mathrm{BF}_3\), boron shares its 3 valence electrons with three fluorine (F) atoms, each contributing one valence electron to the bond formation.

Fluorine atoms, with 7 valence electrons (as they belong to Group 17), only need one more to achieve a full octet, making them highly reactive and eager to form bonds.

Octet Rule

The octet rule is a principle used to predict the formation of many molecules by suggesting that atoms seek to complete eight electrons in their valence shell. This rule stems from the observation that elements become chemically stable when they have a full set of eight electrons similar to that of noble gases.
This is crucial for understanding molecular structures such as \(\mathrm{BF}_3\). When discussing the octet rule:

• Fluorine atoms in \(\mathrm{BF}_3\) achieve a full set of eight valence electrons by forming one covalent bond each with boron.
• Boron in \(\mathrm{BF}_3\), however, is an exception to the octet rule. It only manages to surround itself with 6 electrons after bonding, thus completing fewer than 8 valence electrons.

This misalignment with the octet rule illustrates a situation called an 'incomplete octet'. It is noteworthy in chemical bonding, especially for elements like boron, aluminum, and other elements in Groups 3A, 3B, and some transition metals.

Covalent Bonds

Covalent bonds are the connectors in a molecule formed from the sharing of valence electrons between atoms. They allow each atom in the molecule to achieve greater stability by reaching a noble gas electron configuration. The formation of covalent bonds can be demonstrated with Lewis structures, which visually represent the sharing of electrons.
In the case of \(\mathrm{BF}_3\):

• A covalent bond forms between each boron and fluorine atom by sharing one valence electron from each, resulting in 3 single covalent bonds formed.
• Through these bonds, each fluorine atom in \(\mathrm{BF}_3\) achieves an octet, making them stable.
• The central boron atom, though stable enough to exist, remains without an octet, having 6 valence electrons.

This type of bonding showcases how atoms can reach an agreeable electron configuration through sharing. Understanding covalent bonds highlights how molecules are constructed and the stability they aim to achieve through electron sharing.