Write a Lewis structure for each of the following molecules or ions. You may wish to review the chapter on chemical bonding. (a) \(\mathrm{H}_{3} \mathrm{BPH}_{3}\) (b) \(\mathrm{BF}_{4}^{-}\) (c) \(\mathrm{BBr}_{3}\) (d) \(\mathrm{B}\left(\mathrm{CH}_{3}\right)_{3}\) (e) \(\mathrm{B}(\mathrm{OH})_{3}\)

Valence electrons are the outermost electrons of an atom and play a crucial role in chemical bonding. They are the electrons involved when atoms form chemical bonds to become more stable. For instance, in the exercise, boron (B) has 3 valence electrons, reflected in how it bonds with other atoms. Understanding the number of valence electrons in an element is the first step in predicting how it will react and bond with other elements.

Using the periodic table can help you quickly identify an element's valence electrons. For example, the members of the alkali metals group have one valence electron, while the noble gases, except helium, typically have eight. In the case of transition metals, the determination of valence electrons becomes a bit more complex due to d-orbitals. Detailed knowledge of how to count valence electrons is vital for drawing Lewis structures accurately.

Chemical bonding refers to the attraction between atoms that enables the formation of chemical substances containing two or more atoms. The bonds form as a result of the electromagnetism between components. There are two main types of bonds: ionic and covalent. Ionic bonds occur when electrons are transferred from one atom to another, typically between metals and non-metals, leading to the formation of ions. Covalent bonds happen when electrons are shared between atoms, usually between non-metals.

In the exercise examples, like H3BPH3 and BBr3, we notice covalent bonding, where boron shares its valence electrons with hydrogen or bromine atoms to achieve stability. Understanding how atoms bond and the difference in electronegativities that leads to ionic or covalent bonding is fundamental in chemistry and essential for properly predicting molecule structures and properties.

The octet rule states that atoms tend to bond in such a way that they have eight electrons in their valence shell, giving them the same electron configuration as a noble gas. The rule is a helpful guideline in understanding the chemical bonding of atoms in molecules such as H3BPH3 and BF4-. Hydrogen (H) is an exception to this rule since it can achieve stability with a duet of electrons.

While the octet rule is widely applicable, there are exceptions. Some elements can have fewer or more than eight electrons. For example, the boron family, including BH3, often has fewer than eight electrons around the boron atom giving it an incomplete octet and unique properties. Similarly, elements in the third period and beyond can expand their valence shell to accommodate more than eight electrons using d-orbital. Understanding the octet rule and its exceptions is key to predicting the likelihood of bond formation and the resulting molecular structure.