In each of these ions and compounds, an atom violates the octet rule. Identify the atom and explain the deviation from the octet rule: (a) \(\mathrm{BrF}_{3} ;\) (b) \(\mathrm{BH}_{3}\).

Valence electrons are the outermost electrons of an atom and play a pivotal role in chemical bonding and the properties of elements. They are the ones involved in forming bonds with other atoms. Each element in the periodic table has a characteristic number of valence electrons that determines how it will react with other elements. For instance, group 1 elements have one valence electron, making them highly reactive, as they tend to lose that one electron to achieve a stable electron configuration.

Understanding valence electrons is key to grasping the octet rule. The rule suggests that atoms are most stable when they have eight electrons in their valence shell, resembling the electron configuration of noble gases. The drive for stability leads atoms to gain, lose, or share electrons during chemical reactions to complete their valence shell with an octet of electrons.

The electron configuration of an atom describes the distribution of electrons among the various orbitals. Electrons fill orbitals starting from the lowest energy level moving up to higher energy levels. For example, the hydrogen atom has one electron, which it places in the 1s orbital, the lowest energy level. On the other hand, a larger atom like bromine has electrons distributed across several orbitals including s, p, and potentially d orbitals.

Within electron configuration, the valence shell is the outermost electron shell, and it can provide insight into an atom's reactivity and how it will bond with other atoms. The significance of understanding electron configuration lies in anticipating the behavior of atoms during reactions, especially when considering deviations from the octet rule. This rule is a guiding principle rather than a strict law, and knowing the electron configuration helps in understanding why certain atoms do not adhere strictly to it.

While the octet rule serves as a useful guideline, several exceptions exist where atoms do not have eight electrons in their valence shell. The deviations from the octet rule generally fall into three categories:

• Atoms with few electrons: Some atoms, like hydrogen and helium, are stable with fewer than eight valence electrons. Boron often falls into this category, as seen in compounds like BH₃ where it has only six valence electrons.
• Atoms with more than an octet: Elements in Period 3 or higher of the periodic table, such as sulfur and phosphorus, can have more than eight valence electrons. This is possible due to the availability of d orbitals, as illustrated by BrF₃ where bromine has ten valence electrons.
• Atoms with an odd number of electrons: Molecules containing an odd number of total valence electrons, such as nitric oxide (NO), cannot distribute electrons to give each atom eight.

Identifying these deviations is crucial for understanding the full spectrum of chemical bonding. Recognizing that the octet rule is not absolute enables a deeper comprehension of complex molecular structures and reactivity.