Problem 59 - Chemistry: The Central Science

Question

Indicate whether each statement is true or false: (a) The octet rule is based on the fact that filling in all \(s\) and \(p\) valence electrons in a shell gives eight electrons. (b) The Si in \(\mathrm{SiH}_{4}\) does not follow the octet rule because hydrogen is in an unusual oxidation state. (c) Boron compounds are frequent exceptions to the octet rule because they have too few electrons surrounding the boron. (d) Compounds in which nitrogen is the central atom are frequent exceptions to the octet rule because they have too many electrons surrounding the nitrogen.

Step-by-Step Solution

Answer

(a) True: The octet rule is based on achieving eight valence electrons by filling the s and p orbitals. (b) False: Si in SiH4 follows the octet rule, as each hydrogen bond contributes an electron for a total of eight valence electrons. (c) True: Boron compounds are often exceptions to the octet rule because they have too few electrons surrounding the boron atom. (d) False: Nitrogen compounds are not frequent exceptions to the octet rule, although rare cases with too many electrons can occur.

Step 1: Statement (a) - Octet Rule

The octet rule states that atoms are most stable when they have a full set of valence electrons. In many cases, this means filling s (2 electrons) and p (6 electrons) orbitals, which makes a total of 8 electrons. Therefore, statement (a) is true.

Step 2: Statement (b) - Si in SiH4

In SiH4, silicon (Si) is bonded to four hydrogen (H) atoms. Each hydrogen atom contributes one electron to form a covalent bond with silicon, giving it a total of four additional electrons in its valence shell. Silicon's ground state has four valence electrons, so with four additional electrons from the hydrogen atoms, it follows the octet rule. Hydrogen in SiH4 does not affect the octet rule's application to silicon (Si). Hence, statement (b) is false.

Step 3: Statement (c) - Boron Compounds

Boron (B) has three valence electrons in its outer shell. In many compounds, it forms three covalent bonds, which means it has a total of six electrons in its valence shell, not eight. Therefore, boron compounds are considered exceptions to the octet rule because they have too few electrons surrounding boron. Statement (c) is true.

Step 4: Statement (d) - Nitrogen Compounds

Nitrogen (N) has five valence electrons in its outer shell. It forms three covalent bonds in many stable compounds, like ammonia (NH3), which gives it eight electrons in total, following the octet rule. However, nitrogen can also sometimes form four or five covalent bonds, in which case there would be too many electrons in the valence shell. These cases are quite rare, and the statement implies that nitrogen compounds are frequently exceptions, which is not entirely accurate. Therefore, statement (d) is false.

Key Concepts

Valence Electrons

Valence electrons are crucial because they determine how atoms interact with each other. These electrons reside in the outermost shell of an atom. They are primarily involved in forming bonds between atoms. Understanding valence electrons helps predict how elements will bond and what compounds they can form.

• For most elements, especially main-group elements, filling the outermost shell with electrons makes them stable.
• For example, oxygen, with six valence electrons, tends to form two bonds to achieve the stable octet configuration.
• Elements in group 18, like neon and argon, naturally have full valence shells, making them non-reactive.

The number of valence electrons varies across the periodic table. For instance, carbon has four valence electrons, sulfur has six, and nitrogen has five. Knowing the number of valence electrons helps us understand and predict chemical behavior.

Covalent Bonds

Covalent bonds are formed when atoms share electrons. This sharing allows atoms to achieve the electron configurations of noble gases, leading to greater stability.

• Each covalent bond typically involves two electrons—one from each atom participating in the bond.
• For instance, a single covalent bond involves two shared electrons, while a double bond involves four.
• Molecules like water (H₂O) and methane (CH₄) are typical examples where covalent bonding occurs.

Atoms form covalent bonds to fulfill the octet rule, where they try to have a total of eight electrons in their valence shell, mimicking the noble gas configuration. In the molecule ewline \( \text{SiH}_{4} \ \ ewline Si \) forms covalent bonds with four \( \text{H} \) atoms, achieving a full outer shell by sharing electrons. This shared family of electrons allows the compound to satisfy the octet rule.

Exceptions

Despite the general rule of achieving a stable octet by sharing electrons, certain elements and compounds do not follow this pattern. These exceptions are key to understanding the broader scope of chemistry.

• Boron, for example, often forms three bonds in compounds like BF₃, resulting in only six valence electrons instead of eight.
• Hydrogen is another notable exception because it seeks to complete a duet (2 electrons), not an octet.
• Furthermore, elements like sulfur and phosphorus can exceed the octet rule due to their ability to expand their valence shell using d-orbitals, making compounds like SF₆ and PF₅ possible.

Such exceptions arise due to the unique electron configurations or the availability of d-orbitals that allow more electrons to be accommodated. In the case of nitrogen, although typically fulfilling the octet rule, it can occasionally exceed eight electrons, although this is less common.