Problem 119 - Chemistry: An Atoms-Focused Approach

Question

Why do \(\mathrm{C}, \mathrm{N}, \mathrm{O},\) and \(\mathrm{F}\) atoms in covalently bonded molecules and ions have no more than eight valence electrons?

Step-by-Step Solution

Answer

Answer: Carbon, nitrogen, oxygen, and fluorine atoms have no more than eight valence electrons in covalently bonded molecules and ions because their outermost (second) energy level can hold a maximum of eight electrons, with four available orbitals: one 2s orbital and three 2p orbitals. These elements follow the octet rule, which states that atoms tend to form stable compounds with eight electrons in their outermost shell, resembling the electron configuration of noble gases. To satisfy the octet rule and achieve a stable electron configuration, these elements form covalent bonds by sharing electrons.

Step 1: Understand the Octet Rule

The octet rule is a chemical rule of thumb that states that atoms of main-group elements tend to combine in such a way that each atom has eight electrons in its valence shell, giving it the same electron configuration as a noble gas. This rule is a result of the observed stability of noble gas electron configurations which have filled s and p orbitals in their outermost shell.

Step 2: Look at the Electron Configurations

To understand why C, N, O, and F follow the octet rule, we need to examine their electron configurations: - Carbon (C): 1s^2 2s^2 2p^2 (atomic number: 6) - Nitrogen (N): 1s^2 2s^2 2p^3 (atomic number: 7) - Oxygen (O): 1s^2 2s^2 2p^4 (atomic number: 8) - Fluorine (F): 1s^2 2s^2 2p^5 (atomic number: 9) These elements have 4, 5, 6, and 7 valence electrons, respectively, in their outermost shell, which is the second energy level (n=2). In this energy level, there are four available orbitals: one 2s orbital and three 2p orbitals. Each orbital can accommodate two electrons, so the second energy level can hold a maximum of 8 electrons.

Step 3: Understand Covalent Bonding

Covalent bonding occurs when two atoms share electrons in order to achieve a stable electron configuration. By sharing electrons, each atom can effectively "count" the shared electrons as part of its own outer shell, thus satisfying the octet rule. For example, when a carbon atom shares one electron with each of its four neighboring hydrogen atoms to form methane (CH4), both the carbon and hydrogen atoms achieve stable electron configurations.

Step 4: Describe the Octet Limitation for C, N, O, and F

C, N, O, and F are limited to no more than eight valence electrons in covalent molecules and ions because they have only four available orbitals in their outermost (second) energy level: one 2s orbital and three 2p orbitals. Since each orbital can hold a maximum of two electrons, these elements can have a maximum of eight electrons in their outermost shell. Gaining or losing electrons is energetically unfavorable, so these elements form covalent bonds by sharing electrons to satisfy the octet rule and achieve a stable electron configuration.