Question

Which of these atoms cannot serve as a central atom in a Lewis structure: (a) \(\mathrm{O} ;\) (b) \(\mathrm{He} ;\) (c) \(\mathrm{F} ;\) (d) \(\mathrm{H} ;\) (e) \(\mathrm{P}\) ? Explain.

Short answer

(b) Helium (He), (c) Fluorine (F), and (d) Hydrogen (H) cannot serve as central atoms.

Step-by-step solution

- Understand the concept of a central atom

A central atom in a Lewis structure is typically the atom that can form the most bonds. This atom is usually the least electronegative (other than hydrogen) and has available orbitals to accommodate bonding pairs of electrons.

- Analyze each atom's ability to form bonds

Evaluate the valence electrons and bonding capabilities for each atom:(a) Oxygen (O): Oxygen has 6 valence electrons and can form up to 2 bonds.(b) Helium (He): Helium has 2 valence electrons and is a noble gas with a full valence shell, so it doesn't form bonds. (c) Fluorine (F): Fluorine has 7 valence electrons and can form only 1 bond. (d) Hydrogen (H): Hydrogen has 1 valence electron and can only form 1 bond. (e) Phosphorus (P): Phosphorus has 5 valence electrons and can form up to 5 bonds.

- Determine which atoms cannot serve as central atoms

From the analysis in Step 2, identify the atoms that cannot serve as central atoms because they either do not form bonds or can form only one bond. These atoms are Helium (He), Fluorine (F), and Hydrogen (H).

Key concepts

Valence Electrons

Valence electrons are the electrons in the outermost shell of an atom. These electrons play a crucial role in chemical bonding. Atoms with a full valence shell are generally stable and less likely to participate in bonding.

For instance, helium (He) has 2 valence electrons and a full outer shell, making it stable and unlikely to form bonds. Most other elements, however, seek to fill or empty their valence shell by forming bonds with other atoms. This concept is essential in understanding why certain atoms can or cannot be central atoms in a Lewis structure. Central atoms tend to have more room in their valence shell to accommodate multiple bonds.

Bonding Capabilities

Bonding capability refers to an atom's ability to form chemical bonds with other atoms. This ability is largely determined by the number of valence electrons an atom has. Atoms with fewer than four valence electrons often lose electrons to form positive ions, while those with more than four valence electrons tend to gain electrons to form negative ions.

For example:

• Hydrogen (H) has 1 valence electron and can form only one bond.
• Oxygen (O), with 6 valence electrons, can form two bonds.
• Phosphorus (P), with 5 valence electrons, can form up to five bonds.

The central atom in a Lewis structure is usually the one that can form the most bonds, based on its valence electrons. Hence, hydrogen, fluorine (F) with 7 valence electrons, and helium cannot act as central atoms because they cannot form multiple bonds.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons. In a molecule, electronegativity helps determine how electrons are distributed.

Elements with high electronegativity, such as fluorine (F), tend to attract electrons strongly, whereas elements with lower electronegativity, like phosphorus (P), are less effective at attracting electrons. In a Lewis structure, the central atom is usually the least electronegative atom (except for hydrogen).

This is because less electronegative atoms are more willing to share their electrons to form bonds, making them suitable to act as central atoms. Fluorine, being highly electronegative and able to form only one bond, is not suitable as a central atom, unlike phosphorus, which is less electronegative and can form multiple bonds.