Question

Give an example of a molecule or polyatomic ion that has the following features. (a) four bonded atoms, no unshared electrons on the central atom (b) two bonded atoms, no unshared electrons on the central atom (c) two bonded atoms, one unshared pair of electrons on the central atom

Short answer

The molecules that meet these criteria are (a) methane (CH4), (b) beryllium chloride (BeCl2), and (c) water (H2O).

Step-by-step solution

Example (a)

The first task is to find a molecule or ion with four bonded atoms and no unshared electrons on the central atom. This aligns with the molecular structure of methane (CH4). In methane, carbon is the central atom surrounded by four hydrogen atoms. There are no unshared electrons on carbon as it is utilizing all of its valence electrons in bonds with hydrogen.

Example (b)

The second task is to find a molecule or ion with two bonded atoms and no unshared electrons on the central atom. A molecule that fits this description is BeCl2 (beryllium chloride). Beryllium is the central atom, and it forms covalent bonds with two chlorine atoms. Beryllium does not have any unshared electrons.

Example (c)

The third task is to identify a molecule or ion with two bonded atoms and one unshared pair of electrons on the central atom. This describes water (H2O). In a water molecule, oxygen is the central atom, bonding with two hydrogen atoms. Oxygen also holds one pair of unshared electrons, making its electron domain count three.

Key concepts

Valence Electrons

Valence electrons play a pivotal role in chemical bonding as they are the outermost electrons of an atom and available for forming covalent bonds with other atoms. Understanding valence electrons assists in predicting how atoms will bond and what molecular structure will result. For instance, in methane (CH₄), the carbon atom has four valence electrons, and each hydrogen atom has one valence electron. According to the octet rule, atoms tend to combine in a way that allows them to have eight electrons in their valence shell, thus achieving the electron configuration of a noble gas.
In the case of methane, carbon shares its four valence electrons with four hydrogen atoms, forming four covalent bonds and fulfilling the octet, reaching stability. Beryllium, in BeCl₂, only has two valence electrons, which it shares with two chlorine atoms, forming two covalent bonds and achieving a stable electron configuration in the context of its available electrons.

Molecular Structure

The molecular structure or geometry of a molecule is determined by the spatial arrangement of atoms linked by covalent bonds and influenced by the electron pairs around the central atom. Different molecular structures are governed by the Valence Shell Electron Pair Repulsion (VSEPR) theory, which dictates that electron pairs will arrange themselves as far apart as possible to minimize repulsion.

• In methane (CH₄), the molecule adopts a tetrahedral structure because of the four electron pairs bonding the hydrogen atoms to the central carbon atom.
• Beryllium chloride (BeCl₂) has a linear geometry as the two electron pairs bonding the chlorine atoms to the central beryllium atom repel each other maximally in a straight line.
• Water (H₂O), on the other hand, exhibits a bent structure due to two bonding pairs and an additional pair of non-bonding or 'lone' electrons on the oxygen atom.

By assessing such examples, one can better understand how the valence electron count and distribution around the central atom determines the shape of a molecule, which in turn influences its chemical properties and behavior.

Covalent Bonds

Covalent bonds are strong chemical bonds formed when two atoms share pairs of valence electrons. These shared electrons count toward the octet of each atom involved, thereby stabilizing the molecule.

• In methane, we observe a perfect model of covalent bonding where carbon shares its four electrons with four hydrogen atoms, each bond representing a covalent bond.
• The beryllium chloride molecule exemplifies covalent bonding even though beryllium does not achieve a full octet, demonstrating exceptions to the octet rule.
• Water showcases a situation where covalent bonds and lone pairs of electrons coexist, influencing its molecular geometry to bend slightly as opposed to being linear.

The strength of a covalent bond is determined by the electronegativity of the atoms involved and the degree of electron sharing. The bond length—distance between the nuclei of the two bonded atoms—also impacts the stability and strength of the bond. In these examples, the nature of the covalent bonds significantly affects the physical and chemical properties of the compounds, including their reactivity and interaction with other substances.