Question

How many lone pairs are on the underlined atoms in these compounds: \(\mathrm{HBr}, \mathrm{H}_{2} \underline{\mathrm{S}}, \underline{\mathrm{C}} \mathrm{H}_{4} ?\)

Short answer

Sulfur has 1 lone pair; carbon has 0 lone pairs.

Step-by-step solution

Identify the Valence Electrons

To determine the number of lone pairs on the underlined atoms, we first need to know the number of valence electrons for each atom:- Sulfur (\(\mathrm{S}\)): Located in group 16, sulfur has 6 valence electrons.- Carbon (\(\mathrm{C}\)): Located in group 14, carbon has 4 valence electrons.

Determine the Number of Bonding Electrons

We need to determine how many of the valence electrons are involved in bonding.- Sulfur (\(\mathrm{S}\)) in \(\mathrm{H}_2\underline{\mathrm{S}}\) forms two single bonds with hydrogen.- Carbon (\(\mathrm{C}\)) in \(\underline{\mathrm{C}}\mathrm{H}_4\) forms four single bonds with hydrogen.

Calculate Lone Pairs for Sulfur

For sulfur in \(\mathrm{H}_2\underline{\mathrm{S}}\):- Total valence electrons = 6- Electrons in bonds (2 H-S bonds) = 2 bonds \(\times 2 \text{ electrons/bond} = 4 \text{ electrons}\)- Lone pair electrons = \(6 - 4 = 2\) electrons.Therefore, sulfur has 1 lone pair (\(\frac{2}{2}\) pairs).

Calculate Lone Pairs for Carbon

For carbon in \(\underline{\mathrm{C}}\mathrm{H}_4\):- Total valence electrons = 4- Electrons in bonds (4 C-H bonds) = 4 bonds \(\times 2 \text{ electrons/bond} = 8 \text{ electrons}\).Since carbon shares all its electrons with hydrogens in bonds, it has 0 lone pairs.

Key concepts

Valence Electrons

Valence electrons are the electrons found in the outermost shell of an atom. They are crucial for chemical reactions and bonding because they can be gained, lost, or shared to create bonds with other atoms. The number of valence electrons is determined by the group number of the element in the periodic table. For example, sulfur is in group 16, meaning it has 6 valence electrons, while carbon, in group 14, has 4 valence electrons. Understanding this helps us predict how atoms interact in chemical bonding.
Valence electrons guide the formation of bonds, as atoms tend to achieve a stable electron configuration, often seen in noble gases. This is known as the octet rule, which suggests that atoms are most stable when they have eight electrons in their valence shell. Knowing the number of valence electrons is key to drawing Lewis structures and determining lone pairs.

Bonding Electrons

Bonding electrons are the valence electrons involved in forming chemical bonds between atoms. These are typically shared in covalent bonds or transferred in ionic bonds. For covalent bonds, like those in (CH_{4}) and (H_{2}S), the involved electrons are shared evenly between the elements.
In our examples: - Sulfur in (H_{2}S) uses two of its six valence electrons to form single bonds with two hydrogen atoms, meaning four electrons are engaged in bonding. - Carbon in (CH_{4}) uses all four of its valence electrons to form single bonds with four hydrogen atoms, engaging eight electrons in total bonding.
Bonding electrons directly impact the molecule's shape and physical properties since they determine how atoms stick together once bonded. Identifying these electrons assists in understanding molecular geometry and properties.

Lewis Structure

A Lewis structure is a simple way to represent the distribution of electrons in a molecule. It shows how atoms are bonded together and how valence electrons are arranged around them. In Lewis structures, dots represent electrons, and lines represent covalent bonds.
To draw a Lewis structure, you must:

• Identify each atom's valence electrons.
• Predict how many electrons will be shared to form bonds.
• Display any remaining unshared electrons as lone pairs.

This representation helps visualize the molecular arrangement and predict the chemical properties. For example, the Lewis structure of (H_{2}S) would show two single bonds with hydrogens and two lone pairs on sulfur, confirming its tetravalent nature. Meanwhile, (CH_{4}) shows a symmetric arrangement without lone pairs.

Chemical Bonding

Chemical bonding refers to the force that holds atoms together within a molecule. There are different types of bonds, including covalent, ionic, and metallic. In the examples discussed, both (CH_{4}) and (H_{2}S) involve covalent bonds where electrons are shared between atoms.
Covalent bonds occur when atoms share electrons to complete their valence shells, adhering to the octet rule. For instance:

• (CH_{4}) forms a stable tetrahedral shape as carbon shares electrons equally with four hydrogen atoms.
• (H_{2}S) forms an angular shape as sulfur shares electrons with two hydrogen atoms, making it more polar due to differing electronegativities.

Understanding chemical bonding is fundamental to predicting molecule behavior, reactivity, and interaction, as these bonds determine how molecules interact with their environment and other chemicals.