Unit 4 Covalent Bonding Review

What is the oxidation number on the chlorine atom in chlorate?

• -1

• -2

• -3

• +1

• +5

Answer explanation

In chlorate (ClO3-), the oxidation state of oxygen is -2 (3 oxygens contribute -6). To balance the -1 charge of the ion, chlorine must have an oxidation number of +5. Thus, the correct answer is +5.

Which formula represents a substance that contains covalent bonds?

• LiCl

• CaCl₂

• K₂O

• SiO₂

Answer explanation

SiO₂ represents silicon dioxide, which contains covalent bonds between silicon and oxygen atoms. In contrast, LiCl, CaCl₂, and K₂O are ionic compounds, formed by the transfer of electrons.

3. Bonds in molecular compounds are __, which means that electrons are __.

• ionic, shared

• covalent, shared

• ionic, lost or gained to form ions

• covalent, lost or gained to form ions

Answer explanation

Bonds in molecular compounds are covalent, meaning that electrons are shared between atoms. This distinguishes them from ionic bonds, where electrons are lost or gained to form ions.

In a nonpolar covalent bond, electrons are

• transferred from one atom to another

• located in a mobile “sea” shared by many ions

• shared equally by two atoms

• shared unequally by two atoms

Answer explanation

In a nonpolar covalent bond, electrons are shared equally by two atoms, meaning there is no significant difference in electronegativity between the atoms involved.

Which of these shows the molecule with the correct number of sigma and pi bonds?

• hydrogen, 1 sigma, 0 pi

• chlorine, 1 sigma, 2 pi

• oxygen, 1 sigma, 3 pi

• fluorine, 1 sigma, 1 pi

Answer explanation

Hydrogen has one single bond, which is a sigma bond, and no pi bonds. Therefore, the correct choice is hydrogen, 1 sigma, 0 pi, as it accurately represents the bonding in a hydrogen molecule.

This shape will ONLY form polar molecules

• linear

• tetrahedral

• trigonal planar

• trigonal pyramidal

Answer explanation

The trigonal pyramidal shape has a lone pair of electrons, creating an asymmetrical distribution of charge. This results in a dipole moment, making it a polar molecule. Other shapes like linear and tetrahedral can be nonpolar.

This shape will ONLY form polar molecules

• linear

• tetrahedral

• trigonal planar

• bent

Answer explanation

The bent shape results from a central atom with lone pairs, creating an uneven distribution of charge. This leads to a polar molecule. In contrast, linear, tetrahedral, and trigonal planar shapes can be nonpolar if symmetrical.

Which of the following compounds is an exception to the octet rule?

• BH₃

• CH₄

• NH₃

• H₂O

Answer explanation

BH₃ is an exception to the octet rule because it has only six electrons around the boron atom, making it electron-deficient. In contrast, CH₄, NH₃, and H₂O follow the octet rule, having complete octets.

“Like dissolves like” means __ solutes dissolve in __ solvents. Choose all that apply

• polar, polar

• nonpolar, nonpolar

• polar, nonpolar

• nonpolar, polar

Answer explanation

"Like dissolves like" means that polar solutes dissolve in polar solvents and nonpolar solutes dissolve in nonpolar solvents. Therefore, the correct choices are polar, polar and nonpolar, nonpolar.

Which of the following has an influence on the physical properties of molecular covalent compounds?

• shape of the molecule

• type of intra-molecular bonding (polar v. nonpolar)

• molecular mass

• all of these influence physical properties

Answer explanation

All listed factors—shape, type of bonding, and molecular mass—affect the physical properties of molecular covalent compounds, such as boiling point and solubility, making 'all of these influence physical properties' the correct choice.