Question

(a) What is meant by the term covalent bond? (b) Give three examples of covalent bonding. (c) A substance \(X Y\), formed from two different elements, boils at \(-33^{\circ} \mathrm{C}\). Is XY likely to be a covalent or an ionic substance? Explain.

Short answer

(a) A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms, creating a stable balance of attractive and repulsive forces. (b) Examples of covalent bonding include: 1. Water (H2O) - covalent bonds between oxygen and hydrogen atoms 2. Oxygen gas (O2) - covalent bond between two oxygen atoms 3. Methane (CH4) - covalent bonds between carbon and hydrogen atoms (c) Substance XY has a boiling point of -33°C, which is relatively low. This suggests that XY is likely a covalent substance, as covalent substances generally have lower boiling points than ionic substances.

Step-by-step solution

Part (a) - Definition of Covalent Bond

A covalent bond is defined as a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs, also known as shared pairs or bonding pairs, create a stable balance of attractive and repulsive forces between the atoms involved.

Part (b) - Examples of Covalent Bonding

Example 1: Water (H2O) - In a water molecule, one oxygen atom shares electron pairs with two hydrogen atoms, creating covalent bonds between them. Example 2: Oxygen gas (O2) - In an oxygen gas molecule, two oxygen atoms share electron pairs with each other, forming a covalent bond. Example 3: Methane (CH4) - In a methane molecule, one carbon atom shares electron pairs with four hydrogen atoms, establishing covalent bonds between them.

Part (c) - Determining if XY is Covalent or Ionic

To determine if the substance XY is likely covalent or ionic, we can use its boiling point as an indicator. Covalent substances generally have lower boiling points, while ionic substances tend to have higher boiling points due to the strong electrostatic forces between their ions. In this case, XY has a boiling point of -33°C, which is relatively low. Therefore, XY is likely to be a covalent substance.

Key concepts

Chemical Bonding

At its most fundamental level, chemical bonding is the force that holds atoms together in compounds. The atom, the smallest unit of matter with unique chemical identity, seeks a stable electron configuration. To achieve stability, atoms will lose, gain, or share electrons to fill their outer energy levels, which often manifests as bonding.

Two major types of chemical bonds exist: ionic and covalent. Ionic bonds are formed through the transfer of electrons from one atom to another, resulting in a relationship between oppositely charged ions. Covalent bonds, in contrast, involve the sharing of electrons between atoms. This sharing can vary from equal (as in the case of diatomic oxygen, O₂) to unequal (as in the case of water, H₂O). Amidst these, other bond types and interactions like metallic and hydrogen bonds also play significant roles in the chemistry of substances.

Electron Sharing

Electron sharing is the essence of covalent bonding and is a mutual sharing of one or more pairs of electrons between two atoms. This sharing allows each atom to attain the electron configuration of a noble gas, achieving a stable electronic arrangement.

For example, in a water molecule, the oxygen atom shares one pair of electrons with each of the two hydrogen atoms, forming two covalent bonds. This leads to a complete outer shell for oxygen, which needs two additional electrons to achieve stability, and also fills the outer shell of hydrogen, which requires just one.

Molecular Structure

Molecular structure refers to the three-dimensional arrangement of atoms within a molecule. It is shaped by a variety of factors, including covalent bonds and the electron pair repulsions. The molecular structure influences the molecule's physical properties, reactivity, phase of matter, and much more.

For instance, methane (CH₄) has a tetrahedral structure, with carbon at the center and hydrogen atoms at the corners of an imaginary tetrahedron. This spatial arrangement is a direct consequence of the electron sharing between carbon and hydrogen atoms and decides how methane will behave in various chemical interactions.

Boiling Point

Boiling point is the temperature at which a liquid changes to a gas. It's a critical physical property used to identify substances and predict their state under different conditions. The boiling point can indicate the type of bonding present in a substance: higher for ionic compounds, lower for covalent compounds, due to the differences in the strength of their intermolecular forces.

For molecules with covalent bonds, intermolecular forces like hydrogen bonding, dipole-dipole interactions, and London dispersion forces are typically weaker than the ionic bonds in salts, leading to lower boiling points. For example, with a boiling point of -33°C, a molecule XY would be expected to consist of covalent bonds due to the relatively low energy required to change its state from liquid to gas.

Ionic versus Covalent Substances

Differentiating between ionic and covalent substances is key in understanding their properties and behaviors. Ionic substances consist of metal and nonmetal ions held together by the strong electrostatic forces of ionic bonds, usually forming crystalline solids with high melting and boiling points.

Covalent substances, on the other hand, are made up of nonmetal atoms bonded by shared electrons. These substances can be gases, liquids, or solids and have lower melting and boiling points compared to ionic substances. They also exhibit a variety of physical properties depending on the polarity of the molecules involved. By examining properties such as boiling points, one can infer if a substance, like XY with its low boiling point, is likely an ionic or covalent compound.