Question

For each of the following bonds, draw a figure indicating the direction of the bond dipole, including which end of the bond is positive and which is negative. a. \(\mathrm{Si}-\mathrm{H}\) b. \(P-H\) d. \(\mathrm{Cl}-\mathrm{H}\)

Short answer

For each bond, the bond dipole points from the positive end to the negative end: a. Si-H Bond: Si (positive) → H (negative) b. P-H Bond: P (positive) → H (negative) d. Cl-H Bond: H (positive) → Cl (negative)

Step-by-step solution

Determine electronegativity values

Consult a periodic table or reference material to find the electronegativity values for each element. Electronegativity values are as follows: - Silicon (Si): 1.90 - Hydrogen (H): 2.20 - Phosphorus (P): 2.19 - Chlorine (Cl): 3.16

Identify bond dipole directions based on electronegativity values

Compare the electronegativity values of the bonded atoms to determine which atom has a partial positive charge and which has a partial negative charge. The bond dipole points from the positive end towards the negative end. a. Si-H Bond: Since H (2.20) has a higher electronegativity value than Si (1.90), the bond dipole points from Si to H. So, Si is the positive end, and H is the negative end. b. P-H Bond: Both P (2.19) and H (2.20) have nearly the same electronegativity value. However, since H has a slightly higher electronegativity, the bond dipole points from P to H. So, P is the positive end, and H is the negative end. d. Cl-H Bond: Cl (3.16) has a higher electronegativity value than H (2.20), so the bond dipole points from H to Cl. Thus, H is the positive end, and Cl is the negative end.

Draw figures indicating bond dipole directions

Here are the figures for each bond, with arrows representing bond dipoles pointing from the positive end to the negative end: a. Si-H Bond: Si (positive) → H (negative) b. P-H Bond: P (positive) → H (negative) d. Cl-H Bond: H (positive) → Cl (negative)

Key concepts

Electronegativity

What exactly is electronegativity? It's a measure of the tendency of an atom to attract a bonding pair of electrons. Think of it as a tug-of-war contest between atoms. In the context of our exercise, where students are asked to determine the direction of bond dipoles, electronegativity is fundamental. Here's what you need to know:

Elements in the periodic table have assigned electronegativity values that you can find in most reference materials. These values can range from around 0.7 for elements like francium up to around 4.0 for fluorine, which is considered the most electronegative element.

When two atoms form a bond, the pair of electrons that make up that bond aren't always shared equally. The atom with the higher electronegativity will pull harder on the electrons, gaining a slight negative charge, while the other atom will have a slight positive charge. This is where the bond dipole comes into play. The bond dipole is essentially the direction of this 'tug-of-war' victory. As shown in the step-by-step solution, the bond dipoles for the given exercises point from the less electronegative atom to the more electronegative atom.

Chemical Bonding

How do atoms stick together to form molecules? This is where chemical bonding comes into play, a core concept that every student needs to get a firm grasp on. Chemical bonds can be classified into three main types: ionic, covalent, and metallic.

In our textbook exercise, we are concerned with covalent bonds, where two atoms share one or more pairs of electrons to achieve a more stable electron configuration. However, not all covalent bonds are created equal. If the sharing is equal, as it would be between two atoms of the same element, it is a nonpolar covalent bond. Conversely, when the sharing is unequal due to differences in electronegativity, we get polar covalent bonds, which lead to the formation of bond dipoles.

An illustrative analogy for understanding chemical bonding is two people sharing a blanket. If they share evenly, both are warm and content—nonpolar covalent bond. But if one person keeps pulling the blanket towards themselves, the other gets cold—a classic case of a polar covalent bond, much like our Si-H, P-H, and Cl-H bonds from the exercise.

Polarity of Molecules

Explaining the polarity of molecules is like understanding a group of individuals and their influence on one another. Just as people have different levels of influence in a group, atoms within a molecule can have different levels of electronegativity. If all the atoms in a molecule have equal electronegativity and are arranged symmetrically, the molecule is nonpolar. But let's say one of the atoms is significantly more electronegative, and the structure is not symmetrical: this results in a polar molecule.

The degree of polarity in a molecule has significant consequences on its properties, such as solubility, melting and boiling points, and how it interacts with other molecules. For example, water (H2O) is a polar molecule, which is why it dissolves other polar substances so well. Our exercise helps students visualize molecular polarity on a smaller scale: by drawing bond dipoles, they get a snippet view of the overall 'personality' of the molecule.

By understanding the direction of bond dipoles—like those in the Si-H, P-H, and Cl-H bonds—students will have a decoding ring for predicting molecular behavior, a fundamental skill for mastering chemistry.