Please help me understand how to use a diagram to identify which period of elements is most likely to have the strongest intermolecular forces.

Question

Please help me understand why period 5 is the right answer to the question - use a diagram to identify which period of elements is most likely to have the strongest intermolecular forces.

Answer 1

Intermolecular forces hold particles like atoms and molecules together. Kinetic energy shakes the particles apart. Boiling point is an easy way to indirectly measure the intermolecular forces because it is the process of separating particles into a gas phase. Stronger intermolecular forces require more kinetic energy to separate the particles - so higher boiling points represent stronger intermolecular forces. In the diagram, the highest boiling points have the greatest intermolecular forces.

There are three basic types of intermolecular forces: Hydrogen bonding, Dipole-Dipole forces, and London Dispersion forces. London Dispersion Forces (LDF) increase as the number of electrons increase in the particle. This is due to random movements of electrons creating temporary dipoles in the electron clouds.

The diagram is showing periods and groups of elements. Water has one of the highest boiling points because of its hydrogen bonding - a very strong attraction. After water, we can see the boiling point go down with the next element (loss of hydrogen bonding) but then a gradual increase over the next set of elements. Each line going across is tracking elements in the same group (up and down): top line - H₂O, H₂S, H₂Se, H₂Te - O, S, Se and Te are in the same group. As you increase periods (rows) in a group, the atomic number is increasing as well as the number of electrons. So, Period 2 (sulfur) has fewer electrons than Period 5 (Te) thus Period 5 will have the greatest LDF attractions albeit not as great as a particle with hydrogen bonding.

Answer: More electrons with each increasing period of the Periodic Table = greater LDF intermolecular forces.

Answer 2

The graph appears to be a boiling point trend of hydrogen compounds of elements across different periods. Since boiling point correlates with intermolecular forces (higher boiling point = stronger intermolecular forces), we can use this data to infer the strongest intermolecular forces.

Why Period 5?

1. Boiling Point Trends

The graph shows an increasing trend in boiling points for Period 5 elements (such as tellurium in H2Te) compared to lower periods.

Elements in Period 5 have larger, more polarizable electron clouds, leading to stronger London dispersion forces, which dominate in heavier molecules.

Hydrogen bonding effects also play a role but are strongest in lighter elements (like Period 2 for HF, NH, and H2O). However, for heavier elements, dispersion forces become the dominant factor.

2. Comparison with Other Periods:

Period 2 (e.g., HF, NH, H2O) has strong hydrogen bonding, which leads to high boiling points, but dispersion forces are weak because of the smaller size.

Period 3 and 4 (e.g., H‚2S, H2Se) have moderate boiling points, but their intermolecular forces are weaker than those in Period 5 due to less polarizability.

3. Lastly

Period 5 elements, such as tellurium compounds (H2Te), experience the strongest combined intermolecular forces (dipole-dipole, London dispersion), leading to the highest boiling points in their trend.

Definitively:

Period 5 is identified as having the strongest intermolecular forces because the diagram shows that its hydrogen compounds have the highest boiling points due to strong London dispersion forces.