Problem 19 - Chemistry: The Central Science

Question

(a) Because an exact outer boundary cannot be measured or even calculated for an atom, how are atomic radii determined? (b) What is the difference between a bonding radius and a nonbonding radius? (c) For a given element, which one is larger? (d) If a free atom reacts to become part of a molecule, would you say that the atom gets smaller or larger?

Step-by-Step Solution

Answer

(a) Atomic radii are determined by examining atomic distances, particularly in bonded atoms in a molecule, and using methods like electron diffraction, X-ray diffraction, and spectroscopy. (b) Bonding radius (covalent radius) is the distance between nuclei of two identical atoms joined by a single covalent bond, whereas nonbonding radius (van der Waals radius) is the distance between nuclei of two non-bonded atoms. (c) For a given element, the nonbonding radius is generally larger than the bonding radius. (d) When a free atom reacts to become part of a molecule, it forms chemical bonds and moves closer to other atoms, causing its overall size to decrease.

Step 1: Understand the problem

Atoms are very small and their outer boundary is not a strict line we can measure. Instead, atomic radii are determined based on how atoms behave in certain situations.

Step 2: Examine atomic distance

When two atoms are in a bonded state in a molecule, the distance between their nuclei is the measure of their combined radii. To find the atomic radius, the distance is divided by two. This method mainly applies to covalently bonded atoms.

Step 3: Use other methods

Atomic radii can also be determined using electron diffraction, X-ray diffraction, or spectroscopy methods. These techniques involve studying the behavior of atoms with electromagnetic radiation and provide more accurate results. #b) Bonding radius vs Nonbonding radius#

Step 4: Define bonding radius

Bonding radius, also known as covalent radius, is the distance between the nuclei of two identical atoms joined by a single covalent bond. It represents half the distance between the two bonded atoms.

Step 5: Define nonbonding radius

Nonbonding radius, also known as van der Waals radius, is the distance between the nuclei of two non-bonded atoms. It is the distance at which the attractive and repulsive forces between the atoms balance each other. #c) Comparing bonding and nonbonding radii#

Step 6: Comparison

For a given element, the nonbonding radius is generally larger than the bonding radius. When two atoms are bonded, there's an attractive force between them, pulling them closer together. In contrast, when two atoms are not bonded, there are no attractive forces and they generally stay farther apart. #d) Size change in reaction#

Step 7: Understand the scenario

When a free atom reacts to become part of a molecule, it forms chemical bonds with other atoms.

Step 8: Size change

During bond formation, the electron cloud of the atoms overlaps, and the atoms move closer to minimize the repulsion forces. This causes the overall size of the atom to decrease. So, when an atom becomes part of a molecule, it gets smaller.

Key Concepts

Bonding Radius

The term "bonding radius" essentially refers to how close atoms get when they share electrons in a covalent bond. Imagine two identical atoms coming together and forming a bond by sharing electrons. The bonding radius is half the distance between the nuclei of these two bonded atoms.

• This distance is measured when atoms are connected, like holding hands in a partnership.
• The smaller the bonding radius, the closer the atoms are to one another, meaning a stronger pull from the shared electrons.

The bonding radius can give important insights into how atoms interact, influencing the structure and stability of molecules formed together.

Nonbonding Radius

Nonbonding radius, also known as the van der Waals radius, describes the distance between the nuclei of two neighboring, non-bonded atoms. This concept focuses on how atoms behave when they are part of the same ensemble, but not directly bonded.

• The nonbonding radius tells us how atoms naturally space themselves when not held together by a chemical bond.
• This radius is generally larger than the bonding radius because there are no shared electron pairs to pull them closer.

It represents an equilibrium point where the forces between atoms are balanced – a mix of repulsive forces due to overlapping electron clouds and attractive forces due to van der Waals forces.

Atomic Bonds

Atomic bonds are the forces that hold atoms together in molecules. There are several types of bonds, with covalent bonds being one of the most important forms where atoms share electrons equally or unequally.

• When an atom becomes part of a molecule, it engages in atomic bonding, usually forming covalent bonds where electrons are shared.
• This sharing of electrons causes the electron clouds of the atoms to overlap, leading to smaller atomic sizes within molecules.
• The reduction in size occurs because atoms move closer to minimize repulsion and balance the attraction of their shared electrons.

Understanding atomic bonds helps in predicting molecular shapes and key properties like reactivity and phase of matter in different conditions.