Problem 71 - Chemistry Matter and Change

Question

Contact Lenses Soft contact lenses are made of silicon and oxygen atoms bonded together. Create a table listing the atomic and ionic electron configurations, and the atomic and ionic radii for silicon and oxygen. When silicon bonds with oxygen, which atoms become larger and which become smaller? Why?

Step-by-Step Solution

Answer

Silicon becomes smaller, and oxygen becomes larger when bonding due to electron loss and gain, respectively.

Step 1: Determine Atomic Electron Configurations

First, we need to determine the atomic electron configurations for both silicon and oxygen:- **Silicon (Si)**: Atomic number 14. Electron configuration: \(1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^2\).- **Oxygen (O)**: Atomic number 8. Electron configuration: \(1s^2 \, 2s^2 \, 2p^4\).

Step 2: Determine Ionic Electron Configurations

Next, we find the electron configurations for the ions formed by these elements:- **Silicon ion (Si^{4+})**: Loses four electrons, configuration: \(1s^2 \, 2s^2 \, 2p^6\).- **Oxygen ion (O^{2-})**: Gains two electrons, configuration: \(1s^2 \, 2s^2 \, 2p^6\).

Step 3: Find Atomic Radii

Atomic radii need to be listed for both elements: - **Silicon**: Atomic radius is approximately 111 pm. - **Oxygen**: Atomic radius is approximately 60 pm.

Step 4: Find Ionic Radii

Now, list the ionic radii: - **Si^{4+} ion**: Ionic radius approximately 40 pm. - **O^{2-} ion**: Ionic radius approximately 140 pm.

Step 5: Analyze Changes in Atomic Size

Compare atomic and ionic sizes to determine which atoms become larger or smaller upon bonding: - **Silicon**: Becomes smaller from 111 pm (atomic) to 40 pm (ionic). - **Oxygen**: Becomes larger from 60 pm (atomic) to 140 pm (ionic). This occurs because silicon loses electrons and reduces size, while oxygen gains electrons and increases size.

Key Concepts

Atomic Radius

The atomic radius is a critical concept in understanding how atoms behave when forming bonds. It refers to the size of an isolated, neutral atom's electron cloud, which can be tricky to measure precisely because it isn't a fixed quantity but rather a probabilistic boundary where electrons are likely to be found.
The atomic radius is affected by factors such as:

• Atomic number: As you move from left to right across a period in the periodic table, the atomic number increases along with the positive charge of the nucleus. This greater pull causes the electrons to be drawn closer to the nucleus, reducing the atomic radius.
• Electron shielding: As you move down a group, the number of electron shells increases, leading to more shielding of the outer electrons from the nuclear charge. This results in a larger atomic radius.

Understanding the atomic radius helps explain the nature of an element's chemical bonding and reactivity, as seen with silicon and oxygen in contact lenses.

Ionic Radius

The ionic radius is significant when discussing how atoms change size during ion formation. Ions form when atoms gain or lose electrons, resulting in an overall charge change. This affects the size of the ion compared to the original atom.
Tendencies in ionic radii include:

• Cations: When an atom loses electrons to form a cation (positively charged ion), like the silicon ion ( Si^{4+} ), it typically becomes smaller. This is because there are fewer electrons, allowing the nucleus to pull the remaining electrons closer.
• Anions: When an atom gains electrons to form an anion (negatively charged ion), like the oxygen ion ( O^{2-} ), it typically becomes larger. Additional electrons increase electron-electron repulsion within the cloud, expanding the radius.

Understanding ionic radii is crucial when predicting and explaining the properties of compounds, such as their physical state and reactivity.

Bonding

Bonding is the process by which atoms combine to form compounds. The nature of bonding determines the properties of the resulting substance.
Key types of bonding:

• Covalent bonding: Involves the sharing of electron pairs between atoms, usually nonmetals, as seen in silicon-oxygen bonds in contact lenses. Covalent bonds are strong and result in molecules.
• Ionic bonding: Involves the transfer of electrons from one atom to another, resulting in attraction between oppositely charged ions. Despite forming different structures, the bonds in contact lenses highlight how electron sharing and attraction result in durable and flexible materials.

Bonding explains why elements like silicon and oxygen can create stable compounds despite their changes in size and charge, playing a fundamental role in chemical reactions.

Size Change in Atoms

When atoms form ions or compounds, they typically undergo size changes that are important for understanding phenomena like lattice stability and molecule formation.
The reasons for size change are influenced by:

• Electron loss or gain: Silicon ( Si^{4+} ) loses electrons, becoming smaller as the electron cloud contracts. Oxygen ( O^{2-} ) gains electrons, becoming larger due to increased repulsion among added electrons.
• Resulting structure: The ionic structures formed by these size changes affect properties like solubility and melting point in materials. A decrease in size or increase can determine how tightly ions pack together in a solid structure, influencing its overall properties and uses, such as in contact lenses.

These size changes are key to the properties of the compounds formed through atomic bonding, explaining differences in physical and chemical properties.