Characteristics of Covalent Compounds

TL;DR

Covalent compounds form by sharing electrons, leading to weak intermolecular forces. This results in generally low melting/boiling points and poor electrical conductivity. They are often insoluble in water but soluble in organic solvents.

1. The Mental Model

Imagine atoms holding hands instead of exchanging things. This 'hand-holding' is strong within a molecule, but between molecules, they're just loosely connected. This loose connection explains most of their properties.

2. The Core Material

You've learned that covalent compounds form when atoms share electrons to achieve a stable electron configuration. This sharing creates a strong intramolecular bond (within the molecule). However, the forces between different molecules (intermolecular forces) are generally much weaker. It's these weak intermolecular forces that primarily dictate the physical properties of covalent compounds.

Melting and Boiling Points

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Because the forces between molecules are weak, you don't need much energy to overcome them. Think of it like trying to separate LEGO bricks that are just stacked on top of each other versus bricks that are snapped together. This means most covalent compounds have relatively low melting and boiling points compared to ionic compounds. Many are gases or liquids at room temperature, and solids tend to be soft.

Electrical Conductivity

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For a substance to conduct electricity, it needs free-moving charged particles (ions or free electrons). In covalent compounds, electrons are shared and held tightly within the molecules. There are no free ions or mobile electrons available to carry a charge. Therefore, covalent compounds are generally poor conductors of electricity, both in solid and molten states, and in solution.

Solubility

Covalent compounds tend to dissolve best in solvents with similar bonding characteristics. This is often summarized as "like dissolves like." Generally, polar covalent compounds (like sugar, which has some uneven electron distribution) can dissolve in polar solvents (like water). However, many simple covalent compounds, especially non-polar ones (like oil or methane), are insoluble in water (a very polar solvent) but soluble in organic solvents (which are often non-polar or less polar), such as petrol, alcohol, or ether.

Here's a quick summary of how the bonding affects properties:

graph TD
    A["Covalent Bonding"] --> B{"Electron Sharing"}
    B --> C["Strong Intramolecular Bonds"]
    B --> D["Weak Intermolecular Forces"]
    D --> E["Low Melting/Boiling Points"]
    D --> F["Soft Solids (if solid)"]
    C & D --> G["No Free Ions/Electrons"]
    G --> H["Poor Electrical Conductors"]
    B --> I["Polarity Determines Solubility"]
    I --> J["Insoluble in Water (often)"]
    I --> K["Soluble in Organic Solvents (often)"]

3. Worked Example

Let's compare methane (CH₄) and sodium chloride (NaCl). Methane is a covalent compound. Its melting point is -182.5°C and its boiling point is -161.5°C, making it a gas at room temperature. It doesn't conduct electricity. Sodium chloride, an ionic compound, has a melting point of 801°C and a boiling point of 1413°C. It conducts electricity when molten or dissolved in water. This stark difference in properties clearly shows the impact of weak intermolecular forces in methane versus strong electrostatic forces in ionic NaCl.

4. Key Takeaways

• Covalent compounds are formed by sharing electrons between atoms.
• They have strong bonds within molecules but weak forces between molecules.
• This leads to generally low melting and boiling points.
• They are typically poor conductors of electricity due to the absence of free charged particles.
• Many covalent compounds are insoluble in water but dissolve in organic solvents.
• Their physical state at room temperature is often gas or liquid, or soft solids.
• The strength of intermolecular forces dictates many of their physical properties.

Common Mistakes to Avoid:
- Don't confuse strong intramolecular bonds with weak intermolecular forces; both exist.
- Don't assume all covalent compounds are insoluble in water; some polar ones are.
- Don't think covalent compounds have no electrical conductivity because they have no charges; it's about free-moving charges.
- Don't confuse physical properties (like melting point) with chemical reactivity.

5. Now Try It

Think about water (H₂O) and carbon dioxide (CO₂). Both are covalent compounds. Why is water a liquid at room temperature while carbon dioxide is a gas? Write down your explanation based on what you've learned about intermolecular forces and polarity. What does this suggest about the relative strengths of their intermolecular forces?