Types of Covalent Bonds and Electron Dot Structures

TL;DR

Covalent bonds form when atoms share electrons to achieve a stable electron configuration, typically an octet. We can show these shared electrons using electron dot structures, which help us visualize how atoms connect. The number of shared electron pairs determines if it's a single, double, or triple covalent bond.

1. The Mental Model

Think of atoms as needing to fill their outermost "shell" of electrons to become happy and stable. They do this by sharing electrons with other atoms, forming a connection, rather than giving or taking them completely.

2. The Core Material

When atoms get together, their main goal is usually to achieve a stable outer electron shell, often with eight electrons (the "octet rule"). For carbon and many other non-metals, they do this by sharing electrons, creating a covalent bond.

The number of electron pairs shared determines the type of covalent bond:

Single Covalent Bond

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In a single bond, two atoms share one pair of electrons. Each atom contributes one electron to that shared pair. You'll see this represented by a single line (–) in structural formulas, or two dots (..) in electron dot structures.

Double Covalent Bond

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A double bond happens when two atoms share two pairs of electrons. Each atom contributes two electrons to the two shared pairs. This is stronger and shorter than a single bond. In structural formulas, it's shown as two parallel lines (=), and in electron dot structures, as four dots (::) between the atoms.

Triple Covalent Bond

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A triple bond is formed when two atoms share three pairs of electrons. This is the strongest and shortest type of covalent bond. It's represented by three parallel lines (≡) in structural formulas, or six dots (::: or two sets of three dots) in electron dot structures.

Electron Dot Structures (Lewis Structures)

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These diagrams show an atom's valence electrons (outermost electrons) as dots. When atoms form a covalent bond, the shared electrons are placed between them, while unshared electrons (lone pairs) are placed on the atom itself. This helps you visualize if each atom has achieved its stable octet (or duet for hydrogen).

Let's look at how atoms bond:

graph TD
    A["Atom 1 (Needs electrons)"] --> B{{"Electron Sharing"}}
    B --> C["Atom 2 (Needs electrons)"]
    C --> D{{"Stable Outer Shell"}}
    D --> E["Covalent Bond Formed"]
    E --> F{"Type of Bond?"}
    F --> G["One Pair Shared (Single Bond)"]
    F --> H["Two Pairs Shared (Double Bond)"]
    F --> I["Three Pairs Shared (Triple Bond)"]

3. Worked Example

Let's draw the electron dot structure for Carbon Dioxide ($\text{CO}_2$).

1. Count total valence electrons:

   • Carbon (Group 14) has 4 valence electrons.
   • Oxygen (Group 16) has 6 valence electrons. There are two oxygen atoms.
   • Total valence electrons = $4 + (2 \times 6) = 4 + 12 = 16$ electrons.

2. Determine central atom: Carbon is less electronegative than oxygen and can form more bonds, so it's usually central. The structure is O-C-O.

3. Place two electrons between bonded atoms to form single bonds:

   • O-C-O. We've used 4 electrons (2 for each single bond).
   • Remaining electrons: $16 - 4 = 12$ electrons.

4. Distribute remaining electrons as lone pairs to outer atoms first to satisfy octets:

   • Each oxygen needs 6 more electrons to get to 8 (2 are already shared).
   • Place 6 electrons on the left oxygen, 6 on the right oxygen.
   • Remaining electrons: $12 - 6 - 6 = 0$.

5. Check octets for all atoms:

   • Left Oxygen: Has 6 lone pair electrons + 2 shared electrons = 8 electrons (octet satisfied).
   • Right Oxygen: Has 6 lone pair electrons + 2 shared electrons = 8 electrons (octet satisfied).
   • Carbon: Has 2 shared electrons (with left O) + 2 shared electrons (with right O) = 4 electrons. Carbon's octet is NOT satisfied.

6. Form double/triple bonds by moving lone pairs from outer atoms to shared positions until central atom (and others) have octets:

   • Since carbon only has 4 electrons, it needs 4 more. Take one lone pair from each oxygen and move it into the bonding region between carbon and that oxygen, creating double bonds.
   • Original: :Ö–C–Ö:
   • Move one lone pair from left O to C: :Ö=C–Ö: (Now 4 shared electrons between C and left O, 4 lone electrons on left O)
   • Move one lone pair from right O to C: :Ö=C=Ö: (Now 4 shared electrons between C and right O, 4 lone electrons on right O)

7. Final check:

   • Left Oxygen: 4 shared electrons + 4 lone pair electrons = 8 electrons. (Octet satisfied)
   • Carbon: 4 shared electrons (left) + 4 shared electrons (right) = 8 electrons. (Octet satisfied)
   • Right Oxygen: 4 shared electrons + 4 lone pair electrons = 8 electrons. (Octet satisfied)

This gives us the electron dot structure for $\text{CO}_2$ with two double bonds.

4. Key Takeaways

• Covalent bonds involve sharing electron pairs between atoms, mostly non-metals.
• The goal of covalent bonding is usually to achieve a stable octet (8 valence electrons) for each atom, or a duet (2 valence electrons) for hydrogen.
• Single bonds share one pair of electrons, double bonds share two pairs, and triple bonds share three pairs.
• Electron dot structures visually represent valence electrons and how they are shared in covalent bonds.
• Lone pairs are unshared electrons that also contribute to an atom's octet.
• Always check that all atoms (except hydrogen, which aims for 2) have 8 electrons around them in the final electron dot structure.
• If the central atom doesn't have an octet after forming single bonds and adding lone pairs to outer atoms, convert lone pairs from outer atoms into multiple bonds.

5. Now Try It

Draw the electron dot structure for $\text{N}_2$ (nitrogen gas). What type of covalent bond does it have? What success looks like: You'll have two nitrogen atoms, each surrounded by 8 electrons (shared and unshared), with a specific number of shared electron pairs between them indicating the bond type.