IB DP Chemistry: HL复习笔记14.1.1 Covalent Bonds

Bond overlap in covalent bonds

• A single covalent bond is formed when two nonmetals combine
• Each atom that combines has an atomic orbital containing a single unpaired electron
• When a covalent bond is formed, the atomic orbitals overlap to form a combined orbital containing two electrons
  • This new orbital is called the molecular orbital
• The greater the atomic orbital overlap, the stronger the bond
• Sigma (σ) bonds are formed from the head-on/ end-to-end overlap of atomic orbitals
• The electron density is concentrated between the two nuclei
• S orbitals overlap this way as well as p to p, and s with p orbitals

Sigma orbitals can be formed from the end-on overlap of s orbitals

Hydrogen fluoride has sigma bonds between s and p orbitals

Fluorine has sigma bonds between p orbitals

•  The electron density in a σ bond is symmetrical about a line joining the nuclei of the atoms forming the bond
• The pair of electrons is found between the nuclei of the two atoms
• The electrostatic attraction between the electrons and nuclei bonds the atoms to each other

π bonds

• Pi (π) bonds are formed from the sideways overlap of adjacent p orbitals
• The two lobes that make up the π bond lie above and below the plane of the σ bond
• This maximises overlap of the p orbitals
• A single π bond is drawn as two electron clouds one arising from each lobe of the p orbitals
• The two clouds of electrons in a π bond represent one bond containing two electrons

π orbitals are formed by the end-on overlap of p orbitals

Examples of sigma & pi bonds

Ethene

• Each carbon atom uses three of its four electrons to form σ bonds
• Two σ bonds are formed with the hydrogen atoms
• One σ bond is formed with the other carbon atom
• The fourth electron from each carbon atom occupies a p orbital which overlaps sideways with another p orbital on the other carbon atom to form a π bond
• This means that the C-C is a double bond: one σ and one π bond

Overlap of the p orbitals results in the forming of a π bond in ethene

Ethyne

• This molecule contains a triple bond formed from two π bonds (at right angles to each other) and one σ bond
• Each carbon atom uses two of its four electrons to form σ bonds
• One σ bond is formed with the hydrogen atom
• One σ bond is formed with the other carbon atom
• Two electrons are used to form two π bonds with the other carbon atomEthyne has a triple bond formed from two π bonds and one σ bond between the two carbon atoms

• Whether sigma (σ) or pi (π) bonds are formed can be predicted by consideration of the combination of atomic orbitals 

Answer

• • Nitrogen contains a triple bond and a lone pair on each nitrogen atom
  • Nitrogen atoms have the electronic configuration 1s²2s²2p³
  • The triple bond is formed from the overlap of the s orbitals on each N to form a σ bond and the overlap of two sets of p orbitals on the nitrogen atoms to form two π bonds
  • These π bonds are at right angles to each other

The triple bond is formed from two π bonds and one σ bond

• • Hydrogen cyanide contains a triple bond
  • One σ bond is formed between the H and C atom
  • A second σ bond is formed between the C and N atom
  • The remaining two sets of p orbitals of nitrogen and carbon will overlap to form two π bonds at right angles to each other

    Hydrogen cyanide has a triple bond formed from a σ bond and the overlap of two sets of p orbitals of nitrogen