Edexcel A Level Chemistry:复习笔记1.4.6 Covalent Dot-and-Cross Diagrams

Covalent Dot-and-Cross Diagrams

 

Covalent compounds

  • The atoms in covalent compounds will share their outer valence electrons to achieve a noble gas configuration

1.3-Chemical-Bonding-Covalent-Dot-Cross-11.3-Chemical-Bonding-Covalent-Dot-Cross-2-11.3-Chemical-Bonding-Covalent-Dot-Cross-2-21.3-Chemical-Bonding-Covalent-Dot-Cross-3-1

Dot-and-cross diagrams of covalent compounds in which the atoms share their valence electrons

 

Double covalent bonding

Oxygen, O2

 

1.3-Chemical-Bonding-Double-Covalent-Bonding-Oxygen

Covalent bonding in oxygen

 

 

Carbon dioxide, CO2

1.3-Chemical-Bonding-Double-Covalent-Bonding-CO2

Covalent bonding in carbon dioxide

 

Ethene, C2H4

1.3-Chemical-Bonding-Double-Covalent-Bonding-Ethene

Covalent bonding in ethene

 

Triple covalent bonding

Nitrogen, N2

1.3-Chemical-Bonding-Triple-Covalent-Bonding

Covalent bonding in nitrogen

 

Dative covalent bonding 

  • In simple covalent bonds, the two atoms involved share electrons
  • Some molecules have a lone pair of electrons that can be donated to form a bond with an electron-deficient atom
    • An electron-deficient atom is an atom that has an unfilled outer orbital

     

  • So both electrons are from the same atom
  • This type of bonding is called dative covalent bonding or coordinate bonding
  • An example with a dative bond is in an ammonium ion
    • The hydrogen ion, H+ is electron-deficient and has space for two electrons in its shell
    • The nitrogen atom in ammonia has a lone pair of electrons which it can donate to the hydrogen ion to form a dative covalent bond

     

 1.3-Chemical-Bonding-Dative-Covalent-Bonding-Ammonium-ion

Ammonia (NH3) can donate a lone pair to an electron-deficient proton (H+) to form a charged ammonium ion (NH4+)

 

Aluminium chloride

  • Aluminium chloride is also formed using dative covalent bonding
  • At high temperatures aluminium chloride can exist as a monomer (AlCl3)
    • The molecule is electron-deficient and needs two electrons to complete the aluminium atom’s outer shell

     

  • At lower temperatures the two molecules of AlCl3 join together to form a dimer (Al2Cl6)
    • The molecules combine because lone pairs of electrons on two of the chlorine atoms form two coordinate bonds with the aluminium atoms

     

1.3-Chemical-Bonding-Dative-Covalent-Bonding-Aluminium-chloride

Aluminium chloride is also formed with a dative covalent bond in which two of the chlorine atoms donate their lone pairs to each of the aluminium atoms to form a dimer

 

Exam Tip

Covalent bonding takes place between nonmetal atoms. Remember to use the Periodic Table to decide how many electrons are in the outer shell of a nonmetal atom.

 

 

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