Covalent Bond: Definition

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• A covalent bond occurs when two electrons spend much of their time

  Electrons are constantly moving, but they spend more time in areas of lower potential energy. In other words, areas where the electron is less repulsed by negative charge and more attracted by positive charge. Locations where electrons are more likely to be found are often depicted as probability densities

  Example:

  In a covalent bond, the area of highest probability density is between the bonded atoms

  between two atoms – this is often referred to as ‘sharing’ the electrons. The positively charged nuclei

  The nucleus of every atom contains one or more protons. Since protons carry positive electric charge, all nuclei are positively charged

  Example:

  Hydrogen has the smallest nucleus of any atom, but still has one proton

  of both atoms are attracted to the ‘shared’ electrons which results in both atoms being attracted together
• The electrons localize between atoms because they are attracted to the positive charge of both nuclei and the area of highest positive charge

  Positive electric charge exerts a 'pull' on negative charges. The closer a negative charge is to the positive charge, in this case the nucleus, the stronger the 'pull'. Electrons near two nuclei experience the 'pull' of both nuclei added together

  Example:

  When two hydrogen atoms covalently bond to one another, their electrons localize between the two nuclei. This places the electrons close to both positively charged nuclei at the same time

  is between them
• Multiple covalent bonds can form between the same two atoms permitting double

  Two pairs of electrons (four total electrons) are shared between two atoms. Double bonds are stronger and shorter than single bonds

  Example:

  Oxygen gas consists of sets of two oxygen atoms double bonded together

  and triple

  Three pairs of electrons (six total electrons) are shared between two atoms. Triple bonds are stronger and shorter than double bonds

  Example:

  Nitrogen gas consists of sets of two nitrogen atoms triple bonded together

  covalent bonds
• Although the nuclei of both atoms are drawn towards the shared electrons, they do not come closer than the bond length

  The bond length is the distance between the center of one atom involved in a bond and the center of the other

  Example:

  The bond length of two oxygen atoms bonded together (diatomic oxygen) is 120.75 picometers

  . This is because the positively-charged nuclei repulse each other increasingly as the atoms get closer, as do the electron clouds
• Covalent bonds are classified as polar

  A polar covalent bond is one where the bond electrons spend more time near one atom than the other. This happens because some atoms attract electrons more strongly than other (electronegativity)

  Example:

  In hydrochloric acid (HCl) the bond electrons are more strongly attracted by chlorine and spend most of their time near it

  , non-polar

  The electrons in a non-polar covalent bond form an orbital which is symmetrical between the bonding atoms. This is in contrast to polar covalent bonds

  Example:

  The electrons bonding two oxygen atoms together are as likely to be near either oxygen atom at any moment. This is because both oxygen atoms attract the electrons with equal strength (equal electronegativity)

  and coordinate (dative)

  In coordinate (also known as dative) covalent bonds both electrons in the covalent bond come from the same atom. This is opposed to most covalent bonds where one electron comes from each atom in the bond

  Example:

  When a covalent bond forms between a hydrogen ion and ammonia to form ammonium, both electrons in the bond were orignally in ammonia, making the bond coordinate covalent

  covalent bonds