Chapter 3 - Bonding

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Covalent Bonding

What is a Covalent Bond?

  • The sharing of electrons between two atoms to create a ful outer shell of electrons for both atoms involved in the bond.
  • The nucleus of each atom is attracted to the electron pair, holding the two atoms together.

A Hydrogen Molecule:

  • A covalent bond is represented using dot and cross diagrams.
  • the first electron shell needs two electrons to be complete so each hydrogen atom needs another electron to have a complete shell.
  • Consequently, two hydrogen atoms pair up to share their electrons, meaning that each H atom now has two electrons in its outer shell.
  • This covalent bond is drawn like this:
  • Other examples of covalent bonds are, Hydrogen Chloride, Chlorine, Methane and Ethene.
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Multiple Covalent Bonding

What is Multiple Covalent Bonding?

  • It is exactly the same as a normal single covalent bond except there is more than one spare electron for each atom, meaning that it has to make two or possibly more coalent bonds so that it can have a full outer shell.

Double Covalent Bonding in an Oxygen Molecule:

  • Oxygen has only 6 electrons in its outer shell so it would need 2 more to complete the shell.
  • Consequently, 1 covalent bond would not fill the shell so 2 covalent bonds are needed.
  • One atom joins with another oxygen atom and makes 2 covalent bonds.
  • This molecule would look like this when drawn:
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Ionic Bonding

What is an Ionic Bond?

  • In a covalent bond, the electrons are shared, whereas in an ionic bond, they are taken.
  • One atom gives an electron and one atom takes it. This means that these atoms now have a charge of either -1 or +1.
  • Once they have been charged, the ions can be attracted to the oppositely charged ion.

Ionic Bonding in Sodium Chloride:

  • Sodiun (Na) has oly one electron in its outer shell and Chlorine (Cl) has seven.
  • If these two atoms were to share an electron, Cl would have a full outer shell but Na wouldn't.
  • Therefore it must have an ionic bond.
  • The spare electron from Na is transferred to Cl so that both atoms now have a full outer shell.
  • Na now has a +1 charge because it has lost an electron. Cl now has a -1 charge because it has gained an electron. The new ions then are attracted to each other because they have opposite charges.
  • The molecule can be drawn like this, using a dot and cross diagram:
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Metallic Bonding

What is Metallic Bonding?

  • When atoms bond together to form metal, they lose some electrons, making positive ions.
  • The positive ions are held together in a tight lattice structure due to all the free electrons, known as delocalised electrons.
  • The delocalised electrons are free to move between the metal ions. This is what allows metals to conduct electricity.
  • The more electrons are lost, the stronger the ion made and therefore the stronger the metal is. Metals with stronger ions tend to have higher melting points due to the strong bonds.

Metallic Bonding of Sodium and Magnesium:

  • Sodium loses one electron and therefore is relatively weak and soft with a low melting point.
  • Magnesium loses two electrons so the ions are doubly positive with more electrons in the sea of delocalised electrons. This means that Mg is stronger with a higher melting point.
  • It is key to remember that the metal has no overall charge as all of the electrons are still within the metal, meaning that none are lost or gained.
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