Learning Objective

In this lesson we will learn about the different types of chemical bonds that can exist between atoms and the different types of chemical structures atoms can form.

Learning Outcomes

By the end of this lesson you will be able to:

  • Describe metallic, ionic and covalent bonding.

  • Describe properties of monatomic, metallic, ionic, covalent molecular and covalent network structures.

  • Identify the type of chemical bonding in different elements and compounds.

  • Explain how the type of bonding and structures an element forms is related to its location on the periodic table.

 

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Introduction

  • Most atoms do not exist as separate particles.
    More often, they are connected to other atoms by chemical bonds, forming molecules or lattices.
    These molecules and lattices are fixed arrangements of atoms, consisting of either one type of atom or different types of atoms.

 

Chemical Bonds

  • Chemical bonds are strong forces of attraction between atoms that hold them together.
  • There are three types of chemical bonds:
    1. Metallic bonds.
    2. Ionic bonds.
    3. Covalent bonds.

chemical bond atom molecule

Chemical bonds are strong forces of attraction between atoms.

 

Metallic Bonding

  • In metals, the valence electrons are free to move between atoms, forming a ‘sea’ of delocalised electrons.
  • These electrons surround a lattice of positive metal ions.
  • Metallic bonds are electrostatic forces of attraction between positively charged metal ions and the negatively charged electrons surrounding them.
  • Metallic bonding exists in all metal elements, except for mercury.
    It also exists in metal alloys, which are mixtures of two or more metals, such as brass, which is a mixture of copper and zinc.

 
metallic bonding atom lattice

Metallic bonding

 

Metallic Structures

  • Atoms joined by metallic bonds form metallic structures.
  • Metallic structures have similar properties, including:
    • They are solids at room temperature.
    • They conduct electricity and heat.
    • They are malleable (can be bent into shape) and ductile (can be drawn into wires).
    • They have a shiny metallic lustre.

 
metal properties shiny lustre malleable

Metals have distinctive properties, such as being shiny and malleable.

(Image: Qimono, Pixabay)

 

Ionic and Covalent Bonding

  • Apart from metallic bonding, atoms tend to combine with other atoms in ways that result in full valence shells.
    Having full valence shells makes atoms more stable.
  • Atoms can combine in two different ways to achieve full valence shells:
    1. They can gain or lose electrons to form ionic bonds.
    2. They can share electrons to form covalent bonds.
  • Some atoms can form either ionic or covalent bonds, depending on the type of atom they are combining with.
    For example, all non-metals that can form ionic bonds with metals can also form covalent bonds with other non-metals.

 

Ionic Bonding

  • Many atoms achieve full valence shells by gaining or losing valence electrons.
    As a result, they form charged particles called ions.
  • When an atom loses one or more electrons, it forms a positively charged ion (cation).
    Atoms that can form positive ions include all metals.
  • When an atom gains one or more electrons, it forms a negatively charged ion (anion).
    Atoms that can form negative ions include non-metals in groups 15-17.
  • Ions formed from single atoms, such as Mg2+ and Br, are known as monatomic ions.
    Ions can also be composed of more than one atom, for example, ammonium (NH4+) and sulfate (SO42-); these ions are known as polyatomic ions.
    Within a polyatomic ion, atoms are held together by covalent bonds.
  • Ionic bonds are electrostatic forces of attraction between positively charged ions and negatively charged ions.
  • Since ionic bonds exist between different types of ions, they can exist in compounds only – ionic bonding does not exist in elements.

 
ionic bonding atom arrangement

Ionic bonding

 

Ionic Structures

  • When metals and non-metals combine chemically, they form ionic structures.
  • Ionic structures are compounds with an orderly crystalline lattice structure.
    These lattices consist of alternating positive and negative ions joined by ionic bonds.
  • There are no individual molecules within this type of structure.
    Chemical formulas for ionic compounds represent ratios of positive and negative ions, rather than structures of molecules.
  • Ionic structures have similar properties, including:
    • They are hard, brittle solids with high melting points.
    • They are often soluble in water.
    • They do not conduct electricity as solids, but do as molten liquids or aqueous solution.

 
copper sulfate crystal ionic compound

Ionic compounds, such as copper sulfate, are brittle solids with crystalline structures.

(Image: Ra’ike, Wikimedia Commons)

 

Covalent Bonding

  • Covalent bonds involve the sharing of valence electrons between non-metal atoms.
  • The sharing of electrons enables these atoms to have full outer shells.
  • For hydrogen, a full outer shell consists of two electrons.
    For all other non-metals, a full outer shell consists of eight electrons, known as an octet.
  • Covalent bonds only form between non-metal atoms.
    Two non-metal atoms are unable to form an ionic bond because an ionic bond requires a positive ion and a negative ion, and non-metals can only form negative ions.
    Hydrogen atoms can form positive ions, but only in solution; therefore hydrogen does not form ionic bonds. When part of a molecule, hydrogen always forms covalent bonds.
  • The atoms involved in a covalent bond can be the same type or different types.
    Therefore, covalent bonds can exist in both elements and compounds.
  • Covalent bonds may result in discrete molecules (covalent molecular structures) or continuous lattices (covalent network structures).

 
covalent molecule bond

Covalent bonding

 

Covalent Molecular Structures

  • Covalent molecular structures consist of discrete molecules held together by covalent bonds.
  • Covalent molecular structures may be elements or compounds.
  • Examples of covalent molecular elements include all non-metals in groups 15-17, which contain varying numbers of atoms in each molecule.
    For example, nitrogen (N2) exists as molecules containing two nitrogen atoms, phosphorus (P4) exists as molecules containing four phosphorus atoms and sulfur (S8) exists as molecules containing eight sulfur atoms.
  • Examples of covalent molecular compounds include water (H2O), ammonia (NH3) and methane (CH4).
  • Covalent molecular structures have similar properties, including:
    • They have low melting and boiling points and are usually gases or liquids at room temperature.
    • They do not conduct electricity.

 
chlorine covalent molecular compound

Covalent molecular substances, such as chlorine, have low melting and boiling points.

(Image: Larenmclane, Wikimedia Commons)

 

Covalent Network Structures

  • Covalent network structures consist of continuous lattices of atoms joined by covalent bonds.
  • Covalent network structures may be elements or compounds.
  • An example of a covalent network element is carbon, which exists as continuous networks of carbon atoms covalently bonded to each other.
  • An example of a covalent network compound is silicon dioxide, which exists as continuous networks containing silicon atoms and oxygen atoms joined by covalent bonds in a 1:2 ratio.
  • Covalent network structures have similar properties, including:
    • They are hard solids with high melting points.
    • They are insoluble in water.
    • They do not conduct electricity.
    • They are chemically unreactive.

 
silicon dioxide quartz covalent network compound

Covalent network structures, such as quartz (silicon dioxide), are hard crystalline solids with very high melting points.

(Image: Stux, Pixabay)

 

Monatomic Structures

  • Monatomic structures consist of individual atoms, with no chemical bonds between them.
  • The only elements that exist as separate atoms are the noble gases, which are located in group 18 of the periodic table.
    These include helium, neon and argon.
  • Monatomic structures have similar properties, including:
    • They have very low boiling points and are gases at room temperature.
    • They are chemically very unreactive.

 
neon gas monatomic element

Neon gas does not react, but glows red when an electric current is passed through it.

(Image: Pslawinski, Wikimedia Commons)

 

Bonding and Electron Configuration

  • The type of bonding that forms between two atoms depends on the electron configurations of the atoms involved.
  • Metals readily lose valence electrons to become positive ions and form ionic bonds with negative ions.
  • Non-metals (excluding the noble gases) can form either ionic bonds or covalent bonds.
    When combining with metals, they gain valence electrons to become negative ions and form ionic bonds.
    When two non-metals combine, they share valence electrons form covalent bonds.
  • Noble gases do not form bonds with other atoms because they already have full valence shells and are therefore very stable.

 
sulfur covalent and ionic bond

Sulfur has six valence electrons. It can attain a full valence shell by receiving electrons from metals or sharing electrons with non-metals.

 

Bonding and the Periodic Table

  • The types of bonding and structures that atoms may form can be predicted from their location on the periodic table.
  • The periodic table can be divided into noble gases, other non-metals, metals and metalloids, as shown below:

 
periodic table bonding non-metal metal

The periodic table, showing noble gases, other non-metals, metals and metalloids.

 

  • Noble gases are located in group 18.
    None of these elements form chemical bonds (under most conditions).
  • Other non-metals are located in the upper sections of groups 14-17.
    All of these elements form covalent bonds with themselves and other non-metals.
    Most will also form ionic bonds with metals.
  • Metals are located in groups 1-12 and the lowers sections of groups 13-16.
    All of these elements have metallic bonding and form ionic bonds with non-metals.
  • Metalloids are located in a diagonal band from the top of group 13 to the bottom of group 17.
    Metalloids have intermediate properties of metals and non-metals.
    These elements can usually form both ionic and covalent bonds.

 

Summary

  • Chemical bonds are strong forces of attraction between atoms that hold them together.
  • Atoms in elements and compounds can be joined by three different types of chemical bonds:
    1. Metallic bonds
    2. • Electrostatic forces of attraction between positively charged metal ions and the negatively charged electrons surrounding them.

    3. Ionic bonds
    4. • Electrostatic forces of attraction between positive and negative ions.

    5. Covalent bonds
    6. • Bonds involving the sharing of valence electrons between non-metal atoms.

  • The different types of chemical bonds, or the lack thereof, result in five different types of chemical structures:
    1. Monatomic structures
    2. • Individual atoms, with no chemical bonds between them.
      • Include elements only.

    3. Metallic structures
    4. • Rigid but malleable lattices of metal ions surrounded by delocalised valence electrons, connected by metallic bonds.
      • Include elements and alloys.

    5. Ionic structures
    6. • Hard lattices of positive and negative ions, connected by ionic bonds; composed of monatomic metal and non-metal ions, or polyatomic ions.
      • Include compounds only.

    7. Covalent molecular structures
    8. • Discrete molecules consisting of non-metal atoms joined by covalent bonds.
      • Include elements and compounds.

    9. Covalent network structures
    10. • Hard lattices of non-metal atoms connected by covalent bonds; may also involve metalloid atoms.
      • Include elements and compounds.

 
graphene carbon lattice

(Image: Carbophiliac, Wikimedia Commons)

 

(Header image: AlexanderAlUS, Wikimedia Commons)

 

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