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Chemical Bonding . Overview . Bonding and structure explains the properties of a substance! . Physical & Chemical Properties . ‘property’: ‘what something is like’

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Overview

Bonding and structure explains the properties of a substance!


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Physical & Chemical Properties

  • ‘property’: ‘what something is like’

  • Physical properties of a substance are those that can be observed or measured without the substance changing into another substance.

  • Chemical properties of a substance describe the change of a substance into another substance.

  • Some examples of physical properties:

Solubility in water (or other solvents)

Melting and boiling points

Electrical conductivity


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Ionic Bond: The Definition

An ionic bond…

is the force of attraction between opppositely charged ions in a compound.


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Giant Ionic StructuresPhysical Properties: M.P. & B.P.

Ionic compounds have high melting and boiling points.

  • strongelectrostaticforces of attractionbetween the ions in an ionic compound

  • large amount of heat is needed to break the strong ionic bonds holding the ions together


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Giant Ionic StructuresPhysical Properties: Electrical Conductivity

Ionic compounds do not conduct electricity in the solid state. It conducts electricity in the molten and aqueous state.

Why do ionic compounds only conduct electricity in the molten or aqueous state, but not in the solid state?

  • ions can move in the molten or aqueous state

  • moving ions carry the electric current


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Giant Ionic Structures Physical Properties: Solubility

Ionic compounds are soluble in water but insoluble in organic solvents.

  • ions attract water molecules

  • disrupts the crystal structure

  • cause the ions to separate and go into solution


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Giant Ionic StructuresPhysical Properties: Volatility

Ionic compounds are not volatile and hence, have no smell.

  • cannot evaporate easily because of strong ionic bonds holding the ions together

    A volatile substance evaporates easily.


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Giant Ionic StructuresPhysical Properties: Physical State

Ionic compounds are hard, crystalline solids.

  • ions held in place by strong ionic bonds, make the crystal hard

  • ions are arranged in straight rows and form structure with flat sides, resulting in a crystalline structure (flat sides & regular shapes)



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Ionic VS Covalent

  • Similarities

    • Electronic configuration of a noble gas

      (PS: Everybody wants to be noble!!)

  • Differences




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Covalent Bond: The Definition

A covalent bond is…

a bond formed by the sharing of a pair of electrons.


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More about COVALENT BONDING

  • Each atom acquires a stable octet structure

    • Electronic configuration of noble gas (Full shell) = Energetically stable

  • Formed between atoms of non-metals (but there are exceptions!)

    • “Spectrum” of bonds


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Simple Molecular Substances

  • Consists of small molecules, e.g. bromine


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Simple Molecular Substances

  • Within the molecule

    Atoms are held together by strong covalent bonds

  • Between molecules

    Weak intermolecular forces(van der Waals’ forces)


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Iodine, I2

Within each iodine molecule, the iodine atoms are held together by strong covalent bonds.

Between the iodine molecules, there are only weak van der Waals’ forcesholding the molecules together.


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Methane, CH4

In a molecule of methane, CH4, the four C–H covalent bonds are strong.

However, weak intermolecular forces between methane molecules hold them together loosely. Therefore, methane exists as a gas at room temperature and pressure.


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Physical Properties

  • Physical state

    • Most substances are liquids or gases at room temperature.

    • Forces between molecules are weak, allowing molecules to move freely.

  • Low M.P. & B.P.

    • Little energy required to overcome the (weak) intermolecular forces

    • (Usually <200°C)



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Physical Properties

  • Volatility

    • Low B.P. = Volatile

    • Evaporate easily (to give a smell)

  • Electrical conductivity

    • Do not conduct electricity. (Some exceptions, e.g. graphite)

    • No free-moving ions or electrons to conduct electricity

  • Solubility

    • Most molecular substances are insoluble in water, but dissolve in organic solvents. (Some exceptions, e.g. alcohol and sugar, hydrogen chloride)


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