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Atoms, Compounds, and Mixtures

  • An atom is the smallest particle of a chemical element that can exist alone
  • John Dalton, a British scientist in 1806 put forward a number of statements about atoms which made up what was called Dalton’s Atomic Theory.
  • The main ideas of the theory are:
    • Matter is made up of minute indivisible particles.
    • Atoms can neither be created nor destroyed.
    • All atoms of an element are alike and different from the atoms of all other elements.
    • Chemical combination takes place between small number of atoms of different elements.
  • These old ideas have been modified over the years in the light of new scientific knowledge. These modifications are:
    • Atoms have been found to be made up of different units like electrons, protons, and neutrons.
    • Radioactivity and nuclear reactors show that atoms can be created and destroyed during some chemical reactions and little changes in mass because of energy changes.
    • A large number of atoms can combine to form organic compounds.
    • Atoms of the same elements are alike but may differ in masses (isotopy).
    • Atomic number = Number of protons = Number of electrons
    • Mass Number = Number of protons + Number of neutrons
Components of Atoms
  • Protons: They are positively charged.
  • Electrons: They are negatively charged.
  • Neutrons: They have no charge.

The Atom (Photo Credit: EnergyWaveTheory.com)

  • This is the existence of atoms of the same element having the same atomic number but different mass number due to the number of neutrons. E.g. Chlorine, carbon, oxygen, and hydrogen.
  • Examples:
      • For Chlorine:
      • For Carbon:
      • For Oxygen:
  • There are many elements in nature.
  • These elements are usually represented by symbols instead of writing their names in full.
  • You use symbols or abbreviations to work out some rules.
  • The abbreviation for each element is called the symbol for that element.
  • An abbreviated form for the name of an element is referred to as chemical symbol.
  • There are four groups of chemical symbols:
    • Group A – Elements that are classified with the first letter of the name of the element.
      Name of ElementSymbol
    • Group B – Elements that are classified with the first two letters of their name.
      Name of ElementSymbol
    • Group C – Elements that are classified with the first letter and another letter in the name of the element.
      Name of ElementSymbol
    • Group D – Elements that are classified or derived from their Latin names.
      Name of ElementLatin NameSymbol
  • A molecules is the amount of a substance that can exist separately; S, NaCl, etc.
  • Molecules contain the number of atoms of elements shown in the subscripts.
    • For example:
      • O2 – it reads one molecule of oxygen contains two atoms of oxygen.
      • NH3 – it reads one molecule of ammonium contains one atom of nitrogen and three atoms of hydrogen.
  • Every molecule and every compound has its own formula.
  • The formula of a compound, in most cases, can be determined by analyzing the content of the molecule or compound, then the chemical formula can be written.
Name of SubstanceFormula
Oxygen moleculeO2
Nitrogen moleculeN2
Carbon IV oxideCO2
Magnesium oxideMgO
Carbon II oxideCO
  • A compound is a substance that is formed from the chemical combination of two or more elements.
  • A compound can only be separated into its constituent elements by chemical means.
  • The properties of compounds differ from the properties of the elements that make up the compound.
  • Compounds always contain the same ratio of its component atoms.
  • A mixture is a combination of two or more substances that are not chemically combined and can be separated by any physical means or methods.
  • Some substances are made up of more than one element but they are not compounds, they are mixtures.
  • Mixtures are made easily. In the formation of mixtures, there is no chemical change involved.
  • It is usually possible to separate mixtures into their constituent components.
  • Some mixtures are not made but occur naturally. Air, for instance, is a mixture of several gases.
  • Mixtures that are uniform throughout are called homogeneous mixtures. Air is a homogeneous mixture.
  • Mixtures that are not uniform throughout are called heterogeneous mixtures. Salt, sugar dissolve in water to form heterogeneous mixtures.
  • Heterogeneous mixtures are called solutions. A good example of a very important mixture is petroleum, generally called crude oil. Petroleum is a mixture of many substances called hydrocarbons. To separate the constituents of petroleum, we apply the method of fractional distillation.
  • The products of fractional distillation from crude oil are:
    • Petroleum gas.
    • Petroleum.
    • Naphtha.
    • Kerosene.
    • Diesel oil.
    • Lubricating oil.
    • Bitumen.
  • Petroleum and kerosene are the two commonest products in the fractional distillation of crude oil.
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