Chemistry Topic One

Chemistry Topic One

1. Atoms consist of electrons surrounding a nucleus that contains protons and neutrons.

   • Neutrons are neutral, but protons and electrons are electrically charged. Protons have a relative charge of +1, while electrons have a relative charge of -1.
     • The number of protons in an atom is called its atomic number. In the periodic table atoms are arranged in atomic number order.
       • Electrons are arranged in energy levels or shells, and different energy levels can hold different numbers of electrons. The electronic structure of an atom is a description of how the electrons are arranged, which can be shown in a diagram or by numbers.
   • Atoms

2. The structure of an Atom

   • Atoms are made from smaller subatomic particles.
     • The total number of electrons in an atom is always the same as the number of protons in the nucleus. This means atoms have no overall electrical charge.
       • The number of protons in an atom is called its atomic number - also called the proton number. Atoms are arranged in the periodic table in order of increasing atomic number
     • • At the centre of an atom is a nucleus containing protons and neutrons. Electrons are arranged around the nucleus in energy levels or shells.

3. Atomic Structure

   • The mass number of an atom is the total of protons plus neutrons. Atoms of the same element with different numbers of neutrons (and hence different mass numbers) are called isotopes of that element.
     • Protons and neutrons have the same mass. Electrons have such a small mass that this can usually be taken as zero.

4. Isotopes

   • Isotopes are the atoms of an element with different numbers of neutrons. They have the same proton number, but different mass numbers.

5. Chemical Equations

   • Balancing Equations
     • Example: write the balanced equation for the reaction between hydrogen and oxygen to produce water.
       • Step 1 Write the formulae for each substance: H2 + O2 → H2O
         • Step 2 Check for an unbalanced element, for example, O. Adjust the number of each atom or molecule needed, but never change a formula. We need two O atoms on each side: H2 + O2 → 2H2O
           • Step 3 Check for another unbalanced element. In this example, there are 2 H atoms on the left and 2 × 2 = 4 atoms on the right. So we need to double the number of hydrogen molecules: 2H2 + O2 → 2H2O

6. Separating solids from liquids – filtration

   • If a substance does not dissolve in a solvent, we say that it is insoluble.
     • Filtration is a method for separating an insoluble solid from a liquid. When a mixture of sand and water is filtered: the sand stays behind in the filter paper (it becomes the residue) the water passes through the filter paper (it becomes the filtrate)

7. Separating the solvent from a solution – simple distillation

   • Simple distillation is a method for separating the solvent from a solution. For example, water can be separated from salt solution by simple distillation.
     • This method works because water has a much lower boiling point than salt. When the solution is heated, the water evaporates. It is then cooled and condensed into a separate container. The salt does not evaporate and so it stays behind.

8. Separating a liquid from a mixture – fractional distillation

   • Fractional distillation is a method for separating a liquid from a mixture of two or more liquids. For example, liquid ethanol can be separated from a mixture of ethanol and water by fractional distillation.
     • This method works because the liquids in the mixture have different boiling points. When the mixture is heated, one liquid evaporates before the other.

9. Separating dissolved solids – chromatography

   • Paper chromatography is a method for separating dissolved substances from one another. It is often used when the dissolved substances are coloured, such as inks, food colourings and plant dyes.
     • It works because some of the coloured substances dissolve in the solvent used better than others, so they travel further up the paper.
       • A pure substance will only produce one spot on the chromatogram during paper chromatography. Two substances will be the same if they produce the same colour of spot, and their spots travel the same distance up the paper.

Bilder und Dateien der Mindmap