Chemistry - Additional

These are additional chemistry notes.

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  • Created by: Nusrath
  • Created on: 01-04-10 21:20

Atomic Structure

  • The nucleus of an atom is made up of protons are neutrons.
  • Protons have a positive charge, electrons have negative charge and neutrons have no charge.
  • The atomic number (proton number) of an element = the number of protons in the nucleus of its atoms.
  • Elements are arranged in order of their atomic numbers in the periodic table.
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Arrangements of electrons in an atom

  • Electrons in atoms can be represented by shells as energy levels.
  • Electrons in the lowest energy level are in the shell closest to the nucleus.
  • All elements in a group of the periodic table have the same number of electrons in their outer shell.
  • Electrons occupy energy levels from the lowest first eg: neon with 10 electrons is 2.8.

8 groups and four periods (http://www.bbc.co.uk/schools/gcsebitesize/science/images/aqaaddsci_01.gif)

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Chemical Bonding

  • Noble gases are unreactive becasue they have stable arrangements of their electrons.
  • Atoms of other electrons can achieve stable electronic structures by gaining or losing electrons to form ions, or by sharing electrons to form colvalent bonds.
  • Group 1 lose their single outer shell electron eg: Sodium Na (2.8.1) forms sodium ions, Na+ (2.8) ---> group 7 gains one electron to form ions eg: Chlorine Cl (2.8.7) forms chloride ions Cl- (2.8.8)
  • Positive and Negative Ions attract and form ionic bonds.
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Ionic Bonding

  • Ionic Bonding holds oppositely charged ions together in giant structures.
  • Strong electrostatic forces of attraction act in all directions.
  • Each ion in the lattice is held together firmly by surrounding ions with the opposite charge.
  • The Sodium Chloride structure contains equal numbers of sodium ions and chloride ions as shown by its formula NaCl.
  • Both ions alternate to form a cubic lattice.
  • The ratio of ions in the formula and structure nof an ionic compund depensd on the charge on the ions eg: Magnesium ions are Mg²+ and Chloride ions are Cl¯ so chloride needs 2 to make the formula balanced.---> It would make MgCl².
  • The ratio would then be 1:2
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Covalent Bonding

  • A covalent bond is a strong bond between two non metal atoms by sharing a pair of electrons from the outer shell.
  • Hydrogen and chlorine can each form one covalent bond, oxygen two bonds, nitrogen three, while carbon can form four bonds.

  • Covalent bonds are strong - a lot of energy is needed to break them. Substances with covalent bonds often form molecules with low melting and boiling points, such as hydrogen and water.

  • Atoms can form more than one covalent bond.

  • There is a quick way to figure out how many covalent bonds an atom has.

Group 4Group 5Group 6Group 7 example carbon nitrogen oxygen chlorine number of bonds 8 - 4 = 4 8 - 5 = 3 8 - 6 = 2 8 - 7 = 1

Hydrogen forms one covalent bond. The noble gases in Group 0 do not form any!

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Covalent Bonding Continued......

Double and triple Bonds:

  • A molecule of oxygen (O2) consists of two oxygen atoms held together by a double bond, like this: O=O. A molecule of nitrogen (N2) has two nitrogen atoms held together by a triple bond, like this: Two capital Ns joined together by three lines (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_112b.gif)

Dot and Cross Diagrams:

Bonding in hydrogen: two hydrogen atoms each share one electron (http://www.bbc.co.uk/schools/gcsebitesize/science/images/diag_hydrogen_2.gif)

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Ionic Compounds

Ionic bonds form when a metal reacts with a non-metal. Metals form positive ions; non-metals form negative ions. Ionic bonds are the electrostatic forces of attraction between oppositely charged ions.

  • The oppositely charged ions are arranged in a regular pattern to form giant ionic lattices. Ionic Compounds often form crystals as a result.

Properties of Ionic Compounds:

  • High Melting and Boiling Points- Ionic bonds are very strong - a lot of energy is needed to break them. So ionic compounds have high melting and boiling points.
  • Conductive when liquid - Ions are charged particles, but ionic compounds can only conduct electricity if their ions are free to move. Ionic compounds do not conduct electricity when they are solid - only when dissolved in water or melted.
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Covalent Compounds

Covalent bonding occurs between non metal atoms. Each bonds consists of a shared pair of electrons, and is very strong. There are two main types of covalently bonded substances: 1.) Simple Molecules and 2.) Giant Covalent Structures.

Simple Molecules:

  • Contain only a few atoms held together by strong covalent bonds.
  • An example of this could be Carbon Dioxide, where one atom of Carbon is bonded with two atoms of Oxygen.

Properties of simple molecular substances:

  • Low Melting and Boiling Points - This is becasue the weak intermolecular forces break down easily.
  • Non - Conductive - Substances with a simple molecular structure do not conduct electricity. This is because they do not have any free electrons or an overall electric charge.
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Continued...(Higher Tier Only)

  • Hydrogen, ammonia, methane and water are also simple molecules with covalent bonds.
  • They all have very strong bonds between the atoms, but much weaker forces holding the molecules together.
  • When one of these subtances melts or boils, it is these 'weak intermolecular forces' that break, not the strong covalent bonds.
  • Simple molecular substances are gases, liquids or solids with low melting and boiling points.
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