Monday 17 February 2014

CHEMISTRY - unit 1 - basic ideas of chemistry


CHEMISTRY - UNIT 1 ATOMS


Atoms consist of ELECTRONS surrounding a nucleus containing PROTONS and NEUTRONS.

  • NEUTRONS are neutral
  • PROTONS have a charge of +1
  • ELECTRONS have a charge of -1


The number of electrons is equal to the number of protons so atoms have no overall charge.

All substances are made up of these tiny particles called atoms.
  • An element is made up of only one type of atom.
Structure of atoms

ATOMIC NUMBER is the number of protons an atom contains (therefore the number of electrons)
MASS NUMBER is the total number of protons and neutrons in the nucleus (usually the bigger no.)


E.g          35
                   Cl    has 17 protons and 35 protons                    ...............................and neutrons.
                17
    
      Chlorine therefore has 18 neutrons (35-17)



Periodic table - shows the elements in order

(part of the periodic table)

  • The horizontal rows are periods and show the number of shells an atom has
  • The vertical columns are groups and show how many electrons an atom has in its outer shell
Therefore all elements in period 4 have atoms that have 4 occupied shells and all elements in group 3 have atoms that have 3 electrons in their outer shell.



Electronic structure

The shells around atoms are energy levels.
The innermost shell can hold 2 electrons and the rest can hold 8, so we write 2,8,8,8 etc. depending on how many electrons an atom has (shown by the atomic number)

If sodium (Na) has an atomic number of 11, it has 11 electrons. We show it's electronic structure as 2,8,1 
  • 2 electrons in the first shell (can only hold 2)
  • 8 electrons in the second shell (can only hold 8)
  • And 1 electron in the third shell (only 1 electron left)
Sodium will be found in group 1 because it has 1 electron in its outer shell.


The electronic structure diagram for sodium will be:



Use the periodic table to work out electronic structures:
  1. Work out the period (rows) and draw that many circles (shells)
  2. Look at the atomic number (bottom one) and draw on dots.  2 in the first and 8 from then on.





















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