Chemistry - topic 1

Chemistry - topic 1

1. Atoms

   • The nucleus: Contains protons and neutrons, overall positive charge. The electrons: They have a negative charge, they're tiny compared to the nucleus, virtually no mass, they occupy shells around the nucleus.
   • Proton: mass = 1. Charge = +1
     • Neutron: mass = 1. Charge = 0
       • Electron: mass = 1/2000. Charge = -1

2. Electron shells

   • 1st shell: 2. 2nd shell: 8. 3rd shell: 8
   • e.g. Magnesium = proton number = 12. 1st shell: 2. 2nd shell: 8. 3rd shell: 2

3. Elements

   • Mass number = This tell you the total number of protons and neutrons in the atom.
     • Atomic number = This tells you the number of protons in the atom (and so the number of electrons too).
     • To find the number of neutrons in the atom: subtract the atomic number from the mass number.

4. Isotopes and relative atomic mass

   • Isotopes are: different atomic forms of the same element, which have the same number of protons but different number of neutrons.
   • Relative atomic mass = First, multiply the mass of each isotope by its relative abundance. Next, add those together. Finally, divide by the sum of the relative abundance (e.g. 3+1)

5. A brief history of the periodic table

   • Dmitri Mendeleev in 1869 arranged 50 or so elements according to their properties of their compounds to make a table of elements. He placed elements with similar chemical properties in the same vertical groups but he found he had to leave gaps to make this work.
     • The gaps in Mendeleev's table were clever because they predicted the properties of undiscovered elements. Since then new elements have been found which fit into the gaps left in Mendeleev's table.

6. The periodic table

   • Metals (on the left) and non metals (on the right) are on opposite sides of the periodic table.
   • Periods: The rows of the periodic table are called periods. The elements are arranged in order of increasing atomic number along each row. The period number is the same number of electron shells. The properties of elements change as you go along a period.
   • Groups: The columns of the periodic table are called groups. Elements in the same group have similar properties because they have the same number of electrons in their outer shell. The group number is equal to the number of electrons in the outer shell. The properties of elements often gradually change as you go down a group.