Chemistry

Chemistry

1. Structure of an atom

   • Protons and neutrons in nucleus
     • Electrons and shells
     • Charges on the proton balances the - charges with the + charges so they atom is neutral
   • Isotopes
     • Atomic number is the number of protons in the nucleus
     • The mass number is the total number of protons and neutrons in the nucleus
     • Number of neutrons = mass number - atomic number
     • Isotopes are atoms of the same elements with different numbers of neutrons

2. The modern periodic table

   • Non metals on the right hand side
   • Horizontal are periods
   • Metals are on the left hand side
   • Vertical groups have similar properties

3. Electron shells

   • 1st shell holds 2 electrons, 2nd shell holds 8, 3rd shell holds 8
   • Electrons shown with dots or crosses
   • Circles represent shells

4. Ions

   • Atoms have the same number of protons and neutrons
   • No overall charge
   • Becomes an ion by gaining or losing one or more electrons
   • Cations are positive
   • Non metals atoms form negative ions
   • Non metals gain electrons to form negatively charged anions

5. Ionic compound

   • Groups of atoms can form ions these are called compound ions have opposite charges and they attract to each other
     • This creates ionic bonds between the ions and a compound is formed.
   • Sodium and chlorine atoms
   • An ionic compound between a metal and an non metal is called a salt
   • Finding the formula
     • All charges must cancel out
     • Swap the number that give the charge for each ion and write them after after the relevant symbol
       • If the number is 1 don't write it in the formula
       • If both the number are the same, don't write them in the formular

6. Properties of ionic compounds

   • Melting and boiling points
     • Ionic substances have high melting points and high boiling points
     • This is because the ions are held together in the lattice be very strong electrostatic forces
     • Conducting electricity
       • Only conducts if it contains charged particles
       • Ionic structures don't conduct when they are solid because the ions are held together in a lattice
         • They do conduct when they are molten or dissolved in water, because particles can move around

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