Chemistry unit two Revision_2

Chemistry unit two Revision_2

1. Atoms

   • Atoms are made up of three types of small particles... PROTONS, NEUTRONS and ELECTRONS
   • The atomic and the mass number describe an ATOM
     • The mass number is at the top - The mass number is the total number of PROTONS and NEUTRONS
     • The Atomic number is at the bottom - the atomic number is the number of PROTONS
   • The atomic number teaks you how many protons the atom has
   • Atoms of the same element have the same number of PROTONS - atoms of different elements will have different PROTONS to NEUTRONS

2. Compounds

   • Compounds Elements which are chemically bonded together .
   • Compounds are formed when atoms of two or more elements are chemically combined together. For example, carbon dioxide is a compound between carbon and oxygen.
   • It's difficult to separate the two original elements out again.

3. Isotopes

   • Isotopes are the same as compounds except for an extra NEUTRON or two
   • ISOTOPES ARE: different atomic forms of the same element, which have the same number of protons but a different number of neutrons
   • Isotopes must have the same atomic number but different mass numbers

4. Ionic Bonding

   • Ionic bonding is transferring electrons
   • In ionic bonding, atoms lose or gain electrons to form charged particles (called ions) which then strongly attracted to one another (because of the attraction of opposite charges, + and -).
   • A shell with just one Electron is well keen to get rid...
     • All the atoms over at the left-hand-side of the periodic table, e.g. sodium, potassium, calcium etc. have just one or two electrons in their outer shell (highest energy level). And they're pretty keen to get shot of them, because then they'll only have full outer shells left, which is how they do get rid of it.

5. Ions have the electronic structure of a noble gas

6. Formulas

   • Ionic compounds are made up of + charge part and a - charged part
     • The overall charge will be 0
     • All the - charges must balance the + charges
   • You can use the charges on the individual ions present to work out the formula of the ionic compound
     • NaCl
       • Na (+1) Cl (-1) the charges are balanced with one of each ion.
     • Mgcl2
       • Mg (+2) and Cl (-1) ions two - ions are needed to balance out the 2+ charges of a magnesium ion.

7. Covalent bonding

   • Some elements bond ironically but others form covalent bonds. This is where atoms share electrons with each other so that they've got full outer shells
   • They only share electrons in their outer shells
   • Having a full outer shell makes them happy and gives them the electronic structure of a NOBLE GAS
   • Each covalent bond gives one extra share electron for each atom.
   • a covalent bond is a shared pair of electrons
   • Each atom involved has to make up enough covalent bonds to fill up its outer shell

8. Ions

   • The elements that most readily form ions are those in groups 1, 2, 6 and 7
     • Group 1 and 2 are metals and lose electrons to form + ions
     • group 6 and 7 are non - metals and gain electrons to form - ions
     • the charge on the + ions is the same as the group number of the elements.
     • Any of the positive ions which are in group 1 or 2, can combine with any of the - ions to form an ionic compound.
     • Only elements at opposite sides of the periodic table will form ionic compounds, e.g Na & Cl, where one of them forms a + ion and one becomes a - ions.
   • ELECTRONIC STRUCTURE OF IONS
     • Na has one free + ion Cl needs one more ion to complete the outer shell. 1 ion passes from Na to Cl to complete the shell so it's just NacL
     • Mg has 2 free + ions. O needs 2 ions to complete the outer shell so 2 ions pass from Mg to O to complete the outer shell, so it's just MgO
     • Ca has 2 free + ions, Cl needs just 1 more ion to complete the outer shell. 2 atoms of Cl are needed to take the 2 free + ions from Ca. So it's CaCl2

9. metallic structure

   • Properties
     • metals consist of a giant structure
     • metallic bonds involve the all-important 'free electrons' which produce all the properties of metals. These delocalised (free) electrons come from the outer shell of every metal atom in the structure
     • These electrons are free to move through the whole structure and so metals are good conductors of heat and electricity
     • These electrons also hold the atoms together in a regular structure. These are strong forces of electrostatic attraction between the + metal ions and the - electrons
     • They also allow the layers of atoms to slide over each other, allowing metals to be bent and shaped
   • Alloys are harder than pure metals
     • Pure metals often aren't quite right for certain jobs. So scientists mix two or more metals together - creating an alloy with the properties they want
     • Different elements have different size atoms. So when another metal is mixed with the pure metal, the new metal atoms will distort the layers of metal atoms, womaking it more difficult for them to slide over each other. So alloys are harder
   • Identifying the structure of a substance by its properties
     • You should be able to easily identify most substances just by the way they behave as either: giant ionic, simple molecular, giant covalent or giant metallic.
     • The way they might test you in the exam is by describing the physical properties of a substance and asking you to describe which type of structure it has.

10. New materials

    • Smart material behave differently depending on the conditions e.g. temperature
    • A good example is nitinol - a "shape memory