Atomic Number and Mass Number Describe an Atom
The Mass Number= TOTAL NUMBER OF PROTONS AND NEUTRONS
The Atomic Number= NUMBER OF PROTONS
- To get the number of neutrons, subtract the atomic number from the mass number.
-Electrons are not counted in the mass number because their relative mass is very small.
- Compounds are formed when atoms of two or more elements are chemically combined together.
- It is difficult to seperate the two original elelments out again.
ISOTOPES= DIFFERENT ATOMIC FORMS OF THE SAME ELEMENT, WHICH HAVE THE SAME NUMBER OF PROTONS BUT A DIFFERENT NUMBER OF NEUTRONS.
IONIC BONDING= Lose or gain electrons to form charged particles (called ions) which are then strongly attracted to one another (because the attraction of opposite charges + -)
- A shell with just one electron wants to lose its electron (sodium, potassium, calcium), because then they will only full shells left - which is what they want because they want to have the same electronic structure as a noble gas.
- This leaves the atom as an ion. They attach themselves to an appositely charged ion.
- A nearly full shell (oxygen and chorine - Group 6/7) wants to get an extra one or two electrons to fill up its outer shell.
- They become an ion and are attracted to a oppositely charged ion.
- Ionic compounds always have giant ionic lattices.
- The ions form a closely packed regular lattice arrangement.
- There are very strong electrostatic forces of attraction between oppositely charged ions, in all directions.
- A single crystal of sodium chloride is one giant ionic lattice.
- The Na+ and Cl- ions are held together in a regular lattice.
Ionic Compounds- Properties
- High melting points.
- High boiling points.
- Strong attraction between ions.
- When they are melted the ions are free to move and carry electric current.
- Dissolve easily in water.
- Ions seperate and are free to moe in the solution and carry electric current.
Ions and Formulas
- Ions have an electronic structure of a noble gas.
- The elements that most readily form ions are Groups 1, 2, 6 and 7.
- Group 1 and 2 elements are metals and they lose electrons to form positive ions.
- Group 6 and 7 elements are non-metals. They gain electrons to form negative ions.
- The charge on the positive ions is the same as the group number of the element.
Ionic Compounds- Charges
- Ionic compounds are made up of positively charged part and a negatively charged part.
- The overall charge of any compound is zero.
- So all the negative charges in the compound must balance all the positive charges.
- You can use the charges on the individual ions present to work out the formula of the ionic compound.
Example: Sodium chloride contains Na+ and Cl- ions. (+1)+(-1)=0. The charges are balanced with one of each ion. so the formula for sodium chloride= NaCl
Electronic Structure of Ions
- Sqaure bracket and a + or a - to show the charge.
Electronic Structure of Ions
- Atoms make covalent bonds by sharing electrons with other atoms.
- Share electrons in their outer shells.
- Having a full outer shell gives them the electronic structure of a noble gas.
- Each covalent bond provides one extra shared electron for each atom.
- A covalent bond is a shared pair of electrons.
- Each atom involved has to make enough covalent bonss to fill up its outer shell.
Covalent Bonding - Examples
- Hydrogen atoms have just one electron. They only need one more to complete the first shell.
- Chlorine atoms only need one more electron to complete their outer shell.
Covalent Bonding - Examples
- Carbon has for outer electrons, which is half full shell. So it forms four covalent bonds to make up its outer shell.
Covalent Bonding - Examples
- This is very similiar to H2 and Cl2-. Again both atoms only need one more electron to complete their outer shells.
- Nitrogen has five outer electrons so it needs to form three covalent bonds to make up the extra three electrons needed
Covalent Bonding - Examples
- Oxygen atoms have six outer electrons. They sometimes form ionic bonds by taking two electrons to complete their outer shell.In water molecules, the oxygen shares electrons with the two H atoms.
- In oxygen gas, oxygen shares two electrons with another oxygen atom to get a full outer shell. A double covalent bond is formed.
Covalent Substances- Simple
- The atoms form very strong covalent bonds to form small molecules for several atoms.
- The forces of attraction between these molecules are very weak.
- The melting points and boiling points are very low. because the molecules are easily parted from eachother.
- It is the intermolecular forces that get broken when simple molecular substances melt or boil- not the strong covalent bonds.
- Most molecular substances are gases or liquids at room temperature, but they can be solids.
- Molecular substances do not conduct electricity- there are no ions so there's no electrical charge.
CHLORINE OXYGEN WATER
Covalent Substances- Giant
- These are similiar to giant ionic structures (lattices) except that there are no charged ions.
- All the atoms are bonded to each other by strong covalent bonds.
- This means that they have very high melting and boiling points.
- They do not conduct electricity - not even when molten (except for graphite).
- The main examples are diamond and grpahite, which are both made only from carbon atoms, and silicon dioxide (silica).
Giant Covalent- Diamond.
- Each Carbon atom forms four covalent bonds in a very rigid giant covalent structure. This structure makes diamond the hardest natural substance, so it's used for drill tips.
Giant Covalent- Silicon Dioxide ( Silica)
- Sometimes called silica, this is what sand is made of. Each grain of sand is one giant structure of silicon and oxygen.
Giant Covalent- Graphite
- Each Carbon atom only forms three covalent bonds.
- This creates layers which are free to move over eachother.
- The layers are held together so loosely that they can be rubbed off onto paper- that's how a pencil works. This is because there are weak intermolecular forces between the layers.
- Graphite is the only non-metal which is a good conductor of heat and electricity. Each carbon atom has one deloacalised (free) electron and it is these free eletrons that conduct heat and electricity.
- Metals also consist of a giant structure.
- Metallic bonds involve 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. There are strong forces of electrostatic attraction between the positive metal ions and the negative electrons.
- They also allow the layers of atoms to slide over each other, allowing metals to be bent and shaped.
- Scientists mix two or more metals together- creating an alloy with the properties they want.
- Different elements have different sized atoms.
- When another metal is mixed with a pure metal, the new metal atoms will distort the layers of metal atoms, making it more difficult for them to slide over each other.
- ALLOYS ARE HARDER
- Smart materials behave differently depending on the conditions (temperature).
- Nitinol- Shape memory alloy. It is a metal alloy. When it is cool you can bend it and twist it. However, if you heat it above a certain temperature, it goes back to a 'remembered shape'.
- It is used for dental braces. In the mouth it warms and tries to return to a 'remembered' shape and so gently pulls the teeth with it.
- Really tiny particles. 1-100 nanometres across, are called 'nanoparticles' (1nm=0.000 000 001m).
- Nanoparticles contain roughly a few hundred atoms.
- Nanoparticles contain fullrenes- molecules of carbon shaped liked hollow balls or closed tubes.
- The carbon atoms are arranged in hexagonal rings. Different fullrenes contain different numbers of carbon atoms.
1) Fullrenes can be joined together to form nanotubes - teeny tiny hollow carbon tubes, a few nanometres across.
2) All those covalent bonds make carbon nanotubes very strong. They can be used to reinforce graphite in tennis rackets.
Uses of Nanoparticles
- Using nanoparticles is known as nanoscience.
- They have a huge surface area to volume ratio, so they could help make new industrial catalysts.
- To make sensors to detect one type of molecule and nothing else. These highly specific sensors are already being used to test water purity.
- Nanotubes can be used to make stronger, lighter building materials.
- New cosmetics (e.g. suncream and deodrant) have been made using nanoparticles.
- Nanomedicine. The idea that tiny fullrenes are absorbed more easily by the body than most particles. This means they could delever drugs right into the cells where they are needed.
- New lubricant coatings are being developed using fullrenes. These coatings reduce friction a bit like ball bearings and could be used in artificial joints to gears.
- Nanotubes conduct electricty, so they can be used in tiny electric circuits for computer chips.
FORCES BETWEEN MOLECULES DETERMINE THE PROPERTIES OF PLASTICS
Strong covalent bonds hold the atoms together in long chains. But it is the bonds between the different molecule chains that determine the properties of the plastic.
WEAK FORCES = Individual tangled chains of polymers, held together by WEAK INTERMOLECULAR FORCES and are free to slide over each other.
- THERMOSOFTENING POLYMERS---> do not have cross-linking between chains. The forces between the chains are really easy to overcome (to melt). When it cools, the polymer hardens into a new shape. You can melt and remould these plastics as many times.
STRONG FORCES = STRONG INTERMOLECULAR FORCES between the polymer chains, called CROSSLINKS, that hold the chains firmly together.
- THERMOSETTING POLYMERS---> have crosslinks. These hold the chains together in a solid structure. The polymer does not soften when it is heated. Thermosetting polymers are strong, hard and rigid.
Polymers - Properties
- The starting materials and reaction conditions will both affect the properties of a polymer.
- Two types of polythene can be made using different conditions:
1) Low density (LD) polythene is made by heating ethene to about 200 degeress C under high pressure. It is flexible and is used for bags and bottles.
2) High density (HD) polythene is made at a lower temperature and pressure (with a catalyst) . It is more rigid and is used for water tanks and drainpipes.
Relative Formula Mass
RELATIVE ATOMIC MASS: compares the mass of elements with carbon-12. It is the average mass of all the isotopes of the element.
ISOTOPES: different forms of an element, they have the same number of protons (atomic number), but a different number of neutrons (so different mass number).
RELATIVE FORMULA MASS: Mr. the mass of a compound compared to carbon-12. To calculate this add up the Ar. of all the atoms in the compound. e.g. Mg (NO3)2= 24+(2x14)+(6x16)=148
A food colouring may contain one dye or it might be a mixture of dyes. Here's how you can tell:
- Extract the colour from a food sample by placing it in a small cup with a few drops of solvent ( can be water, ethanol, salt water, etc.).
- Put spots of the coloured solution on a pencil baseline on filter paper.
- Roll up the sheet and put it in a beaker with some solvent - but keep the baseline above the level of the solvent.
- The solvent seeps up the paper, taking the dyes with it. Different dyes form spots in different places.
- A chromatogram with four spots means at least four dyes, not exactly four dyes. There could be five dyes, with two of them making a spot in the same place. It cannot be three dyes though, because one dye cannot split into two spots.
You can identify elements and compounds using instrumental methods.
Advantages of Using Machines
- Very Sensitive- can detect even the tiniest amounts of substances.
- Very Fast and tests can be automated.
- Very accurate
Can seperate out a mixture of compounds and help you identify the substances present.
- A gas is used to carry substances through a column packed with a solid material.
- The substances travel through the tube at different speeds, so they're seperated.
- The time they take to reach the detector is called the retention time. It can be used to help identify the substances.
- The recorder draws a gas chromatograph. The number of peaks shows the number of different compounds in the sample.
- The position of the peaks shows the retention time of each substance.
- The gas chromatography column can also be linked to a mass spectrometer. This process is called GC-MS and can identify the substances leaving the column very accurately.
- You can work out the relative molecular mass of each of the substances from the graoh it draws. You just read off from the molecular ion peak.