- Created by: Rebeccax
- Created on: 12-05-15 21:26
Atoms and compounds
- Atoms of the same element have the same number of protons - so atoms of different elements will have different numbers of protons.
- To get the number of neutrons, just subtract the atomic number from the mass number. Electrons are not counted in the mass number because their realtive mass is very small.
- Compounds are substances in which two or more elements are chemically combined together.
Isotopes are: different atomic forms of the same element, which have the SAME amount of PROTONS but a DIFFERENT number of NEUTRONS.
Therefore isotopes have the same atomic number butdifferent mass number.Carbon - 12 and carbon - 14 are a very popular pair of isotopes.
Ionic bonding is one of the ways of the atoms form compounds.
Ionic bonding - transferring electrons
In ionic bonding, atoms lose or gain electrons to form charged particles (ions) which are then strongly attracted to one another (because of the attraction of opposite charges, + and -).
Atoms of element on the left hand side of the periodic table e.g. sodium, potassium, calcium only have one or two electrons on their outer shell (highest energy level). This means that they want to get rid of their outer electron to have a full outer shell (same as noble gas - group 0).
Elements in group 6 and group 7 (the other side of the periodic table) e.g. oxygen and chlorine, have outer shells which are nearly full. So elements from group 1 (the alkalis) gives (loses) an electron to become a positive metal ion and elements from group 7 (the halogens) which gets the spare electron and becomes a negative halide ion.
Ionic bonding 2
Ionic compounds have regular structures (giant ionic lattices). Ionic compounds are held together by strong electrostatic forces of attraction between oppositely charged ions. These forces act in all direction in the lattice and this is called ionic bonding.
- The ions form a closely packed regular lattice arrangement.
- There are very strong electrostatic forces of attraction between oppositely charged ions, in all directions.
- These compounds have high melting points of boiling points because of the large amounts of energy needed to break the many strong bonds.
- When melted or dissolved in water, ionic compounds conduct elelctricity because the ions are free to move and carry the current.
- atoms that have lost or gained an eletron/electrons are ions
- ions have the electronic structure of a noble gas
- the elements that most readily form are those in group 1, 2 6 and 7
- group 1 and 2 elements are metals and they lose electrons to form positive ions
- for example group 1 elements form ionic compounds with non- metals where the metal ion has a 1+ charge e.g. K+Cl-
- group 6 and 7 elements are non-metals. They gain electrons to form negative ions
- for example, group 7 elements form ionic compounds with the alkali metals where the halide ion has a 1- charge e.g. Na+Cl-
- the charge onthe positive ions is the same as the group number of the element
- any of the positive ions can cpmbine with any of the negative ions to form an ionic compound
- only elements at opposite sides of the periodic table will form ionic compounds e.g. Na and Cl, where one of them becomes a positive ion and one becomes a negative ion
When atoms share pais of electrons, they form covalent bonds. These bonds betweeen atoms are strong. Some covalently bonded substances consist of simple molecules such as H2, Cl2, O2, HCl, NH3 and CH4. Others have giant covalent structures (macromolecules), such as diamond and silicon dioxide.
Covalent bonding - single covalent bonds
H - H H - Cl
Cl - Cl
More covalent bonding
Covalent substances - simple molecular
- substances with covalent bonds (electron sharing) can form simple molecules
- the atoms form very strong covalent bonds to form small molecues of several atoms
- the intermolecular forces are very weak
- because of this ^ melting and boiling points are very low bc molecules are easily parted from each other
- when simple molecular substances melt/boil it's the inter-molecular forces that get broken (not covalent bonds)
- most molecular substances are gases/liquids at room temp but can be solid
- molecular substances don't conduct electricity - there are no ions so no electrical charge
Covalent substances - Giant covalent
Substances with covalent bonds can also be giant covalent structures aka macromolecules.
- similar to giant ionic structures (lattices) except that there are not charged ions
- all atoms bonded to each other by strong covalent bonds - so v. high melt and boil point
- don't conduct elect. - not even when molten (except for graphite)
diamond: - each C atoms forms 4 covalent bonds in a v rigid giant covalent stuc. - this struc. makes the diamon the hardest natural substance so it's used for drill tips
graphite: - each C atoms only form 3 covalent bonds
- this creates layers which are free to slide over each other (like pack of cards) so graphite is soft and slippery
- the layers are held together so losely that they can be rubbed off onto paper (how pencil work) this is because there r weak intermolecular forces between layers
silicon dioxide: - aka silica this is what sand is made of, each grain of sand is 1 giant struc. of silicon and oxy.
Metal properties are all due to the sea of free electrons
- metals also consist of a giant struc.
- metallic bonds involve free /delocalised electrons
- which produce all of the properties of metals.
- they come from the outer shell of every metal atom in the struc.
- they are free to move through the whole sruc. so metals are gd conductors of heat and elec.
- these electrons also hold the atoms together in a reg. struc. there are strong forces of electrostatic attraction between positive metal ions and negative electrons
- they allow the layers of atoms to slide over each other so metals can be bent and shaped
The electrons in the highest occupied energy levels (outer shell) of metal atoms.
alloys are harder than pure metals bc diff. elements = diff. sized atoms so when metals are mixed, the new metal atoms distort layers of metal atoms so more difficult to slide over so a
alloys = 2 or more metals mixed together - to create properties they want
1. smart materials e.g. nitinol (shape memory alloy) - glasses frames, dental braces
2. nanoparticles - 1-100 nanometers e.g. fullerenes (carbon - hollow balls/closed tubes), joined together to form carbon nanotubes used to reinforce graphite in tennis rackets
Carbon can also form fullerenes with different numbers of carbon atoms. Fullerenes can be used for drug delivery into the body, in lubricants, as catalysts, and in nanotubes for reinforcing materials, eg in tennis rackets. structure of fullerenes is based on hexagonal rings of carbon atoms.
Nanoparticles show different properties to the same materials in bulk and have a high surface area to volume ratio, which may lead to the development of new computers, new catalysts, new coatings, highly selective sensors, stronger and lighter construction materials, and new cosmetics such as sun tan creams and deodorants.
The properties of polymers depend on what they are made from and the conditions under which they are made. For example, low density (LD) and high density (HD) poly(ethene) are produced using different catalysts and reaction conditions.
weak forces: - individual tangled chains of polymers, held together by weak intermolecular forces, are free to slide over each other.
1. thermosoftening polymers don't have cross-linking between chains
2. the forces between the chains are really easy to overcome, so it's easy to melt the plastic.
3. when it cools, the polymer hardens into a new shape
4. you can melt these plastics and remould them as many times as you like
strong forces: - some plastis have stronger intermolecular forces between the polymer chains, called crosslinks, that hold the chains firmly together.
1. thermosetting polymers have crosslinks
2. these hold the hains together in a solid struc.
3. the polymer doesn't soften when it's heated
4. thermosetting polymers are strong, hard and rigid