elements, compounds and isotopes
elements consist of one type of atom only.
compounds are chemically bonded;
compounds are formed when two or more elements chemically react together.
it's difficult to separate the two original elements out again.
the properties of compounds are totally different from those of the original elements.
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 proton number but different mass numbers.
The Periodic Table
The periodic table is a table of all known elements;
there are 100ish elements, which all materials are made of.
the modern periodic table shows the elements in order of ascending atomic number.
the periodic table is laid out so that elements with similar properties form columns. These vertical columns are called groups.
the group to which an element belongs corresponds to the number of electrons it has in its outer shell. e.g. group 7 have 7 electrons in their outer shell.
some of the groups have special names. group 1 elements are called alkali metals. group 7 elements are called the halogens, and group 0 elements are called the noble gases.
The periodic table continued
elements in a group have the same number of outer electrons.
the element in each group all have the same number of electrons in their outer shell.
That's why they have similar properties, and we arrange them in this way.
when only a hand-full of the elements were known, the periodic table was made by looking at the properties and arranging them in groups - the same groups that they are in today.
electron shell rules;
1. electrons always occupy shells (sometimes called energy levels)
2. the lowest energy levels are always filled first - these are the ones closest to the nucleus
3. only a certain number of electrons are allowed in each shell:
first shell:2 second shell:8 third shell:8
4. atoms are much happier when they have a full outer shell - like the noble gases
5. in most atoms the outer shell is not full and this makes the atom want to react
ionic bonding - transferring electrons
in ionic bonding, atoms lose or gain electrons to form charged particles (called ions) which are then strongly attracted to one another (because of the attraction of opposite charges, + and - ).
a shell with just one electron is 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. and they're pretty keen to get rid of them, because then they'll only have full shells left, which is how they like it. so given a chance they do get rid of them, and that leaves the atom as an ion instead. now ions aren't the kind of things that sit around quietly,they leap at the first passing ion with an opposite charge ans stick to it like glue.
Ionic bonding continued..
a nearly full shell is keen to gain the extra electrons...
on the other side of the periodic table, the elements in group 6 and 7, such as oxygen and chlorine, have outer shells which are nearly full. they're obviously keen to gain that extra one or two electrons to fill the shell. when they do become ions, they latch onto the atom (now ion) which gave them the extra electron(s), and a reaction occurs.
giant ionic structures don't melt easily but when they do...
ionic bonds always produce giant ionic structures
the ions form a closely packed regular lattice arrangement
there are very strong chemical bonds between all the ions
a single crystal of salt is one giant ionic lattice, which is why salt crystals tend to be cuboid in shape
Properties of Giant Ionic Structures
they have hight melting and boiling points
due to the very strong chemical bonds between all the ions in the giant structure.
they dissolve to form solutions that conduct electricity
when dissolved the ions separate and are all free to move in the solution, so obviously they'll carry electric current.
they conduct electricity when molten
when it melts, the ions are free to move and they'll carry electric current.
Electronic shells and ions
groups 1&2 and 6&7 are the most likely to form ions
atoms that have lost or gained and electron(s) are ions.
the elements that most readily form ions are those in groups 1,2,6 and 7
group 1 and 2 elements are metals and they lose electrons to form positive ions or cations
group 6 and 7 elements are non-metals and they gain electrons to form negative ions or anions
when any of these react they form ionic bonds
only elements at opposite sides of the periodic table will form ionic bonds.
covalent bonds - sharing electrons
sometimes atoms prefer to make covalent bonds by sharing electrons with other atoms.
this way both atoms feel that they have a full outer shell, and that makes them happy
each covalent bond provides one extra shared electron for each atom
each atom involved has to make enough covalent bonds to fill up its outer shell.
covalent substances: two kinds
simple molecular substances
the atoms form very strong covalent bonds to form small molecules of several atoms
by contrast, the forces of attraction between these molecules are very weak
the result of these feeble inter-molecular forces is that the melting and boiling pints are very low, because the molecules are easily parted from each other
most molecular substances are gases or liquids at room temperature
molecular substances don't conduct electricity, simply because there are no ions.
you can usually tell a molecular substance just from its physical state, which is always kinda "mushy" ie. liquid or easily melted solid.
covalent substances: two kinds continued
giant covalent structures
these are similar to giant ionic structures except that there are no charged ions
all the atoms are bonded to each other by strong covalent bonds
they have very high melting and boiling points
they don't conduct electricity - not even when molten
they're usually insoluble in water
the main examples are diamond and graphite, which are both made from carbon atoms