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Transcript of S3 Bonding
The first shell can hold two electrons,
the others hold eight
These electron arrangements are very stable
Bonds hold the atoms together
Only the outer (valence) electrons are involved
When bonded, atoms have filled electron shells,
e.g. chlorine's electron arrangement becomes the same as argon's
this type of bonding involves atoms sharing one or more pairs of electrons
There are seven elements which exist as diatomic molecules in which the two atoms are joined by covalent bonds
We can draw pictures showing the outer electrons being shared
Usually only non-metal elements form covalent bonds
(Atoms of the elements in Groups 1-3 tend to lose electrons rather than share them)
The two positively charged nuclei attract the negatively
charged pair of electrons which holds the atoms together
Covalent bonds are strong forces of attraction!
this type of bonding involves metal atoms losing electrons to become positively charged
ions and non-metal atoms gaining electrons to become negatively charged ions
A molecule is made up of covalently bonded atoms
The chemical formula of a covalent molecular substance gives the number of atoms of each element found in each molecule,
e.g. H2O molecules consist of two hydrogen atoms and one oxygen atom bonded together in a molecule
Ions are charged particles with stable electron arrangements (the same as one of the noble gases)
The chemical formula of an ionic substance gives the simplest ratio of ions in the ionic lattice,
e.g. MgCl2 has the ratio of 1 magnesium ion : 2 chlorine ions
Li 2,1 Li+ 2
Mg 2,8,2 Mg2+ 2,8
Al 2,8,3 Al3+ 2,8
positive ions usually contain a metal (ammonium is an exception, NH4+)
negative ions usually contain a non-metal
group ions such as sulfate, SO42-, contain more than one element
Ions are held together in a giant ionic lattice
The oppositely charged ions are attracted to each other and this electrostatic force of attraction holds them together
Ionic bonds are strong!
charged particles can conduct electricity - these could be electrons or ions
MELTING AND BOILING POINT
The remaining four diatomic elements are: fluorine, chlorine, bromine and iodine
Elements such as chlorine have an incomplete outer electron shell which means that they are able to bond with other atoms
single covalent bond double covalent bond triple covalent bond
Note that the atoms are now surrounded by a filled outer electron shell so have a stable electron arrangement identical to one of the noble gases!
Some covalently bonded substances do not exist as small molecules. Instead they have a giant network of atoms joined by covalent bonds.
Examples include graphite, diamond and silicon dioxide.
To achieve a stable electron arrangement like those of the noble gases, sodium needs to lose its one outermost electron whilst chlorine needs to gain one electron to fill its outer shell
If sodium gives its outer electron to chlorine, both atoms become stable and form IONS
Sodium forms an ion with one positive charge, this is because the ion has 11 (+ve) protons and 10 (-ve) electrons
Chlorine forms an ion with one negative charge as it has 17 (+ve) protons and 18 (-ve) electrons
Compare with the noble gases!
F 2,7 F- 2,8
S 2,8,6 S2- 2,8,8
N 2,5 N3- 2,8
Ionic compounds can only conduct electricity when molten or in solution as the ions are free to move in these states
Ionic compounds have high melting and boiling points because the strong ionic bonds must be broken
Covalent network structures have very high melting and boiling points because the strong covalent bonds must be broken
Covalent compounds cannot conduct electricity in any state (except for graphite)
Shapes of molecules
Diatomic molecules (two atoms) are linear shaped. Molecules with three atoms can be linear or angular (bent planar) depending on which elements are joining together
SOLUBILITY (not in S3 course)
Ionic compounds usually dissolve in water and aqueous solvents
Covalent substances usually dissolve in covalent solvents
Covalent molecules have low melting and boiling points because the covalent bonds are not broken, only weak forces between the molecules need to be overcome