ATOMS, MOLECULES AND STOICHIOMETRY

ATOMS, MOLECULES AND STOICHIOMETRY
Relative Atomic Mass is defined as the mass of one atom of an element compared with 1/12 the mass of one atom of carbon-12.
Relative Isotopic Mass is defined as the mass of one atom of an isotope compared with 1/12 the mass of one atom of carbon- 12.
Relative Molecular Mass of a compound is the sum of the relative atomic masses of all the atoms in one molecule of a covalent compound.
Relative Formula Mass is the sum of all relative atomic masses from the formula of any given substance (atoms, molecules, ions, ionic compounds, etc.)
Avogadro Constant is a large mathematical constant defined as the number of carbon-12 atoms in 12g of carbon-12 (one mole).
Empirical Formula shows the simplest whole-number ratio for the atoms of different elements in a compound.
Molecular Formula shows the actual number of the different elements in one molecules of a compound.



ATOMIC STRUCTURE
Atoms are made up of protons, neutrons, and electrons.
Protons are positively charged. Each proton has a relative charge of +1 and a relative mass of 1 atomic mass unit.
Neutrons are uncharged. Each neutron has a relative charge of 0 and a relative mass of 1 atomic mass unit.
Electrons are negatively charged. Each electron has a relative charge of -1 and a relative mass of 1/1840 atomic mass unit.

Proton
Neutron
Electron
Relative charges
+1
0
-1
Relative masses
1
1
1/1840

A nuclide is a specific combination of protons and neutrons in a nucleus.
Atomic/Proton number---Number of protons
Mass/Nucleon number---Number of neutrons and protons
Shell
No. of orbitals (n2)
No. of each type of orbital
Maximum no. of electrons in shell (2n2)
s
p
d
f
n = 1
1
1



2
n = 2
4
1
3


8
n = 3
9
1
3
5

18
n = 4
16
1
3
5
7
32

? A sub-shell is a group of orbitals with the same energy level, but differ in their orientation in space, e.g. the second shell (n = 2) contains two sub-shells:
Ø A sub-shell containing one s orbital (2s), and
Ø A sub-shell containing three p orbitals (2pxy, 2py,2pz).
?
In an atom, the orbitals have definite
amounts of energy called energy levels.
These energy levels have a convergent pattern.

[NB. 4s orbital has slightly lower energy
than the 3d orbitals.]




Shapes of orbitals
s orbitals
? Each s sub-shell ha 1 orbital only
? Spherical shape
? Size increases as the principal quantum number increases.


p orbitals
? Each p sub-shell has 3 orbitals
? Dumb-bell shape
? Different axes of symmetry for each p orbital (each orbital is mutually perpendicular to the other)
? Orbitals with the same quantum number have the same energy i.e. they are degenerate.
? Size increases as the principal quantum number increases.