The Periodic Table
Mendeleev (1869) used valence and atomic
weight to organize elements into periods.
He did this through recognition that the
valences rose and fell in regular patterns
Meyer (1870) recognized a periodicity
between atomic volumes and atomic
weights
• Rows are periods numbered in relative to the first
quantum number of the orbitals being filled with
increasing atomic number
–
–
–
–
First period – electrons in 1s orbital
Second period – electrons in 2s, 2p orbitals
Third period – electrons in 3s, 3p orbitals
Fourth period – electrons in 4s, 4p, plus 10 elements filling 3d
orbitals
– Fifth and sixth periods – similar to fourth in terms of filling d
orbitals
– 6th period also contains 14 extra elements called the rare earths,
containing electrons in the 4f orbital
– 7th period has 14 extra elements called the actinides containing
electrons in the 5f orbital
• Elements in s and b blocks form the A
Group
• Elements in d block formed the B Group
• Group IA - +1 valence – Alkali metals,
except Hydrogen
• Group IIA - +2 valence – Alkaline metals
• Group VIIA - +7 valence: five in p orbital
and 2 in s orbital
– In nature these actually pick up one electron
to form a -1 valence
• Noble Gases – all orbitals filled – valence
=0
• Transition metals
– Group IB – d10s1 – valence +1
• 1s2,2s2, 2p6,3s2,3p6,3d10,4s1 = Cu
– Group IIB +2 ions
– Group IIIB – valence +3
• 3d1,4s2 = Sc
– Group VIIB – valence +7,+6,+4,+3,+2
– Sd5,4s2
• Group VIIIB- Fe, Co, and Ni are
anomalous
– Should be valence +8 based on 3d6,4s2
However, valence is actually +2 and +3
• Remainder of platinum group elements
have valence of +2, +3, +4, +6, +8
Periodic Law
• Chemical and Physical properties of
elements are similar with increasing
atomic number
• Elements in Groups have similar valences
because they have similar electronic
structures
– This leads to similar physical and chemical
properties within groups
Atomic Number and Atomic Mass
Number
• Atomic Number = z = number of protons in
nucleus
• Atomic Mass Number = A = Z+N
– 126C
• Atomic Mass Unit – AMU = 1/12 of the
mass of 126C
– The most abundant stable isotope of carbon
Atomic Weight, Gram Atomic
Weight, Gram Molecular Weight
• Atomic weight = sum of mass ratio of
naturally occurring isotopes
– Atomic weight is dimensionless because it is
expressed relative to the mass of carbon
• Gram atomic weight = atomic weight in
grams
• Gram molecular weight and gram formula
weight = molecular weight in grams
• Both Gram Atomic Weight and Gram Molecular
weight are referred to as the mole (mol)
– Basic unit of mass of elements and compounds
• 1 mol = fixed number of atoms or molecules
• NA = 6.022045 x 1023 atoms or molecules/mole –
Avagadro’s Number
– 1 mol of a substance contains as many basic units as
there are atoms in 0.012 kg of carbon-12
• The relationship between moles, atomic
weight and Avagadro’s Number allows us
to represent reactions by equations
2H2 + O2 >>>> 2H2O
2 mol = 1 mol >>>>>>>>> 2 mol
2 g + 1g does not yield 2 gram water
Molecular Weight of Compounds
•
•
•
•
•
•
BaSO4 (barite)
Ba – 137.3400 g/mol
S – 37.064 g/mol
O – 15.9994 g/mol
Barite – 233.4016 g/mol
Solubility in cold water for barite = 2.22x
10-4 g/100 mL
Molecular Weight of Compounds
• BaSO4 >>>>>>>>>> Ba2+ + SO421mol
1 mol + 1 mol
2.22 g/L divided by 233.4016 g/mol = 9.512 x 10-6
mol/L
9.512 x 10-6 mol/L x 137.34 g/mol = 1.3064 g Ba2+
Number of ions in solution = 9.512 x 10-6 x
6.022045 x 1023 = 5.778 x 1018 ions/L
Gram Equivalent Weight
• Gram equivalent weight = gram atomic
weight or gram molecular weight divided
by valence
• For acids and bases:
• Gram equivalent weight = gram molecular
weight divided by the number of hydrogen
ion or hydroxyl ion groups when the acid
or base is dissociated in fluid
Gram Equivalent Weight for Ca2+
• Atomic weight calcium = 40.08 g/mol
• Valence = +2
• Gram equivalent weight = 40.08 (g/mol)/2
= 20.04 equivalents (eq)
For Acids
• H2SO4 it is ½ the molecular weight
because 2 H will dissociate in fluid
– ½ molecular weight = 49.04 eq
• H3PO4 it is 1/3 the molecular weight
because 3 H will dissociate in fluid
– 1/3 molecular weight = 32.67 eq