Download 1 Atomic Mass

Atomic Mass
Because the mass of a single atoms is so small, for
convenience, chemists use the unit called atomic mass
unit (amu) also known as dalton (Da).
One amu is defined as 1/12 the mass of an atom of 12C
and is equal to 1.66054 x 10 -24 g.
Mass of one 12C atom = 12.000 amu
Because electrons mass is negligible, the protons and
neutrons each have a mass of ~1 amu
An element s atomic mass (listed in Periodic Table)
are weighted averages for the naturally occurring
mixtures of different isotopes of that element
Atomic mass of an element =
(mass of each isotope x the fractional
abundance of the isotope)
Carbon has two natural isotopes, C-12 and C-13.
At. mass of C = (fraction of 12C )(mass of 12C) + (fraction of 13C)(mass of
13C)
Given that: mass of 12C = 12.000 amu, mass of 13C = 13.0034,
and abundance of 12C = 0.9889, abundance of 13C = 0.0111
Atomic mass of C= (0.9889 x 12.000 amu) + (0.0111 x 13.0034) = 12.011
The element C in the Periodic
Table is shown as:
Problem ?: Copper has two isotopes: copper-63 (69.17%; isotope mass 62.94) and
copper- 65 (30.83%; isotope mass 64.94 amu).
Calculate the atomic mass of copper.
Ans: 63.55 amu
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Compounds
Chemical Reactions- atoms from two or more different
elements combine, creating new materials called
compounds.
Reactants (starting substances)
Products
(new compounds)
O 2 + 2 H2
pure elements
2 H2 O
water molecule
H2O means two H atoms attached to one O atom
Molecular Compounds
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Molecular Compounds
Covalent Bonding (in Molecules): The most common
type of chemical bond is formed when two atoms share
some of their electrons
Naming Molecules
Molecules are made up of two or more non-metals ( C, N, O, H, S, P,
B, halogens, Se, Si)
Naming Binary Molecular Compounds:
1)
Elements towards the left (cationic-like) of the periodic table uses its
elemental name
2)
Elements towards the right (anionic-like) of the periodic table
substitutes the second half of its elemental name with ide.
3)
Use the Greek prefixes to express the number of each element
present.
(mono, di, tri, tetra, penta, hexa, hepta, octa, etc.)
Examples:
N2O4 dinitrogen tetraoxide
SF6 sulfur hexafluoride
PCl3 phosphorus trichloride
Note that hydrogen (H2) and oxygen (O2) are also molecules.
We do not use prefixes here.
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Ionic Compounds
Ionic Bonding (Ionic Solids): These are formed by a
transfer of one or more electrons from one atom to another.
Between metal (gives up electrons) and a nonmetal
(accepts electrons)
2 Na (s) + Cl2(g)
metal
nonmetal
2 NaCl (s) (sodium chloride)
ionic compound
(note: we use (s), (g) and (aq) in chemical equations to denote the
physical state of the compound)
ionic crystal structure
of NaCl
Formation of sodium chloride by
loss and gaining of electrons
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Ions in Ionic Compounds
Ionic compounds contain charged species called
ions. The ions may be charged atoms or molecules
but overall the charges cancel to give a neutral
compound.
For NaCl, Ca(NO3)2 and Al2(SO4)3
the ions are:
Cations (+)
Anions (-)
+
Na sodium ion
Cl
chloride ion
Ca2+ calcium ion
NO3- nitrate ion
Al3+ aluminum ion
SO42- sulfate ion
Naming Binary Ionic Compounds
a)
Identify the positive ion and then the negative ion.
b)
The positive ion uses its elemental name.
c)
The negative ion substitutes the second half of its elemental name
with ide. [note: do not use prefixes]
Ions
Na+ and Cl
combine to form
Ca2+ and Br
combine to form
Al3+ and oxide combine to form
Compound
NaCl sodium chloride
CaBr2 calcium bromide
Al203 aluminum oxide
Eg. The ion of chlorine is chloride.
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