Chapter 5.
Atoms and Molecules
Atoms are the smallest particles of an
element that can exist and still have
the properties of that element.
Atomic Theory
Proposed by John Dalton, 1808
All matter is made up of small particles called atoms.
All atoms of a given type are similar, and different
from atoms of other types.
The relative number and arrangement of different
types of atoms in a substance determine its identity.
Chemical change is a union, separation, or rearrangement of atoms to give new substances.
Atoms are indivisible, so only whole atoms can
participate in or result from any chemical change.
Atomic Theory
Proposed by John Dalton, 1808
How big are atoms?
Not very!
Diameter ~ 10-10 m
~ 1 million in diameter of printed period
Mass ~ 10-22 g
~ 1025 would weigh 1 pound
This was figured out in early 1900's
Atomic Theory
Proposed by John Dalton, 1808
Molecules
A molecule is a group of two or more atoms
that functions as a unit because the atoms
are tightly bound together.
Molecules
Some elements exist as molecules.
Also H2, N2, O2, F2, Br2, I2
Molecules
In molecules, elements are bound in a specific
order and geometry. They act as a unit.
Molecules
Not all compounds exist as molecules!
Chemical Formulas
A chemical formula is a notation that shows
the chemical symbols of the elements present
in a compound, with numerical subscripts to
the right of each symbol to show how many
atoms of each element are present in the
compound.
Water H2O Methane CH4 Aspirin C9H8O4
Calcium hypochlorite Ca(OCl)2
Tetraethyl lead Pb(C2H5)4
Chemical Formulas
Writing chemical formulas:
Subscripts are omitted if only one atom of
that element is present.
Elements are usually listed in order from left
to right as they appear on the periodic table.
Sometimes a group of atoms acts as a unit
within a compound. Such groups are enclosed in parentheses, followed by a subscript to show how many groups are present.
Chemical Formulas
Types of chemical formulas:
Empirical formulas show the smallest
whole-number ratio of the elements found in
a compound. They are always used for nonmolecular compounds.
Molecular formulas are usually used for
molecular compounds. They are wholenumber multiples of empirical formulas.
Chemical Formulas
How many of each type of atom are found in
the following compounds?
Na2O
NH3
Al(NO3)3
COCl2
C6H12O6
N2H4
Fe2O3
CoCl2
Which of these are empirical formulas? Which
are molecular formulas?
Subatomic Particles
For almost 100 years, atoms were thought to
be indivisible. In the late 1800's pieces of
them started showing up!
By 1935, it was known that atoms are made up
of three types of subatomic particles:
Electrons
Protons
Neutrons
Subatomic Particles
Diagram of a gas
discharge tube
used to study
the behavior of
“cathode rays.”
Subatomic Particles
Particle
Mass,
amu
Charge
Notes
Electron
0.0005
-1
Exists on periphery
of atom
Does chemistry
Proton
1.007
+1
Exists in nucleus
Defines element
Participates in
nuclear reactions
Neutron
1.008
0
"Glue" in nucleus
Gives isotopes
Subatomic Particles
Rutherford’s gold foil experiment
Subatomic Particles
Where things are:
Subatomic Particles
Relative sizes:
If the nucleus were the
size of a pea, placed in
the center of a stadium,
the electrons would be
in the outer seats!
Subatomic Particles
Fun facts:
The atom is mostly
empty space!
Most of the mass is
packed into the nucleus
which has a density of
~1014 g/cm3!
Complete Atomic Symbols
The complete atomic symbol shows the number of protons and neutrons in the nucleus,
and the charge (if any) on the atom.
M
C
E
Z
Complete Atomic Symbols
E = Symbol for Element
Z = Atomic number = # of protons in nucleus
Atomic number defines element
Above symbol for element in Periodic Table
M = Mass number = sum of protons and
neutrons in nucleus
C = Charge; Zero for neutral atom, when
# of electrons = # of protons
Complete Atomic Symbols
Reading symbols of neutral atoms:
14
56
C
6
238
Fe
26
Alternate notation:
U-235
P-31
U
92
C-14
Isotopes
Isotopes are atoms of the same element
that have different numbers of neutrons
in their nuclei.
Isotopes of an element have the same normal chemistry (that's a job for electrons!)
but different nuclear behavior. This can
show up in magnetic properties and
nuclear reactions.
Isotopes
Isotopes of some common elements:
1
2
H
3
H
H
1
1
1
12
13
14
C
6
C
6
C
6
Isotopes
Most elements have several isotopes.
These are exceptions:
9
19
Be
23
F
4
Na
9
27
11
31
Al
13
P
15
Atomic Masses
The atomic mass of an element is a
weighted average mass of the isotopes
of that element.
It appears below the element's symbol
on the periodic table.
It represents the mass one would
obtain by measuring the mass of a
group of atoms.
Atomic Masses
Atomic mass units (amu) are used to
give masses of atoms.
Atomic mass units are defined relative
to 12C. One atom of 12C has a mass of
exactly 12 amu.
1.00 amu = 1.99 x 10-23 g
Atomic Masses
Atomic mass of Magnesium
Isotope
Atom % Mass,
amu
Contribution
to total
24Mg
78.70
23.985
18.88 amu
25Mg
10.13
24.985
2.531 amu
26Mg
11.17
25.983
2.902 amu
Total = 24.31 amu
Charges
M
C
E
Z
C = Charge; Zero for neutral atom,
when # of electrons = # of protons
# of electrons = # of protons - charge
Charges
Examples:
23
1+
35
Na
11
1-
63
Cl
17
2+
Cu
29
If we're working with a collection of ions,
all isotopes are present, and we don't
need M (or Z). Simplify:
Na1+
Cl1-
Cu2+