Download Atoms

What is an atom?

Atoms: are the smallest unit of an
element that still maintains the chemical
properties of that element.

Eg. An Oxygen atom
The Atom … old school…
Recall… The Atom…

The atom
is made
up of a
central
nucleus
(protons
and
neutrons),
and
electrons
orbiting
around it.
Another way to look at the Atom!!
Recall… Chemical Symbols
How do you always know which one is
the Mass number, and which one is
the atomic number?
1.
Recall that the atomic number tells us the
number of protons in an atom. If an atom is
neutral – this is also the number of Electrons
(Atom = Neutral, Ion = Not Neutral)
a. Since the atom is neutral we know that
the number of protons (p+) must equal the
number of electrons (e-).
e.g. Mercury’s atomic number is 80 so it
has 80 p+ and 80 e-.
2.
Atoms also contain a neutral subatomic
particle in the nucleus called the neutron.
a. The mass of the atom is made up of the mass
of the p+ and n0. ( Electrons way nothing!)
b. Atomic mass (rounded to the nearest whole
number) is equal to the number of p+ + n0.
c. Chemists use symbols to show the atomic
mass and number for an atom.
e.g. Mercury 200
Bismuth 209
80 Hg
83
Bi
d. Once you know the atomic number and
mass, then you can find the number of p+, n0, or
e-, or any combination of these.
Let’s try and fill in the table using the elemental
symbols given! (Find # of P+, n, and e-)
# of Protons
3
9
8
56
# of Neutrons
(mass numberatomic number)
4
10
8
80
# of electrons
3
9
8
56
Ions are formed when atoms gain or lose
electrons during a chemical reaction (and
therefore have a + or – charge on them.
2. This can happen when an atom is heated to
high temperatures, when exposed to high
voltage, or when ionic compounds are put in
water.
3. Metals tend to lose(give away) electrons and
form positive ions because they will have
more p+ than e- . Non-metals tend to gain
(take) electrons and form negative ions
because they have more e- than p+.
1.
If you see and element has a charge of 2+ - what
did the atom do to get that charge?
Gave away 2 electrons
If you see a charge of 3-, what did the atom do to
get that charge?
Gained 3 electrons
Why do atoms want to be ions?
They all want to look like Noble Gases because
Noble Gases are extremely stable (have a full
valence shell of electrons). Every atom wants to
be stable, so they will give away, take, or share
electrons to give the appearance of a Noble Gas.
5. Ions of atoms are shown using the symbol and
superscripts.
24
2
Eg. 12 Mg
6. When ions form, only the e- are affected, so the
number of p+ and n0 in an ion are the same as they
were in the atom.
2
4
Pb
Mg
7. To find the charge on an ion (see pg. 106):
Charge on the ion = # of protons - # of electrons
e.g. Mg has 12 p+ and 12e-, but loses two
electrons to become an ion; ion charge= 1210= 2+
1. Isotopes are atoms of the same element with the
same number of p+ and e-, but different number of
n0 (and therefore different mass)
2. Most of the naturally occurring elements have at
least one isotope.
e.g. hydrogen has three isotopes (pg. 108); they
are hydrogen-1, deuterium ( hydrogen-2), and
tritium ( hydrogen-3)
1
1
H
2
1
H
3
1
H
Isotopes: Video Clip
Notice that the masses of the elements are not
whole numbers. This is because the masses
take into account the masses of their isotopes
and their fractional abundance
(p. 110)—this is a weighted average.
A weighted average takes into account (1) the
magnitude of the numbers and (2) how
“important” those numbers are:
Example: A student’s grade is calculated by crediting each
3.
term with 40% value and their final exam with 20% value.
If James scores 85% and 76% in first and second term
respectively, and 60% on the final exam what is his final
grade?
Grade  40%(Term1)  40%(Term2)  20%( Final )
 0.40(85%)  0.40(76%)  0.20(60%)
 34%  30.4%  12%
 76%
To find the atomic mass of an element: - multiply each
isotope fractional abundance(in decimal form) by its
mass and then add the products.
Eg. Cl has two isotopes Chlorine-35 and Chlorine-37
Isotope
Mass
Abundance
Chlorine-35
34.969
75.53%
Chlorine-37
36.966
24.97%
Atomic mass  0.7553(34.969)  0.2497(36.966)
atomic mass  35.46