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Transcript
```The Atom: basic unit of all
matter
The Atom is composed of many small
particles. We are going to concentrate
on the THREE MAIN ONES:
•Inside the nucleus there are TWO ……the
Proton (has a positive charge) and the
Neutron (neutral)
•Outside of the nucleus, roams the
electron (has a negative charge)
The Nucleus:
The central core of the atom.
Provides the mass of the atom.
Has an overall positive charge due to the
protons within.
Mass number: the number of protons + the number of neutrons
Atomic number: the number of protons
You should notice that each has its own number of
proton(s) and thus, an equivalent number of
electrons, since all atoms are neutral until they
react with another.
# of
protons
electrons
# of
=
Determining an Atom’s Mass Number and
Atomic Number by Using the
Periodic
Table
7
N
14.00674
Atomic Number
Element’s
symbol
Relative Atomic Mass
So, from the periodic table you can determine the
number of protons since it is the same as the atomic
number. But, how do you determine the number of
neutrons? Well, remember the definition of isotopes?
Each atom has at least two isotopic forms. So, we
have two ways to determine the number of protons
for an individual atom. First, from a special symbol,
like the C-14 or C-12. The 14 and the 12 are the
given mass numbers for that atom. So, given this
info, all you have to do is:
Remember the mass number = #p + #n and, so
mass number - #p = # neutrons
#14 - 6p = 8 neutrons and
#12 - 6p = 6 neutrons
You try some:
Li -7: #p = ____, # e = _____, # n = ____
P- 31: #p = ____, # e = _____, # n = ____
Pb-207: #p = ____, # e = _____, # n = ____
Cl-35:
#p = ____, # e = _____, # n = ____
Answers: Li-7 has 3 protons, 3 electrons, 4 neutrons
P-31 has 15 protons, 15 electrons, 16 neutrons
Pb-207 has 82 protons, 82 electrons, 125 neutrons
Cl-35 has 17 protons, 17 electrons, 18 neutrons
If you are not given this symbol with the mass
number, you can still determine the mass
number for the most abundant isotopic form
for that atom. For example, if you look on the
periodic table for Carbon, you will see a value
underneath its symbol - the RELATIVE
ATOMIC MASS of 12.011. This mass is a
special average mass for all of the isotopes
that exist for that atom. It takes into
consideration HOW abundant each isotope is.
So, the most abundant isotope has the most
influence on this mass! So, if you round this
number to the nearest whole number you have
the MASS NUMBER FOR THE MOST
COMMON ISOTOPE.
A few examples, determine the #p, #e, #n:
53
16
13
I
S
Al
32.066
26.981539
126.90447
Iodine has 53 protons, 53 electrons, 74 neutrons
Sulfur has 16 protons, 16 electrons, 16 neutrons
Aluminum has 13 protons, 13 electrons, 14 neutrons
More on
those
electrons!!!
Electrons exist outside of the nucleus,
are extremely small and so are nearly
weightless, and move at the speed of
light!!!!!! So, they do not add much
to the mass of the atom but they do
control the chemical activity of the
atom and the SIZE of the atom
because of all that motion.
Electron Clouds
Electrons are moving constantly. So, it is
impossible to know, at any given moment, their
exact position. So, we say that they exist in a
region of space known as the electron cloud.
Since electrons are negative they want to
get close to the positive nucleus. But, since
negative repels negative, electrons have to
arrange themselves in special orbits called
energy levels. They do this in order to be
close to the positive nucleus yet, not
repelling each other too much.
Shapes of the Electron Clouds
Energy Levels
The first energy level can only hold up to
TWO electrons to be STABLE. The second
energy level can hold up to EIGHT electrons.
The third can hold up to 18 electrons. This
can be determined by using the following
formula: 2(n2) ; where the n = energy level #
So, what could the 4th hold?____the 5th?___
Valence electrons – outermost electrons from the
nucleus
*Most atoms want to have 8 valence
electrons because it is the most stable
configuratiion!!!! (The Great Eight = an octet)
Only the simplest of atoms can be realistically drawn.
Drawing an atom of Fluorine (F-19)
e- ee-
____#protons
____# electrons
____#neutrons
e-
e-e- e-e-
e-
How many electrons would fluorine like to
get, to be stable?__________
e-
e-
Number of Valence Electrons in a Family
1e 2e
3e 4e
5e 6e
7e 8e
Draw an atom of:
hydrogen(H-1)
oxygen (O-16)
e-
e-
e-
hydrogen (H-1)
e-
ee-
e-
ee-
e-
Let’s bond these 2 hydrogen and 1 oxygen together to make
……….
e-
ee-
ee-
e-
Partial charges should
be drawn on water
since it is polar. More
on this later!
e-
ee-
e-
We just
created a
COVALENT
BOND. This
is a bond in
which two or
more atoms
SHARE
electrons so
that they all
become more
stable.
Ionic Bonding
Some elements do not want to share electrons because
their nucleus is so strong it actually takes the electrons
from another atom. This strong atom which GAINS
electrons (which are negative) becomes a NEGATIVEcharged ION, OR ANION.
Ion: an atom that has lost or gained ELECTRONS!
The weaker atom, which loses an electron or more,
becomes a POSITIVE-charged ion, a CATION.
Once these atoms become IONS, they are now
attracted to one another and so you have what is
called an IONIC BOND.
Diagram of what happens in an Ionic Bond
Draw a sodium (Na)
e- eeeeeeeee-
Draw a chlorine (Cl
e-
e-
e-
ee-
e-
e- eeee- eeeeeeeeee-
Once done, charges should be
drawn in since you no longer have
neutral atoms but IONS!
Ionic Bonds and Cations
What would be easier (require less
energy)?
To give away 1 or 2 electrons or
bring in 6 or 7 electrons in order
to gain that octet?
So: Metals are found on the left side of the
periodic table. Metals in column 1 have only
one valence electron and in column 2 have only
2 valence electrons. These metal families are
most likely to give away their electron(s) to
become positive cations. In other words,
most metals become cations when
involved in ionic bonds, especially those in
Family 1 and Family 2.
Some Metal Examples
Gold
Sodium
Zinc
Mercury
Ionic Bonds and Anions
What columns on the
periodic table have 5, 6, or 7
valence electrons?_________
What would be easier (require less energy)?
To give away 5, 6 or 7 electrons or bring in
1, 2, or 3 electrons to gain that octet?
So: Nonmetals are found on the upper, right
corner of the periodic table. Atoms in these
families, such as families 16 and 17, have 6 and
7 electrons, resp. and are most likely going to
gain electrons when involved in an ionic bond.
In other words, the nonmetals become the
anions when involved in an ionic bond.
How do you know what kind of bond will form?
Once you get into chemistry, you will be given some
rules used to determine this with care. Since you are
in Biology class for now, we are going to use a
general-rule-of-thumb.
It is the following:
If the atoms joining, are on the right hand
side of the stair step (nonmetals), you can
assume the bond(s) will be covalent. If the
atoms joining, are from opposite sides of the
table (a metal with a nonmetal), you can
assume the bond will be ionic.
Examples: NaCl
is ….. ionic
H2O is ….. covalent
CO2
is …..