Atoms and Elements
Chapter 3
Elements are pure substances from
which everything else is built.
An element is a fundamental substance that can
not be broken down,
BY CHEMICAL MEANS,
into a simpler substance.
Ex./ Carbon, Iron, Nitrogen, Calcium, Copper,
Mercury.
The elements have names and symbols.
The first letter of the symbol is always
capitalized.
If there is a second (or third) letter in the
symbol it is never capitalized.
MEMORIZE table 2 page 82 (Names and
symbols of some common elements)
There periodic table has groups.
Arranged in order of increasing atomic number.
Families (groups)
The table is periodic.
There are metals and nonmetals.
Then there's the metalloids ...
Dalton had a theory.
1.) All matter is made up of atoms
2.) All atoms of any element are like each other and
different from atoms of other elements.
3.) Two or more atoms of different elements combine to
form compounds. Any given compound always has the
same number of atoms of its elements.
4.) When a chemical reaction occurs the atoms
rearange, but are never created nor destroyed.
J.J. Thomson discovered the electron!
Particles can have an electric charge.
This charge has certain properties ...
Positive and Negative
Opposites attract, likes repel.
May be transferred
q1q 2
F= k 2
r
These are the properties of the electron:
Mass = 9.11 x 10-28 g
Charge = -1
Symbol = e─
MEMORIZE!
Thomson’s thought the atom looked like
plum pudding!
Never trust an atom, they make up
everything!
Rutherford liked plum pudding, until ...
Rutherford goes nuclear.
Since the theory (plum pudding) didn't match the
data Rutherford came up with his own hypothesis:
Most of the mass and all of the positive charge is in
a very small region in the center of the atom, the
nucleus.
Very little of the mass, and all of the negative
charge occupies most of the volume of the atom,
outside of the nucleus.
Thomson vs. Rutherford!
What's in an atom anyway?
There's the atomic number (Z) and
then there's the mass number (A)
The atomic number (Z) of an element is the number
of protons in the nucleus.
The number of protons in the nucleus is what
determines which element any given atom is.
The mass number (A) is the number of protons +
the number of neutrons in the nucleus.
Isotopes are like cousins.
They have the same number of protons, but
different number of neutrons.
This means they are the same element, but
have different masses.
Mass number = p + n
A
Z
Atomic number = p
E
Elemental symbol
We use a unit of mass for atoms,
called ...
The atomic mass unit (the amu)!
The amu is defined as exactly 1/12 the mass of a
12
C atom.
1 amu = 1.6606 x 10-24 g
The average atomic mass of the elements is on
most periodic tables.
Every element has an average
atomic mass (AAM).
AAM = f1m1 + f2m2 + …
m1 = mass of isotope 1
f1 = fractional abundance of isotope 1
m2 = mass of isotope 2
f2 = fractional abundance of isotope 2
etc.
For example:
Rubidium has 2 naturally occurring isotopes:
Rb-85 which has a mass of 84.9118 amu and
a natural abundance of 72.17%,
and Rb-87 which has a mass of 86.9092 amu and a
natural abundance of 27.83%.
Calculate the AAM of Rubidium.
85.47 amu
Another way to ask about AAM:
Silver (Ag) has 2 naturally occurring isotopes,
Ag-109 and Ag-107.
Ag-107 has a natural abundance of 51.84% and a
mass of 106.905 amu.
What is the isotopic mass of Ag-109?
108.9 amu
Every electron in an atom occupies an
energy level (shell).
Each energy level has a
principal quantum number, n.
Shells
Each energy level consists of one or
more sublevels (subshells).
In each sublevel there are one or
more orbitals.
An orbital is a region where there is a high
probability of finding an electron.
This is what s orbitals look like.
Here are what p orbitals look like.
And the d orbitals ...
These are f orbitals.
Electrons always go into the lowest
energy orbital that has room.
(THE AUFBAU PRINCIPLE)
For a NEUTRAL atom, the number of electrons
is equal to the number of protons.
For an anion, add electrons.
For a cation, subtract electrons.
Orbitals fill in order of lowest energy to
highest energy.
The Pauli exclusion rules tells us that
no more than 2 electrons can be in the
same orbital.
If two electrons are in the same orbital in the
same atom they must have opposite “spins”.
We show electrons as arrows in orbital
diagrams. They point up or down.
Hund’s rule tells us what to do when
putting electrons into orbitals that have the
same energy as each other.
Put one electron into each orbital before pairing them
up.
Whichever way the first arrow (electron) points, the others
must point the same way until they pair up, then they point
in opposite directions.
Here is the hydrogen atom.
Here is helium.
and lithium ...
Here is carbon.
Noble gas (abbreviated) notation is a
shorter way of saying the same thing.
Write the previous noble gas in square
brackets.
This represents all of the electrons through
that noble gas.
To the right of the brackets write the
remaining electrons.
There are core electrons, and valence
electrons.
VALENCE electrons are all electrons that
occupy the highest “n”.
That is, all electrons with the highest number
before their letter in the electron configuration of
an atom are VALENCE electrons.
All other electrons are CORE electrons.
You can read electron configurations
from the periodic table.
Atomic size generally decreases as you
go up and to the right in the periodic
table.
Ionization energy is the energy
required to remove an outer electron
from a gaseous atom.
K(g)
K+(g) + e−
Ionization energy generally increases as you
go up and to the right in the periodic table.
Metallic character generally decreases
as you go up and to the right in the
periodic table.