Atomic Structure
•All matter is made of atoms.
•Every atom has two main regions:
1) Nucleus
*Contains protons – positive charge
Each proton has a mass of 1 atomic mass
unit (AMU)
*Contains neutrons – no charge (neutral)
Each neutron has a mass of 1 AMU
Almost all of an atom’s mass in in the nucleus.
The nucleus has a net positive charge.
2) Electron Cloud
*Contains electrons – negative charge
Almost no mass; we say the mass of an
electron is negligible. Approx. 1/1800 of
an AMU
*Electrons are arranged in energy levels.
At one time these were called shells.
Energy levels are not exactly locations;
they are “categories” associated with
certain frequencies and energies
possessed by the electrons.
Energy levels can be divided into
sublevels:
OMIT THIS
SLIDE
1st energy level
1 sublevel
s
2 e-
2nd energy level
2 sublevels
s
p
2 e6 e-
3rd energy level
3 sublevels
4th energy level
4 sublevels
s
p
d
s
p
d
f
2 e6 e10 e2 e6 e10 e14 e-
2
8
18
32
The arrangement of electrons determines
the chemical properties of an element:
For example:
Helium, element #2, has 2 electrons in the 1st
and only energy level. It is chemically stable
because that energy level is filled.
Other noble gases – Ne, Ar, Kr, Xe, and Rn –
have 8 e- in the outer energy level; these
elements are stable also.
[Electrons in the outer energy level are called
valence electrons.]
The noble gases are found in the
last column of the periodic table. A
column is called a GROUP.
All other elements have partially filled outer
energy levels and are not chemically
stable.
Elements react chemically with each other in
order to lose, gain, or share outer energy
level electrons to become stable.
The closer an element is to having 0 or 8
electrons in the outer energy level, the
more reactive it is likely to be.
Elements in group 1 of the Periodic Table are
called alkali metals. They have one
valence electron and are “anxious” to lose
that electron in order to be stable. We say
the alkali metals have a valence number
of +1 because they have
1 electron to give away.
Group 2 elements are called alkaline earth
metals. They have 2 outer energy level
electrons and a valence number of +2. They
are not quite as chemically active as group 1.
Groups on the periodic table have
2 different types of numbers –
Roman numerals and regular
counting numbers. “A” groups
are numbered IA-VIIIA, to
indicate the number of valence
electrons. “A” groups are also
known as the representative
elements. They are group
numbers 1-2 and 13-18
IA IIA
1
2
“A” groups
IIIA IVA VA VIA VIIA VIIIA
13 14 15 16 17 18
Elements in group IA have one valence electron.
Elements in group IIA have 2 valence electrons.
Elements in group VII A have 7 valence electrons.
How many valence electrons are in
Group IIIA elements? 3
How many in Group VIA elements?
Group IVA?
Group VA?
4
5
Group VIIIA?
8
6
“B” groups are also known as
transition elements. Their
Roman numerals are irregular.
They are in groups 3-12. Their
valence numbers are not
predictable by their groups
numbers alone; they vary.
“B” groups
IIIB IVB VB VIB VIIB
3
4
5 6
7
VIIIB
8
IB IIB
9 10 11 12
In the names of their compounds, Roman
numerals are used to indicate the valence
numbers of transition metals.
Examples include “Iron (III) chloride” for
the compound FeCl3, to indicate that the Fe
in the compound is Fe+3.
Valence numbers are also called oxidation
numbers.
There is a “stair-step line” that cuts
through the periodic table:
It divides the
metals from the
nonmetals.
nonmetals
metals
Elements located along the line are
called metalloids and have properties of
both metals and nonmetals.
Another way of saying this is that they
sometimes act like metals and they
sometimes act like nonmetals.
Metalloids
In
general, sometimes
metals tendtend
nonmetals
lose,
to LOSE
to
sometimes
GAIN
electrons
electrons
gainin in
chemical reactions.
electrons.
11
Na
Sodium
22.9897
The whole number is the
ATOMIC NUMBER. It is
the number of protons in
an atom of this element.
This is the element’s
CHEMICAL SYMBOL.
The decimal number is the
AVERAGE ATOMIC MASS
of all atoms of this element.
Every atom has a MASS NUMBER equal
to the sum of its protons and neutrons.
For example:
16
O
8
127
I
53
has a mass number of 16 and an
atomic number of 8. This means
oxygen has 8 protons and 8 neutrons
(16 minus 8)
has a mass number of 127 and an
atomic number of 53. This means
iodine has 53 protons and 74 neutrons
(127 minus 53)
In a neutral atom (one that has not
reacted chemically), the number of
electrons is equal to the number of
protons.
The number of neutrons in an
element’s atoms may vary. For
example, chlorine may be
37
Cl
17
(20 neutrons)
35
or
17
Cl
(18 neutrons)
Carbon may be:
12
6
C
(6 neutrons)
13
or
C
6
(7 neutrons)
14
or
C
6
(8 neutrons)
Notice that they all have 6 protons.
These different forms of an element are
called ISOTOPES.
The AVERAGE ATOMIC MASS on the
periodic table is the weighted average
of all the atom’s isotopes.
For example:
This is the average of all sulfur
isotopes – 32, 31, 33, etc.
Because the average is so close
to 32, we know that most sulfur
atoms are the 32 isotope.
S
16
16
S
Sulfur
32.065
When an atom such as sodium has one
valence electron, it needs to lose that
electron to become stable. The energy level
below that electron is “full” and stable.
valence
electron
nucleus
11 protons
12 neutrons
stable 2nd energy
level; contains
8 electrons
11 electrons
in energy levels
outside the nucleus
Since atoms start out with equal
+
numbers of protons (p ) and
electrons (e ) the loss of one electron
makes the atom have a positive charge.
11 p+
-
10 e
11 protons
12 neutrons
10 electrons
1+ charge
because we
lost one
We call this a POSITIVE
+1
ION and write it as Na
An atom may lose 2, 3, or even 4
electrons to become an ion with a
+2, +3, or +4 charge.
outer energy level
Loses 2 e-
and becomes 2+
Loses 3 e-
and becomes 3+
Loses 4 e-
and becomes 4+
An atom with 6 or 7 valence electrons needs
to gain 1or 2 electrons to become stable,
with 8 electrons in the outer energy level.
This set of 8 electrons is called an OCTET.
-
A gain of 1 e gives
the atom an octet
and a -1 charge.
A gain of 2 e- gives
the atom an octet and
a -2 charge.
outer
energy
level
When atoms lose or gain electrons,
they form IONIC BONDS.
+
-
+
-
-
+
-
+
+
-
+
-
-
+
-
+
+
-
+
-
-
+
-
+
The + and – ions
are attracted, but
not attached, to
each other.
Elements on opposite sides of the
periodic table (metals, nonmetals)
tend to form ionic bonds.
+1 -1
+1 -2
Ex: NaCl
Ag2O
O
Na
Cl
Ag
Sometimes atoms near each other on the periodic
table, especially nonmetals, share valence
electrons and form COVALENT BONDS.
Ex: CO2
N2O3
C N O
In covalent compounds, the atoms are
actually connected to form molecules.
O
C
Carbon
dioxide
molecules,
CO2
O
O
C
O
Water
molecules,
H2O
H
HH
O