Atoms
What is the nature of matter?
Salt can be
divided into
piles.
Is there a
point where salt
can not be split
any further?
So…what is an Atom?
• From Greek
word “atomos”
meaning unable
to cut
• Atom = basic
unit of matter
diameter = 1
That’s pretty small! pm (picometer)
~ 10 -12
A Brief History:
• Democritus (4th Century
BC)
– First named the “atom”
– Lacked evidence
• John Dalton (1766-1844)
– Performed experiments
to discover:
• Dalton’s Atomic
Theory
Dalton’s
Atomic Theory
Um,
who
I’m Oxygen.
is this?
It
sure
is
1. All
composed of tiny
I amindivisible
different
Heelements
looks
nice
particles
from you, but
pretty called atoms.
hanging
Tell me
I’m sure we
different
out with
about it,
could find
from you
you,
Carbon!
some
way to
and me,
2. Atoms
of
the
same
element
are
identical
Carbon.
bond!
Carbon.
Still more of Dalton’s Atomic
Theory…
3. Atoms of different elements can
physically mix together or can chemically
combine to form compounds.
4. Chemical reactions occur when:
– atoms are separated, joined or rearranged;
– however, they are never changed into atoms
of another element.
Reflecting on Dalton’s Theory
Most of Dalton’s theory is still accepted.
Which part do you think has been
rejected?
The Atom is Divisible!
Return to Theory #1 and cross out the word “indivisible”
The three kinds of subatomic
particles
1. Protons (p+)
–
–
–
-
One unit of positive charge
Relative mass = 1 AMU
Actual mass = 1.67 x 10-24 grams
Found in the nucleus of the atom
2. Neutrons (n0)
–
–
–
–
No charge
Relative mass = 1 AMU
Actual mass = 1.67 x 10-24 grams
Found in the nucleus of the atom
The three kinds of subatomic
particles
3. Electrons (e-)
–
–
–
–
Negative charge
Relative mass = 1/1836 AMU
Actual mass: 9.11 x 10-28 grams
Found in the orbitals outside the nucleus
In 1911, Rutherford performed an
experiment:
What did this
prove?
Source of
alpha
particles
Thin sheet of gold foil
Atoms are mostly empty space
• Rutherford proposed that the atomic
structure was like “planets around the sun”
– This did not account for very specific traits
that atoms possessed.
Atomic Structure
Protons and Neutrons make up the nucleus
Electrons spin around in orbitals surrounding
the nucleus
What makes atoms different from
each other, if they have the same
basic parts?
The Atomic Number
Definition: The number of
protons in the nucleus of
an atom, which also
identifies the element
Mass Number
• The total number of protons and
neutrons is called the mass
number.
Based on this nucleus, what
is the mass number? 4
+ +
What element is it? Helium
Atomic Number vs. Mass
Number
 The Mass
Number = the
total # of p+ and
no
 The mass
number is here.
 The atomic number =
the number of
protons
 Usually the # of
p+ = # of e2
He
4. 00
Helium
Why protons?
Why not neutrons?
Or electrons?
• Oxygen’s Atomic Number is 8.
– How many protons does it have? _______
8
8
– How many neutrons does it have? ________
8
– How many electrons does it have? ________
Electron Orbitals
• The 1st orbital of an atom (closest to the
nucleus) can hold 2 electrons
• The 2nd orbital of an atom can hold up to 8
electrons = octet rule
The Atomic
Number
Element’s
Name
Atomic Mass
5
5
P = _____
B
6
N = _____
Boron
10.81
5
E = _____
Bohr Diagram
Lewis
Structure
Valence Number: 3
B
were
HiYou
there!
Greetings, Nitrogen.
Bonding
and Chemical
right!
I am
We
are Hydrogen
I’mReactions
now
Ammonia!
atoms. Perhaps we
Nitrogen.
But, am I an
•willChemical
Bonding:
change the
way
Am
I
an
atom or a
you
think
about
Combining Atoms of Elements
to
form
atom
or
a
compound
yourself.
Chemical Compoundscompound?
now?
Oh, you can’t be
serious!
Chemical Symbols of
Elements
1st Letter of Symbols are Capitalized
Ex. C = Carbon
Ca = Calcium
S = Sulfur
Si = Silicon
Bonding
An atom is most stable when its outer orbital is either
full or empty
To do this, it will either gain, lose or share electrons.
Ionic Bonding
A bond that involves a transfer of electron
Ex. Sodium Chloride (NaCl) = Salt!
Na+1 ion
Sodium
Cl -1 ion
Chlorine
What keeps ionic bonds together?
• One atom loses its electron, therefore
becoming a positive (+) ion.
• The other atom gains an electron, therefore
becoming negative (-) ion.
• Opposites attract!
Luv ya honey!
(+)
Luv ya
too! (-)
Can you guess which
Covalent Bonds
element I am?
will complete
• A covalent bond is What
a chemical
bond my
valence shell?
formed by sharing electrons.
Different types of Covalent
Bonds
Single Bond: One pair of shared electrons
Double Bond: Two pairs
= of
H shared
– H electrons
Triple Bond: Three pairs of shared electrons
O=C=O
C
Hydrogen Cyanide (HCN) can form this.
Can you?
Bonding Basics Practice
1-
Ionic Bonds
1. Mg + Br
2+
Br
2. Pb + S
1-
Mg
2-
4+
S
Pb
Br
MgBr2
2-
S
PbS2
1-
Cl
3. Al + Cl
Cl
Al
Cl
1-
3+
1-
AlCl3
Bonding Basics Practice
Covalent Bonds
1. H + Cl
Cl H – Cl
H
Cl
2. C + Cl
Cl
C
Cl
Cl
O
Si
Cl
C
Cl CCl4
Cl
Cl
3. Si + O
HCl
O
O = Si = O
SiO2
How do you count atoms?
H2O
Subscript =
# of atoms
within that
molecule
2 Hydrogen
1 Oxygen
Al2(SO4)3
Subscript =
multiple of
everything
within the
parenthesis
2 Aluminum
3 Sulfur
12 Oxygen
3Al2(SO4)3
Number in
front
indicates #
of entire
molecular unit
6 Aluminum
9 Sulfur
36 Oxygen
Chemical Equations
• Chemical equations are sentences that describe
chemical reactions
• A chemical reaction involves the breaking and
reforming of chemical bonds (rearrangement of
atoms)
Reactants (ingredients)  Products (what’s made)
2 H2
+ O2

2 H 2O

Equations are “balanced” = same number of atoms
on both sides of the arrow.