The Atom & Periodic Table
How were atoms discovered?
What are atoms made of?
How is the Periodic Table arranged?
Atoms and the Periodic Table
• Atom “Atomos” (Greek) – smallest piece of
matter that still retains those properties
• Matter – anything that has mass and volume,
what all material is made up of
• Model – uses familiar ideas to explain
unfamiliar facts observed in nature
Models of the Atom
1) Greek Model
• 1) Search for a description of matter began with
a Greek philosopher Democritus more than
2400 years ago
▫ He thought that matter was divided over and over
– would eventually result in something so small
that it couldn’t be divided any longer
▫ He named this smallest particle of matter the
Atom.
2) Dalton’s Model
• Early 1800’s English Chemist John Dalton did
several experiments that led to the acceptance of
the atom
• Dalton’s Atomic Theory – 1803
▫ All elements are composed of Atoms, they are
indivisible and indestructible particles.
▫ Atoms of same elements are exactly alike.
▫ Compounds are formed by the joining of Atoms or
two or more elements
▫ *This theory became one of the foundations of
chemistry
3) JJ Thompson
▫ Electrons – negatively charged particles
▫ If there were negative particles he concluded there
had to be positive particles to balance it out
▫ Plum Pudding Model
▫
Negatively charged ‘plums’
▫
Positively charged ‘pudding’
4)Ernest Rutherford
• Gold Foil Experiment – (led to downfall of Plum
Pudding Model)
• Discovered the proton by shooting small alpha
particles through a thin sheet of gold, some were
reflected
• Gives us the “Planetary Model” where particles
surround the small, dense positively charged
center
5) Niels Bohr
• 1913, Danish Scientist – thought the negatively
charged electrons have a definite orbit around
the nucleus – much like the planets revolve
around the sun
• Led to the development of …
▫ Electron Cloud Theory: electrons are everywhere
but nowhere around the nucleus
• Antonius Van den Broek, 1913 – The atomic
number in nuclear charge
• Henry Moseley, 1913 – English Physicist who
proved Rutherford’s theory through experiments
and refined the field of X-ray diffraction
• James Chadwick, 1932 – discovered the neutron
• Cyclotron – allowed for the creation of new
elements heavier than uranium
6) Wave model
• Today’s atomic model is based on wave
mechanics- electrons don’t move in an orbital
but bounce around in a distinct area
▫ The location of the electron depends on how much
energy it has
• Theory of today says that each atom has a small
positively charged nucleus surrounded by a large
region in which there are enough electrons to
make the atom neutral – a happy atom!
Parts of an Atom
Particle
Location
Charge
Mass
Proton
Nucleus
+
1.67 x 10-27
Neutron
Nucleus
0
1.67 x 10-27
-
9.11 x 10-31
Electron
Energy level/
shell/ orbital or
cloud
Sub Atomic Particles
• Proton:
▫ positively charged particle
▫ All protons stay the same and cannot leave
▫ Protons determine the identity of the atom
• Neutron:
▫ Electrically neutral particle, provides mass
▫ Number of these can change – gives us isotopes
Sub Atomic Particles
• Electrons:
▫ In a neutral atom ( no charge) the number of
negatively charged particles is equal to the
number of positively charged particles
▫ Electron Cloud – space in which electrons are
likely to be found, move so fast and are so small
we can’t see them.
▫ Ions occur when there is an imbalance between
protons and electrons, e- are the one that can
move
Quarks – sub, sub-atomic
• There are six types of quarks, or flavors▫ Up, down, strange, charm, bottom and top
▫ They combine to form hadrons – the most stable
of these are protons and neutrons
Forces within an Atom
• Electromagnetic force- can attract or repel
particles (depending on their charge), this is
what keeps them in their orbital
• Strong force – opposes the EM force of repulsion
between protons
▫ Strongest but has limited range, only when
protons are very close together
• Weak Force – responsible for a process known
as radioactive decay
▫ Decay – a neutron in the nucleus changes into a
proton and an electron
• Gravity- weakest force in nature
Atomic Identity
• Atomic Number
▫ The number of protons in the nucleus of an atom
▫ This number stays the same among all atoms of the
same element
▫ Similar to your SSN
• Atomic Mass
▫ Sum of the protons and neutrons in the nucleus of an
atom, deals with one atom at a time
▫ Similar to your weight, it can change
• Average Atomic Mass
▫ Average of all isotopes of an element in existence,
Deals with more than one atom
▫ AMU – mass unit or amu (6 with 23 zeros equal 1
gram)
Isotopes vs. Ions
Isotopes
▫ Isotopes are atoms with a
different mass due to
different number of
neutrons.
▫ Average Atomic Mass is an
average of all occurring
isotopes (this is why there is
a decimal on some)
Ions
• An Ion is an electrically
charged atom
• Ions deal with the charge or
oxidation number
• How? Gain/Lose e-
• Atomic # (AN or Z)
• Symbol
• Name
• Atomic Mass (AMU)
• Pos/Neg number at
top tells the charge
– deals with ions
Quiz…
•
•
•
•
What is the symbol for Lead?
What is the atomic number of Potassium?
What is the atomic mass of a Carbon atom?
What is the average atomic mass of Aluminum?
Why isn’t it exactly double the number?
Hydrogen isotopes
•
Protium
Deuterium
Tritium
Atomic number (Z) = number of protons in nucleus
Mass number (AMU) = number of protons + number of
neutrons = atomic number (Z) + number of neutrons
Isotopes are atoms of the same element (X) with different
numbers of neutrons in the nucleus
Mass Number
Atomic Number
1
1H
2
1H
235
92
U
A
ZX
(D)
Element Symbol
3
1H
238
92 U
(T)
2.3
Examples
Eleme
nt
AN
Avg.Atomic
Mass
Atomic
Mass
Protons
Oxygen
Lithium
15
38
Neutron Electro
s
ns
Ex) Carbon
Particle
Location
Charge
Number
proton
Nucleus
+
6
Neutron
Nucleus
0
6
electron
Cloud/shell
-
6
AMU
12
Octet Rule
• The outer shell of an atom can hold only a
certain amount of electrons
• 1st shell holds up to 2 e• 2nd shell holds up to 8 e• 3rd shell holds up to 8 e• 4th shell holds up to 18 e• 5th shell holds up to 32 e• Since we only study the first few rows, we say the
atom can hold up to 8 – hence, Octet Rule
Electron Orbitals – s p d f orbitals
Compounds and Molecules
Diatomic molecules
Binary Compounds: Covalent and Ionic Bonds
Polyatomic Compounds
Naming and Balancing
A molecule is an aggregate of two or more atoms in a definite
arrangement held together by chemical bonds, electrons are key
to this
H2
H2O
NH3
CH4
Diatomic Molecules: contains only two atoms, short
list of elements that buddy up – not stable enough
alone
H2, N2, O2, Br2, Cl2, I2, F2
A polyatomic molecule contains more than two atoms
O3, H2O, NH3, CH4
2.5
An ion is an atom, or group of atoms, that has a net positive
or negative charge.
cation – ion with a positive charge
If a neutral atom loses one or more electrons
it becomes a cation.
Na
11 protons
11 electrons
Na+
11 protons
10 electrons
anion – ion with a negative charge
If a neutral atom gains one or more electrons
it becomes an anion.
Cl
17 protons
17 electrons
Cl-
17 protons
18 electrons
2.5
2.6
A molecular formula shows the exact number of atoms of each element in
the smallest unit of a substance
An empirical formula shows the simplest
whole-number ratio of the atoms in a substance
molecular
empirical
H2O
H2O
C6H12O6
CH2O
O3
O
N2H4
NH2
2.6