CHAPTER 1
ATOMIC STRUCTURE
A spoonful of sugar would contain about:
602,000,000,000,000,000,000,000 atoms!
ELEMENTS AND ATOMS
• Every substance in the world is made up of chemical elements.
• These elements cannot be broken down into simpler substances
by chemical means
• Some elements can be found in nature, but most are in
combinations with other compounds
• Each element has a symbol found on the periodic table
• Chemical elements only contain one type of atoms, they are
extremely small
Millions of atoms would fit
onto the head of a pin
THE STRUCTURE OF AN ATOM
• Almost all the mass of an atom is concentrated in the …..
• Which is made up of ……
Nucleus
Nucleons – Protons and Neutrons
• Outside the nucleus there are moving particles called ……. Electrons.
• They move in regions of space called ….. Orbitals
• Most of an atom is …… Empty Space
MODEL OF AN ATOM
Electrons
Nucleus
Neutrons
Electron shells
(Energy levels)
Protons
HOW DO WE KNOW THIS?
ATOMIC THEORY
• The idea of an atomic
theory is more than 2000
years old.
• Until recently, scientists
had never seen evidence
of atoms.
STRUCTURE OF THE ATOM
SUB ATOMIC PARTICLES
DISCOVERY OF THE ELECTRON
• J.J. Thomson began experimenting with cathode ray tubes.
• Cathode ray tubes are sealed glass tubes from which most of the air
has been evacuated. You can see this on page 6 of your text books
• A high voltage is applied across two electrodes at one end of the
tube, which causes a beam of particles to flow from the cathode to
the anode.
• Thomson placed two magnets on either side of the tube, and observed
that this magnetic field deflected the ray.
CATHODE RAY TUBE EXPERIMENT
DISCOVERY OF THE ELECTRON
• The electrons are given off from the heated wire and are attracted towards two metal
plates, which are positively charged.
• As they pass through the metal plates, the electrons form a beam.
• When the electron beam hits the screen a spot of light is produced.
• When an electric field is applied across this beam the electrons are deflected.
• The fact that the electrons are so easily attracted to the positively charged anode and that
they are easily deflected by an electric field shows us that:
• electrons have a negative charge
• electrons have a very small mass.
PLUM PUDDING MODEL
• Thomson proposed that the electrons of an atom were spread
evenly throughout a positively charged ball of matter.
• An atom was a positively charged sphere
• Negatively charged electrons embedded in it like a
‘raisin pudding’
CRT TV
DISCOVERY OF THE NUCLEUS
• performed by Ernest Rutherford
• He bombarded a thin gold foil with a beam of fast-moving -particles (+ve
charged)
• Observation:
• most -particles passed through the foil without deflection
• very few -particles were scattered or rebounded back
DISCOVERY OF THE NUCLEUS
1.
The atom is made up of mostly
EMPTY SPACE
2.
The center of the atom contains a
POSITIVE CHARGE
3.
Rutherford called this positive
bundle of matter the NUCLEUS
MASSES AND CHARGES
• There are two properties of subatomic particles that are
especially important:
• Mass
• Electrical charge
Subatomic
Particle
Symbol
Relative
Mass
Relative
Charge
Electron
Neutron
Proton
e
n
p
1/1836
1
1
-1
0
+1
NUCLEON NUMBER
(A)
23
Na
= total number of protons
and neutrons
11
PROTON NUMBER
(Z)
= number of protons
Atom
helium
Nucleon
number
Proton
number
4
2
fluorine
19
9
strontium
88
38
zirconium
91
40
238
92
uranium
Number of
neutrons
2
10
50
51
146
ISOTOPES
• The number of neutrons in an atom can vary
• There are 3 types of carbon
• They all have the same number of protons
• But different numbers of neutrons
• The different numbers of neutrons make no difference to their
chemical properties
• But they have different physical properties
IONS
• Ions are formed when an atom gains or looses an electron
• Ions are not neutral like atoms, they have an electrical charge