The document summarizes the development of atomic models from Dalton's atomic theory to the modern nuclear model. Key developments include Thomson's discovery of electrons within atoms, Rutherford's gold foil experiment showing a small, dense nucleus, Bohr's model of electron orbits, Chadwick's discovery of neutrons, and the modern nuclear model with protons and neutrons in the nucleus and electrons in surrounding shells. The modern model explains the atomic number and mass number and how atoms achieve neutral charge through balancing protons and electrons.

1. Atomic Model of Matter
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2. All matter is composed of tiny particles called
atoms. The combination of the types of
particles in a substance determines what kind
of matter it is – Solid, Liquid or Gas
Particle model of matter

3. Elements, Compounds and Mixtures
Examples of elements include:
copper (Cu), aluminum (Al), iron (Fe), oxygen (O2),
hydrogen (H2)
Examples of compounds include:
water (H2O), table salt (NaCl), carbon dioxide (CO2),
aluminum oxide (Al2O3), sulfuric acid (H2SO4)
Examples of mixtures include:
water and oil = Immiscible (heterogeneous mixture)
oil and detergent = Miscible (homogeneous mixture)
How are compounds different from elements?

4. John Dalton (Early 1803)
Proposed an atomic theory that included the following:
• Each element is composed of extremely small
particles called atoms.
• All atoms of a given element are identical.
• Atoms of one element are different from atoms
of another element.
• Atoms of an element are not changed into
different types of atoms by chemical reactions.
• Compounds are formed when atoms of more
than one kind combine.
• In a given compound, the relative number and
kind of atoms are constant.
8X2Y16 X 8Y+

5. • The first major advances were possible with the
development of gas discharge tubes by Sir William
(mid to late 1800’s) later described as Cathode ray
Modification of Dalton’s Atomic Theory
Crooke's tube demo.

6. Sir J.J. Thomson (1898)
Evidence:
• When cathode rays traveled through an electric field they
were repelled from the negative side of the field and
attracted to the positive side.
• From measuring the deflection of the cathode rays in
combinations of electric and magnetic fields, Thomson was
able to determine that cathode rays had charge and mass.
• Cathode rays were composed of a stream of negatively
charged particles.

7. J.J. Thomson, measured mass/charge of e-
(1906 Nobel Prize in Physics)
A = alpha (α)
B = gamma (γ)
C = beta (β)

8. Thomson’s Model of the Atom
Evidence:
• Thomson had determined that atoms contained
negative particles called electrons.
• However, atoms were electrically neutral
• Atoms contain an equal amount of positive charge
as negative charge
Thomson suggested that
the electrons in an atom
were embedded in a
positively charged, diffuse
sphere.

9. Millikan Oil Drop Experiment to determine the magnitude of
charge on the electron

10. Earnest Rutherford, Geiger, and Marsden
(1909)
Positively charged alpha-particles were
directed at a piece of thin gold foil.

11. • Most of the alpha particles passed through the gold
foil, but some were deflected at varying angles.
• Note: If the “plum pudding” model was correct and the
positive charge was diffuse, then the high energy
alpha particles should be able to pass through the
positive matter of the atoms of gold foil with limited
deflection.
Rutherford’s Model of Atom

12. Rutherford’s Model of Atom
atomic radius ~ 100 pm = 1 x 10-10
m
nuclear radius ~ 5 x 10-3
pm = 5 x 10-15
m

13. 1913: Niels Bohr
Bohr improved Rutherford’s
model by noticing that energy
levels in atoms went up and
down by specific, “pre-set”
amounts. He suggested that
electrons move around the
nucleus of an atom like
planets around the sun, and
that they move from orbit to
orbit as they gain and lose
K = 1st
shell (n = 1) =
innermost shell

14. Particle-Wave Model of Atoms

15. The Nuclear Model of the Atom
• The positive charges in an atom are not diffuse and instead must be
concentrated into a small space
•Most of an atom is empty space
Claim:
Reasoning:
• Most of the alpha particles were able to pass through the atoms with little or
no deflection, therefore the atom was mainly empty space.
• There must be a concentration of positive charge in order to cause the
alpha particles to be deflected by large angles.
• This concentration of positive charge must be very small or more alpha
particles would have been deflected by large angles.
The region of concentration of positive charge in an atom was called the
nucleus.
The positively charged particles in the nucleus were called protons

16. Sir James Chadwick (1932)
• There must be other particles in the
atom.
• These particles must have mass but no
charge.
• In 1932, James Chadwick confirmed
the existence of these particles, called
neutrons.
Claim:

17. Summary of the Nuclear Model of
the Atom
• Atoms contain protons, neutrons, and electrons.
• Protons are positive, electrons are negative, neutrons
have no charge.
• Protons and neutrons contain most of the mass of an
atom.
• Protons and neutrons are located in the nucleus,
which is very small.
• Electrons are located outside the nucleus.

18. Subatomic Particles
Particle
Mass
(g)
Charge
(Coulombs)
Charge
(units)
Electron (e-
) 9.1 x 10-28
-1.6 x 10-19
-1
Proton (p) 1.67 x 10-24
+1.6 x 10-19
+1
Neutron (n) 1.67 x 10-24
0 0
mass p (1 Da)= mass n (1 Da) = 1840 x mass e-
Assignment: Calculate the e/m ratio of each of the subatomic particles

19. Atomic StructureAtomic Structure
Every different atom has a characteristic
number of protons in the nucleus.
atomic number = number of protons
Atoms with the same atomic number
have the same chemical properties and
belong to the same element.

20. the number of protons in an atom
the number of protons and
neutrons in an atom
HeHe
44
22
Atomic mass (A)
Atomic number (Z)
number of electrons = number of protons
ATOMIC STRUCTUREATOMIC STRUCTURE
XA
Z
Mass Number
Element SymbolAtomic Number

21. MathematicallyMathematically
A = p + nA = p + n
Z = pZ = p
Therefore, A = Z + nTherefore, A = Z + n
Atom becomes electrically neutral when:Atom becomes electrically neutral when:
pp++
= e= e--

22. These are:
1.Quarks
2.Positrons
3.Neutrinos
4.Pions
5.Muons
Other sub-atomic particlesOther sub-atomic particles
Assignment: Define each of these particles and state theirAssignment: Define each of these particles and state their
significant importance in the development of atomic modelsignificant importance in the development of atomic model