All matter is composed of atoms. Atoms are composed of protons, neutrons, and electrons. Rutherford's gold foil experiment disproved the plum pudding model of the atom and led to the discovery of the nuclear atom, with a small, dense nucleus surrounded by electrons. Niels Bohr refined this model by suggesting that electrons orbit the nucleus in discrete energy levels. When electrons change orbits, electromagnetic radiation is emitted or absorbed at wavelengths specific to each element.

2. AllAll mattermatter is composed ofis composed of atomsatoms..
Understanding the structure of atoms isUnderstanding the structure of atoms is
critical to understanding the properties ofcritical to understanding the properties of
matter.matter.

3. Atomic StructureAtomic Structure
Neutral atomsNeutral atoms have the same number ofhave the same number of
protons and electrons.protons and electrons.
IonsIons are charged atoms.are charged atoms.
--cationscations – have more protons than– have more protons than
electrons and are positively charged.electrons and are positively charged.
--anionsanions – have more electrons than– have more electrons than
protons and are negatively charged.protons and are negatively charged.

4. HISTORY OF THE ATOMHISTORY OF THE ATOM
John Dalton
suggested that all matter was made up of
tiny spheres that were able to bounce around
with perfect elasticity and called them
ATOMSATOMS

5. 8 X2Y16 X 8 Y+
DALTONS ATOMIC THEORY

6. Subatomic ParticlesSubatomic 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 = mass n = 1840 x mass e-

7. HISTORY OF THE ATOMHISTORY OF THE ATOM
Joseph John
Thompson
found that atoms could sometimes eject a
far smaller negative particle which he called
an
ELECTRONELECTRON

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

9. gold foil
helium nuclei
CHARGE OF AN ELECTRON
Millikan oil drop
experiment

10. HISTORY OF THE ATOMHISTORY OF THE ATOM
Ernest Rutherford
oversaw Geiger and Marsden carrying out his
famous experiment.
they fired Helium nuclei at a piece of gold foil
which was only a few atoms thick.
they found that although most of them
passed through. About 1 in 10,000 hit

12. Rutherford’s experiment.Rutherford’s experiment.

13. Plum Pudding model of an atom.Plum Pudding model of an atom.

14. Results of foil experiment if PlumResults of foil experiment if Plum
Pudding model had been correct.Pudding model had been correct.

15. A nuclear atom viewed in crossA nuclear atom viewed in cross
section.section.

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

17. Atomic StructureAtomic Structure
AtomsAtoms are composed ofare composed of
--protonsprotons – positively charged particles– positively charged particles
--neutronsneutrons – neutral particles– neutral particles
--electronselectrons – negatively charged particles– negatively charged particles
Protons and neutrons are located in theProtons and neutrons are located in the nucleusnucleus..
Electrons are found in orbitals surrounding theElectrons are found in orbitals surrounding the
nucleus.nucleus.

18. HELIUM ATOM
+
N
N
+
-
-
proton
electron neutron
Shell

19. Atomic 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. Atomic StructureAtomic Structure
Each proton and neutron has a mass ofEach proton and neutron has a mass of
approximately 1 dalton.approximately 1 dalton.
The sum of protons and neutrons is the atom’sThe sum of protons and neutrons is the atom’s
atomic massatomic mass..
IsotopesIsotopes – atoms of the same element that have– atoms of the same element that have
different atomic mass numbers due todifferent atomic mass numbers due to
different numbers of neutrons.different numbers of neutrons.

21. ATOMIC STRUCTUREATOMIC STRUCTURE
the number of protons in an atom
the number of protons and
neutrons in an atom
HeHe
22
44Atomic mass
Atomic number
number of electrons = number of protons

22. ATOMIC NUMBER (Z) = number of protons in nucleus
MASS NUMBER (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
ISOTOPS are atoms of the same element (X) with different numbers
of neutrons in the nucleus
XA
Z
H
1
1 H (D)
2
1 H (T)
3
1
U235
92 U238
92
Mass Number
Atomic Number
Element Symbol

23. Atomic StructureAtomic Structure

24. Atomic StructureAtomic Structure

25. Two isotopes of sodium.

26. HISTORY OF THE ATOMHISTORY OF THE ATOM
Niels Bohr
studied under Rutherford at the Victoria
University in Manchester.
Bohr refined Rutherford's idea by adding
that the electrons were in orbits. Rather
like planets orbiting the sun. With each
orbit only able to contain a set number of
electrons.

27. MULTIELECTRON ATOMSMULTIELECTRON ATOMS

29. ELECTRONS IN ORBIT ABOUTELECTRONS IN ORBIT ABOUT
THE NUCLEUSTHE NUCLEUS

30. The Bohr Model of the AtomThe Bohr Model of the Atom

31. Atomic StructureAtomic Structure

32. The Bohr Model of the Atom:The Bohr Model of the Atom:
Ground and Excited StatesGround and Excited States
• In the Bohr model of hydrogen, the lowest amountIn the Bohr model of hydrogen, the lowest amount
of energy hydrogen’s one electron can haveof energy hydrogen’s one electron can have
corresponds to being in thecorresponds to being in the nn = 1 orbit. We call this= 1 orbit. We call this
itsits ground state.ground state.
• When the atom gains energy, the electron leaps to aWhen the atom gains energy, the electron leaps to a
higher energy orbit. We call this anhigher energy orbit. We call this an excited stateexcited state..
• The atom is less stable in an excited state and so itThe atom is less stable in an excited state and so it
will release the extra energy to return to the groundwill release the extra energy to return to the ground
state.state.
• Either all at once or in several steps.Either all at once or in several steps.

35. Line Emission Spectrum of Hydrogen Atoms
Every element has a unique emission spectrum

36. The Bohr Model of the Atom:The Bohr Model of the Atom:
Hydrogen SpectrumHydrogen Spectrum
• Every hydrogen atom has identical orbits, so everyEvery hydrogen atom has identical orbits, so every
hydrogen atom can undergo the same energyhydrogen atom can undergo the same energy
transitions.transitions.
• However, since the distances between the orbits inHowever, since the distances between the orbits in
an atom are not all the same, no two leaps in anan atom are not all the same, no two leaps in an
atom will have the same energy.atom will have the same energy.
– The closer the orbits are in energy, the lowerThe closer the orbits are in energy, the lower
the energy of the photon emitted.the energy of the photon emitted.
– Lower energy photon = longer wavelength.Lower energy photon = longer wavelength.
• Therefore, we get an emission spectrum that has aTherefore, we get an emission spectrum that has a
lot of lines that are unique to hydrogen.lot of lines that are unique to hydrogen.

37. The Bohr Model of the Atom:The Bohr Model of the Atom:
Hydrogen SpectrumHydrogen Spectrum

38. Line spectrum of
some elements

39. LIGHT EMISSION OF SODIUM ATOM
Line spectrum