Atoms are composed of protons, neutrons, and electrons. Protons and neutrons are located in the nucleus, while electrons surround the nucleus in orbitals. Over time, scientists such as Dalton, Thomson, Rutherford, and Bohr contributed to the developing atomic model. Bohr refined Rutherford's model by proposing that electrons exist in specific energy levels or orbits around the nucleus. Elements have unique atomic structures that are indicated by their atomic number and mass number.

1. ATOMIC STRUCTURE

2. Atomic Structure
All matter is composed of atoms.
Understanding the structure of atoms is
critical to understanding the properties
of matter

3. HISTORY OF THE ATOM
1808 John Dalton
suggested that all matter was made up of
tiny spheres that were able to bounce around
with perfect elasticity and called them
ATOMS

4. 8 X2Y16 X 8 Y+
DALTONS ATOMIC THEORY

5. 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 = mass n = 1840 x mass e-

6. HISTORY OF THE ATOM
1898 Joseph John Thompson
found that atoms could sometimes eject a far
smaller negative particle which he called an
ELECTRON

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

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

9. HISTORY OF THE ATOM
1910 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

10. Rutherford’s experiment.

11. Plum Pudding model of an atom.

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

13. Actual Results.

14. A nuclear atom viewed in cross
section.

15. 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

16. Atomic Structure
Atoms are composed of
-protons – positively charged particles
-neutrons – neutral particles
-electrons – negatively charged particles
Protons and neutrons are located in the
nucleus. Electrons are found in orbitals
surrounding the nucleus.

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

18. 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.

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

20. ATOMIC STRUCTURE
the number of protons in an atom
the number of protons and
neutrons in an atom
He
2
4Atomic mass
Atomic number
number of electrons = number of protons

21. 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
H1
1 H (D)2
1 H (T)3
1
U235
92 U238
92
Mass Number
Atomic Number
Element Symbol

22. Atomic Structure

23. Atomic Structure

24. Two isotopes of sodium.

25. HISTORY OF THE ATOM
1913 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.

26. MULTIELECTRON ATOMS

28. ELECTRONS IN ORBIT ABOUT
THE NUCLEUS

29. 1. e- can have only specific
(quantized) energy values
2. light is emitted as e- moves
from one energy level to a
lower energy level
Bohr’s Model of
the Atom (1913)
En = -RH ( )
1
n2
n (principal quantum number) = 1,2,3,…
RH (Rydberg constant) = 2.18 x 10-18J

30. The Bohr Model of the Atom

31. Atomic Structure

32. The Bohr Model of the Atom:
Ground and Excited States
• In the Bohr model of hydrogen, the lowest amount
of energy hydrogen’s one electron can have
corresponds to being in the n = 1 orbit. We call this
its ground state.
• When the atom gains energy, the electron leaps to a
higher energy orbit. We call this an excited state.
• The atom is less stable in an excited state and so it
will release the extra energy to return to the ground
state.
– 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:
Hydrogen Spectrum
• Every hydrogen atom has identical orbits, so every
hydrogen atom can undergo the same energy
transitions.
• However, since the distances between the orbits in
an atom are not all the same, no two leaps in an
atom will have the same energy.
– The closer the orbits are in energy, the lower
the energy of the photon emitted.
– Lower energy photon = longer wavelength.
• Therefore, we get an emission spectrum that has a
lot of lines that are unique to hydrogen.

38. The Bohr Model of the Atom:
Hydrogen Spectrum

39. Ephoton = DE = Ef - Ei
Ef = -RH ( )
1
n2
f
Ei = -RH ( )
1
n2
i
i f
DE = RH( )
1
n2
1
n2
RH is the Rydberg constant
n is the principal quantum number
En = -RH ( )
1
n2
Bohr showed the energy a H
atom can have is equal to:

40. Line spectrum of
some elements

41. LIGHT EMISSION OF SODIUM ATOM
Line spectrum

42. Atomic Structure
Neutral atoms have the same number of
protons and electrons.
Ions are charged atoms.
-cations – have more protons than
electrons and are positively charged
-anions – have more electrons than
protons and are negatively charged

43. An ion is formed when an atom, or group of atoms, has a
net positive or negative charge (why?).
If a neutral atom looses one or more electrons
it becomes a cation.
If a neutral atom gains one or more electrons
it becomes an anion.
Na
11 protons
11 electrons Na+ 11 protons
10 electrons
Cl
17 protons
17 electrons Cl-
17 protons
18 electrons