1. Unit Objectives
1)understand the evolution of models of the atom: Dalton, Thomson, Rutherford, Bohr, the ‘quantum model
2) understand the meaning of mass number, atomic (proton) number.
3) be able to write and work with full atomic symbols
4) be able to calculate relative atomic, molecular and formula mass

2. Refreshing some Old Terminology
Bohr Rutherford Diagrams
Isotopes and ions
R.A.M. (Relative Atomic Mass)

3. What is “atom”?
The Greek philosopher Democritus

5. These are iron atoms!!
Continuous division
Iron

6. Main points of Dalton’s atomic theory
1. All elements are made up of atoms.
2. Atoms are not created or destroyed.
Atoms of the same element have the same mass and chemical properties.
4. Atoms of different elements are different. They have different masses and chemical properties.
5. Atoms of different elements combine to form a compound. The numbers of various atoms combined bear a simple whole number ratio to each other.

7. Discovery of electrons
JJ Thompson Raisin Pudding Model
Discovery of electrons
A beam of rays came out from the cathode and hit the anode.
He called the beam cathode rays.

8. Deflected in the magnetic field
Deflected in the electric field
The beam was composed of negatively charged fast-moving particles.

9. How are the particles distributed in an atom?
Thompson’s Raisin Bun Model
How are the particles distributed in an atom?
An atom was a positively charged sphere
Electron +
Positive charge
- Negatively charged electrons embedded in it like a ‘raisin pudding’

10. Gold foil scattering experiment
- performed by Ernest Rutherford

11. 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

12. Interpretation of the experimental results
- The condensed core is called ‘nucleus’.
- The positively charged particle is called ‘proton’.

13. Chadwick’s atomic model
Provedthe presence of neutrons
Proton
Electron
Neutron

14. Interpret the diagram
Protons are deflected on a curved path towards the negative plate.
Electrons are deflected on a curved path towards the positive plate.
The amount of deflection is exactly the same in the electron beam as the proton beam if the energies are the same - but, of course, it is in the opposite direction.
Neutrons continue in a straight line.

15. Practice Fill in the Chart 27Al3+
Protons
Neutrons
Electrons
Charge
Atomic Number
Mass Number
Symbol A 19 21 B 20 40 C + 11 23 D 6 E 92
235 F 13 G 16 2- H
27Al3+

16. MASS NUMBER AND ATOMIC NUMBER
Protons
Neutrons
Electrons
Charge
Atomic Number
Mass Number
Symbol A 19 21 40
40K B 20
40Ca C 11 12 10 + 23
23Na+ D 6
12C E 92
143
235
235U F 7 13
13C G 16 18 2- 32
32S2- H 14 3+ 27
27Al3+

17. Review of Models of the Atom (7 min)

18. Bohr's Model of the Atom
Niels Bohr (1913):
-studied the light produced when
atoms was excited by heat or electricity

19. Bohr's Model of the Atom
Niels Bohr (1913):
-studied the light produced when atoms were excited by heat or electricity
Rutherford's model couldn't explain why unique colours were obtained by atoms of different elements
Bohr proposed that electrons are in orbits & when excited jump to a higher orbit. When they fall back to the original they give off light

20. Bohr's Model of the Atom
Bohr's model:
-electrons orbit the nucleus like planets orbit the sun
-each orbit can hold a specific maximum number of electrons

22. Atomic Model Development
NAME
CONTRIBUTION
EXPERIMENT
Democritus
Postulated that matter is made up of indivisible particles.
None – he was a Greek philosopher.
John Dalton
Postulated that matter is made up of indivisible particles
Chemical elements are made of atoms identical in mass.
Atoms of different elements have different masses.
Atoms only combine in small, whole-number ratios, such as 1 : 1, or 1 : 2
 None
J. J. Thomson
Discovered the electron
Created a glowing stream in a cathode ray tube that would bend towards a positively charged electric plate. He suggested the stream made up of small particles – pieces of the atoms which carried a negative charge. These particles were later named electrons.
Marie Curie
Discovered that the atom was not indivisible because radioactive elements emitted particles.
Experiments with uranium emitting rays that electrified the air suggested the atom was not indivisible.
Ernest Rutherford
James Chadwick
Discovered that the atom had a very dense positively charged nucleus which made up a very small proportion of the volume of the atom.
Discovered that neutrons were also located in the nucleus of an atoms and that they contain no charge
Bombarded gold foil with -particles. Found that most passed all the way through whilst others ‘ricocheted’.
Rutherford noted the disparity between the atomic number of an atom, or number of positive charges, and its mass computed in atomic mass units. The atomic number of an atom is usually about half its atomic mass.
Bombarded Beryllium with rays from radioactive Polonium, was unable to deflect the
particle so he said it was neutral.

23. Surrounding the nucleus
Characteristics of sub-atomic particles
Sub-atomic particle
Proton
Neutron
Electron
Symbol
p or
n or
e- or
Location in atom
Nucleus
Surrounding the nucleus
Actual charge (C)
1.6 x 10-9
Relative charge +1 -1
Actual mass (g)
1.7 x 10-24
9.1 x 10-28
Approximate mass (a.m.u.) 1 n 1 e -1 H 1

24. Atomic number
The atomic number (Z) of an element is the number of protons contained in the nucleus of the atom.
Atomic number =
Number of protons
Number of electrons
Reason: Atoms are electrically neutral.
WHY?

25. Mass number
The mass number (A) of an atom is the sum of the number of protons and neutrons in the nucleus.
Mass number =
Number of protons
Number of neutrons +

26. Atomic numbers and mass numbers
No. of protons
No. of electrons
No. of neutrons
Atomic number
Mass number
Hydrogen 1
(1 + 0) = 1
Oxygen 8 9
(8 + 9) = 17
Argon 18 22
(18+22) = 40

27. Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Representation: X A Z
Symbol of the element
Mass number
Atomic number

28. Cl 35 17 37 e.g. the two isotopes of chlorine are written as:
OR labelled as Cl-35 and Cl-37.

29. MASS NUMBER AND ATOMIC NUMBER
Atomic Number (Z) Number of protons in the nucleus of an atom
Mass Number (A) Sum of the protons and neutrons in the nucleus
Mass Number (A)
PROTONS + NEUTRONS Na 23 11
Atomic Number (Z)
PROTONS

30. Fuse School Isotopes

31. The
Configuration of
Electrons

32. Maximum electrons per shell
Bohr-Rutherford Energy Diagrams
Ideas about the structure of the atom have changed over the years. The Bohr theory thought of it as a small nucleus of protons and neutrons surrounded by circulating electrons.
Each shell or energy level could hold a maximum number of electrons.
The energy of levels became greater as they got further from the nucleus and electrons filled energy levels in order.
Maximum electrons per shell
1st shell
2nd shell
3rd shell
4th shell
5th shell

33. The arrangement of the electrons
The electrons are found in ENERGY LEVELS
The first level will only hold 2 electrons,
the second holds 8,
and the third also 8 electrons.
These levels can be thought of as getting progressively further from the nucleus.

34. What is the electronic arrangement in chlorine ?
Atomic number = 17.
Therefore there are 17 protons and 17 electrons.
The arrangement of the electrons will be 2, 8, 7
(i.e. 2 in the first level, 8 in the second, and 7 in the third).
The electronic arrangements of the first 20 elements

35. Practice
Handout Please do the handout on energy level diagrams

36. ISOTOPES & AVERAGE ATOMIC OR MOLAR MASS

37. MASS SPECTRA
R.A.M.
Consider neon and its 3 isotopes Ne 21Ne Ne.
We use these values to determine the
average molar mass of an element or
Relative atomic mass, called RAM
m/z
90.92
0.26
8.82
Abundance / %
Calculate the average relative atomic mass of neon using the above information.
Out of every 100 atoms are 20Ne , are 21Ne and are 22Ne
Average = (90.92 x 20) + (0.26 x 21) + (8.82 x 22) =
100
Relative atomic mass =

38. Calculate the relative atomic mass of the following
– give your answers to 3 significant figures
Bromine : 79 Br 50% , 81 Br 50%
Copper : Cu 69% , 65 Cu 31%
Zirconium : Zr 51.5% , Zr 11.2%, Zr 17.1%,
94 Zr 17.4%, Zr 2.8%
Lead : 204 Pb 1.5% , Pb 23.6%, Pb 22.6%, Pb 52.3%
Neon : Ne 90.9% , Ne 0.2%, Ne 8.9%
63.6
91.3
207.
20.2

39. Can we calculate the % composition if we know the isotopes ?
Naturally occurring potassium consists of potassium-39 and potassium-41.
Calculate the percentage of each isotope present if the average is 39.1.
Assume there are x nuclei of 39K in every 100; so there will be (100-x) of 41K
so x (100-x) =
100
therefore x x = 3910
thus x =
and x = 95
ANSWER There will be 95% 39K and
5% 41K

40. Right, ok now you try, see the handout