1. Summary of the Atom
atoms are the smallest particles that can be uniquely associated with an element
each element has unique atoms
atoms are composed of e-, p and n
atoms are electrically neutral (# of e- = # of p)
for a single element, isotopes differ only in number of n (neutrons)
atoms have characteristic masses (atomic weights)
atoms combine with one another in definite, whole number proportions to make compounds

2. ~ 10-10 m electron nucleus Mass > 10-26 kg Mass 9 x 10-31 kg
Here we have arrived at the usual pictorial conception of an atom. It is a misleading picture, but a good aide memoir .
Size 1 angstrom unit m.
Components electrons negative charge. Very light
at centre the nucleus positive to balance the electron charge
essentially all the mass.
It’s the nucleus we want to talk about . So let’s look a little closer at it.

3. ~ 1 – 7 x m (1 – 7 fermi)
The nucleus is times smaller than the atom, and as we said contains essentially all the mass. You can now see that the usual picture of the atom is way out of scale.
A better idea of scale is given if we consider the atom to be the size of the Sydney Olympic stadium.

4. If I represented the nucleus as a small ball, and threw it into the middle of the stadium, we get a better idea of the size of the nucleus relative to the atom.

5. Microcosms of our solar system, atoms are dominantly empty space:
The Spacious Atom
Microcosms of our solar system, atoms are dominantly empty space:
electron orbits
If an oxygen atom had a total radius of 100 km, the nucleus would be a ~1 m diameter sphere in the middle.

6. Electrons in Orbit
In a simplistic model, electrons float around the nucleus in energy levels called shells.
electron orbits
As the number of electrons increases, they start to fill shells farther out from the nucleus.
In most cases, electrons are lost or gained only from the outermost shell.

7. Atom
Nucleus
Not a European football field, but a real Australian rules footy field
This is the Melbourne CG, and is the size of the

8. The Nuclear Model
of the atom

9. Subatomic Particles Particle Symbol Charge Relative Mass
Electron e
Proton p
Neutron n

10. Atomic Number 11 Na
Atomic Number
Symbol

11. All atoms of an element have the same number of protons11 Na
11 protons
Sodium

12. Number of Electrons An atom is neutral The net charge is zero
Number of protons = Number of electrons
Atomic number = Number of electrons in a neutral atom

13. Mass Number
Counts the number of
protons and neutrons
in an atom

14. Atomic Symbols atomic number 11 Show the mass number and atomic number
Give the symbol of the element
mass number
23 Na sodium-23
atomic number 11

15. 12C 13C C Notation for Atoms only one isotope of carbon
all isotopes of carbon

16. Basic Definitions “atomic number” = number of protons in the nucleus;
“atomic weight” = average mass of an atom calculated from the masses and natural abundances of all isotopes
(use atomic weights to calculate the molecular weights of compounds from their constituent elements!)
“mass number” = sum of protons + neutrons in the nucleus
“isotopic mass” = mass of a single isotope

17. 16 31 65 8 15 30 More Atomic Symbols O P Zn 8 p+ 15 p+ 30 p+
O P Zn
8 p p+ 30 p+
8 n 16 n 35 n
8 e e e-

18. Isotopes
Atoms with the same number of protons, but different numbers of neutrons.
Atoms of the same element (same atomic number) with different mass numbers
Isotopes of chlorine
35Cl 37Cl
chlorine chlorine - 37

19. Mass spectrometry

20. Atomic weight measurements
How was the atomic weight measured?
By mass spectrometry
This also measures
% natural abundance
for a given isotope

21. Atomic weight calculation
There are three naturally occuring isotopes of neon (Ne):
20Ne isotopic mass = amu
21Ne isotopic mass = amu
22Ne isotopic mass = amu
the atomic weight is reported in text as:
amu

22. Learning Check 1
Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms.
12C C 14C
#P _______ _______ _______
#N _______ _______ _______
#E _______ _______ _______

23. Solution 6 6 6 12C 13C 14C #P __6___ _ 6___ ___6___
#P __6___ _ 6___ ___6___
#N __6___ _ _7___ ___8___
#E __6___ _ 6___ ___6___

24. Learning Check 2 An atom of zinc has a mass number of 65.
A. Number of protons in the zinc atom
1) ) ) 65
B. Number of neutrons in the zinc atom
C. What is the mass number of a zinc isotope
with 37 neutrons?
1) ) ) 67

25. Solution An atom of zinc has a mass number of 65.
A. Number of protons in the zinc atom
1) 30
B. Number of neutrons in the zinc atom
2) 35
C. What is the mass number of a zinc isotope
with 37 neutrons?
3) 67

26. Learning Check 3
Write the atomic symbols for atoms with the following:
A. 8 p+, 8 n, 8 e- ___________
B. 17p+, 20n, 17e- ___________
C. 47p+, 60 n, 47 e- ___________

27. Solution B. 17p+, 20n, 17e- 37Cl 17 47 16O A. 8 p+, 8 n, 8 e- 8
C. 47p+, 60 n, 47 e- 107Ag 47

28. Learning Check 4 16 14 14 An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) ) ) 34
B. Its mass number is
C. The element is
1) Si 2) Ca 3) Se
D. Another isotope of this element is
1) 34X 2) 34X 3) 36X

29. Solution 14 An atom has 14 protons and 20 neutrons.
A. It has atomic number
1) 14
B. It has a mass number of
3) 34
C. The element is
1) Si
D. Another isotope of this element would be
3) 36X 14

30. Masses of Atoms
A scale designed for atoms gives their small atomic masses in atomic mass units (amu)
An atom of 12C was assigned an exact mass of amu
Relative masses of all other atoms was determined by comparing each to the mass of 12C
An atom twice as heavy has a mass of amu. An atom half as heavy is 6.00 amu.

31. Atomic Mass Na
22.99
Gives the mass of “average” atom of each element compared to 12C
Average atom based on all the isotopes and their abundance %
Atomic mass is not a whole number

32. Calculating Atomic Weight or Mass
Percent(%) abundance of isotopes
Mass of each isotope of that element
Weighted average =
mass isotope1(%) + mass isotope2(%) + …

33. Atomic Mass of Magnesium
Isotopes Mass of Isotope Abundance
24Mg = 24.0 amu %
25Mg = amu %
26Mg = amu %
Atomic mass (average mass) Mg = 24.3 amu Mg
24.3

34. Atomic mass calculation
How was the atomic mass calculated?
multiply each isotopic mass by the reported natural abundance for the isotope, then:
add these individual contributions for each isotope to get the average atomic mass for the element

35. Atomic mass calculation
There are three naturally occuring isotopes of neon (Ne):
20Ne mass # = amu (90.51%)
21Ne mass # = amu (0.27%)
22Ne mass # = amu (9.22%)
the atomic mass is reported in text as:
amu
= amu

36. Learning Check 5
Gallium is a metallic element found in small lasers used in compact disc players. In a sample of gallium, there is 60.2% of gallium-69 (68.9 amu) atoms and 39.8% of gallium-71 (70.9 amu) atoms. What is the atomic mass of gallium?

37. Solution Atomic mass Ga = 69.7 amu 68.9 amu x 60.2 = 41.5 amu for 69Ga
100
Ga-71 (%/100)
70.9 amu x = amu for 71Ga
Atomic mass Ga = amu

38. Finding An Isotopic Mass
A sample of boron consists of 10B (mass 10.0 amu) and 11B (mass 11.0 amu). If the average atomic mass of B is 10.8 amu, what is the % abundance of each boron isotope?

39. Assign X and Y values:
X = % 10B Y = % 11B
Determine Y in terms of X
X Y = 100
Y = X
Solve for X:
X (10.0) (100 - X )(11.0) =
Multiply through by 100
10.0 X X =

40. Collect X terms
10.0 X X =
- 1.0 X = -20
X = = % 10B
- 1.0
Y = X
% 11B = % = 80% 11B

41. Learning Check 6
Copper has two isotopes 63Cu (62.9 amu) and 65Cu (64.9 amu). What is the % abundance of each isotope? (Hint: Check Zumdahl or any other chemistry text for atomic mass)
1) 30% 2) 70% 3) 100%

42. Solution 2) 70% Solution 62.9X + 6490 = 64.9X = 6350 -2.0 X = -140
2) 70%
Solution
62.9X = 64.9X = 6350
-2.0 X = -140
X = 70%

43. 13C 12C Atomic Masses 13.00335 amu (1.11%) 12.0000 amu (98.89%)
atomic weight of C = amu WHY?

44. Calculating masses of atoms relative to 12C
(mass of 12C atom) * = mass of F atom =
reported atomic weight of F =

45. Charged Atoms: Ions
Left to their own devices, atoms are electrically neutral.
That means that they have an equal number of
protons and electrons.
During the course of most natural events,
protons are not gained or lost, but electrons may be.
Atoms with more or fewer electrons than protons are
electrically charged. They are called ions:
an atom that loses electrons takes on a positive charge (cation);
an atom that gains electrons takes on a negative charge (anion).
Complex cations and anions can also occur: (NH4)+1, (SO4)-2

46. An ISOTOPE is one of a set of nuclides with the same Z and consequently different A. (ie isotopes are the same chemical element but different masses). e.g.
An ISOTONE is one of a set of nuclides with the same N and consequently different A. e.g.
An ISOBAR is one of a set of nuclides with the same A but different N and Z.
e.g

47. More on atomic notation, which is based on the nuclear structure:
Isotope: same Z, different A and N
Isobar: same A, different Z and N
Isotone: same N, different Z and A
Example: From the following list of atoms, which are isotopes, isobars, and isotones?
Component
Atom A Z N Xe I Cs