1. Dalton’s Atomic Theory (experiment based!)
All elements are composed of tiny indivisible particles called atoms
Atoms of the same element are identical. Atoms of any one element are different from those of any other element.
John Dalton
(1766 – 1844)
Atoms of different elements combine in simple whole-number ratios to form chemical compounds
In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element.

2. Thomson’s Atomic Model
J. J. Thomson
Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

3. Conclusions from the Study of the Electron:
Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.
Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons
Electrons have so little mass that atoms must contain other particles that account for most of the mass

4. The Rutherford Atomic Model
Based on his experimental evidence:
The atom is mostly empty space
All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus”
The nucleus is composed of protons and neutrons (they make the nucleus!)
The electrons distributed around the nucleus, and occupy most of the volume
His model was called a “nuclear model”

5. QUICK CHECK

6. Subatomic Particles Particle Charge Actual Mass (g)
Relative Mass (amu)
Electron
(e-) -1
9.11 x 10-28
Proton (p+) +1
1.67 x 10-24
Neutron
(no)

7. Measuring Atomic Mass
Instead of grams, the unit we use is the Atomic Mass Unit (amu)
It is defined as one-twelfth the mass of a carbon-12 atom.
Carbon-12 chosen because of its isotope purity.
Each isotope has its own atomic mass, thus we determine the average from percent abundance.

8. HOW DO ATOMS DIFFER
OBJECTIVES:
Explain what makes elements and isotopes different from each other.

9. # protons in an atom = # electrons
Atomic Number
Atoms are composed of identical protons, neutrons, and electrons
How then are atoms of one element different from another element?
Elements are different because they contain different numbers of PROTONS
The “atomic number” of an element is the number of protons in the nucleus
# protons in an atom = # electrons

10. Atomic Number
Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.
Element
# of protons
Atomic # (Z)
Carbon 6
Phosphorus 15
Gold 79

11. Mass Number
Mass number is the number of protons and neutrons in the nucleus of an isotope:
Mass # = p+ + n0
Nuclide p+ n0 e-
Mass #
Oxygen - 10 - 33 42
- 31 15 18 8 8 18
Arsenic 75 33 75
Phosphorus 16 15 31

12. Using a periodic table and what you know about atomic number, mass, isotopes, and electrons, fill in the chart:
Element
Symbol
Atomic
Number
Mass
# of protons
# of neutron
# of electron
charge 8
Potassium 39 Br 45 30 35
Atomic Number = Number of Protons
Number of Protons + Number of Neutrons = Atomic Mass
Atom (no charge) : Protons = Electrons

13. Using a periodic table and what you know about atomic number, mass, isotopes, and electrons, fill in the chart:
A N S W E R K E Y
Element
Symbol
Atomic
Number
Mass
# of protons
# of neutron
# of electron
charge 8
Potassium 39 Br 45 30 35
Oxygen O 8 16 K 19 19 20 19
Bromine 35 80 35 35
Zinc Zn 30 65
Atomic Number = Number of Protons
Number of Protons + Number of Neutrons = Atomic Mass
Atom (no charge) : Protons = Electrons

14. Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.
Protons
Electrons
Neutrons
Nucleus
Hydrogen–1
(protium) 1
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium) 2

15. C Isotopes Mass # Atomic # 12 6
Atoms of the same element with different mass numbers.
Nuclear symbol: 12 6 C
Mass #
Each isotope has a different number of neutrons.
Atomic #
Hyphen notation: carbon-12
Courtesy Christy Johannesson

16. Isotopes + + Carbon-12 Neutrons 6 Protons 6 Electrons 6 Carbon-14
Nucleus
Neutron
Proton +
Electrons
Nucleus
Nucleus
Neutron
Proton +
Carbon-12
Neutrons 6
Protons 6
Electrons 6
Electrons
The chemistry of each element is determined by its number of protons and electrons.
In a neutral atom, the number of electrons equals the number of protons.
Symbols for elements are derived directly from the element’s name.
Nuclei of atoms contain neutrons as well as protons.
The number of neutrons is not fixed for most elements, unlike protons.
Atoms that have the same number of protons, and hence the same atomic number, but different numbers of neutrons are called isotopes.
Carbon-14
Neutrons 8
Protons 6
Electrons 6
Nucleus

17. Cl Isotopes 37 17 Cl Chlorine-37 atomic #: mass #: # of protons:
# of electrons:
# of neutrons: 17 37 20 37 17 Cl
Atoms that have the same number of protons, and hence the same atomic number, but different numbers of neutrons are called isotopes.
Courtesy Christy Johannesson

18. Relative Atomic Mass 12C atom = 1.992 × 10-23 g atomic mass unit (amu)
1 amu = 1/12 the mass of a 12C atom
Neutron +
1 p = amu 1 n = amu 1 e- = amu
Electrons
Atomic mass
1. The mass of any given atom is not simply the sum of the masses of its electrons, protons, and neutrons.
2. Atoms are too small to measure individually and do not have a charge.
3. The arbitrary standard that has been established for describing atomic mass is the atomic mass unit (amu), defined as one-twelfth of the mass of one atom of 12C.
4. Most elements exist as mixtures of several stable isotopes. The weighted average is of the masses of the isotopes is called the atomic mass.
5. Electrons added or removed from an atom produce a charged particle called an ion, whose charge is indicated by a superscript after the symbol for the element.
Nucleus
Proton
Nucleus
Carbon-12
Neutrons 6
Protons 6
Electrons 6

19. 6Li 7Li 3 p+ 3 n0 3 p+ 4 n0 2e– 1e– 2e– 1e– + + Lithium-6 Lithium-7
Nucleus
Neutron
Proton
Nucleus
Neutron
Proton
Electrons +
Electrons +
Nucleus
Nucleus
Lithium-6
Lithium-7
Neutrons 3
Protons 3
Electrons 3
Neutrons 4
Protons 3
Electrons 3

20. Check the atomic weight of elements in the
periodic table.
If the number of protons and neutrons are whole
numbers, why is the atomic mass NOT
a whole number?

21. Calculating averages
You have five rocks, four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks?
Total mass = (4 x 50) + (1 x 60) = 260 g
Average mass = (4 x 50) + (1 x 60) = 260 g
Average mass = 4 x x 60 = 260 g
California WEB

22. Average Atomic Mass Avg. (mass)(%) + (mass)(%) Atomic Mass 100
weighted average of all isotopes
on the Periodic Table
round to 2 decimal places
Avg.
Atomic
Mass
(mass)(%) + (mass)(%) =
100
Courtesy Christy Johannesson

23. Average Atomic Mass
EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O.
Avg.
Atomic
Mass
(16)(99.76) + (17)(0.04) + (18)(0.20)
16.00
amu = =
100
Courtesy Christy Johannesson

24. Isotopes
Because of the existence of isotopes, the mass of a collection of atoms has an average value.
Average mass = ATOMIC WEIGHT
Boron is 20% B-10 and 80% B That is, B-11 is 80 percent abundant on earth.
For boron atomic weight
= (10 amu) (11 amu) = amu

25. Atomic Mass
Calculate the atomic mass of copper if copper has two isotopes % has a mass of amu and the rest has a mass of amu.
Isotope
Percent
Abundance
Mass
Cu-63
69.1
62.93
Cu-65
64.93
30.9
63.548 Cu 29
63.548