Atomic theory

1. Atomic Structure

2. Rutherford’s Gold Foil Experiment
❑ Alpha (α) particles are helium nuclei
❑ Particles were fired at a thin sheet of gold foil
❑ Particle hits on the detecting screen (film) are
recorded
❑https://www.youtube.com/watch?v=y_aQWYnbh6
4&list=PL816Qsrt2Os0K7TZ01RZnC9roJvmf40KJ

3. Rutherford’s Findings
❑ The nucleus is small
❑ The nucleus is dense
❑ The nucleus is positively charged
❑ Most of the particles passed right through
❑ A few particles were deflected
❑ VERY FEW were greatly deflected
“Like howitzer shells bouncing off
of tissue paper!”
Conclusions:

4. • Nucleus: Containing
protons and neutrons, it is
the bulk of the atom and
has a positive charge
associated with it
• Electron cloud:
Responsible for the majority
of the volume of the atom, it
is here that the electrons
can be found orbiting the
nucleus (extranuclear)
The modern atom is composed of two regions:

5. Major Subatomic Particles
Name Symbol Charge Relative Mass
(amu)
Actual Mass
(kg)
Electron e- -1 1/1840 9.11x10-31
Proton p+ +1 1 1.67x10-27
Neutron no 0 1 1.67x10-27
Given in IB data book

6. Elemental Classification
• Atomic Number (Z) = number of protons (p+) in the
nucleus
• Determines the type of atom
• Li atoms always have 3 protons in the nucleus, Hg always 80
• Mass Number (A) = number of protons + neutrons
[Sum of p+ and nº]
• Electrons have a negligible contribution to overall mass
• In a neutral atom there is the same number of
electrons (e-) and protons (atomic number)

7. Nuclear Symbols
• Every element is given a corresponding symbol
which is composed of 1 or 2 letters (first letter upper
case, second lower), as well as the mass number
and atomic number
E
A
Z
elemental symbol
mass number
atomic number

8. Examples
• Find the
• number of protons
• number of neutrons
• number of electrons
• atomic number
• mass number
W
184
74
F
19
9 Br
80
35

9. Ions
• Cation is a positively charged particle.
Electrons have been removed from the
element to form the + charge.
ex: Na loses 1 e- 🡪 Na+
• Anion is a negatively charged particle.
Electrons have been added to the atom to
form the – charge.
ex: Oxygen gains 2 e- 🡪 O2-

10. Isotopes
• Atoms of the same element that have the same
number of protons, but different numbers of
neutrons.
• The atoms of the same element that differ in the
number of neutrons are called isotopes of that
element
• When naming, write the mass number after the name of the
element
H
1
1
Hydrogen-1
H
2
1
Hydrogen-2
H
3
1
Hydrogen-3

11. How heavy is an atom of oxygen?
• There are different kinds of oxygen atoms (different
isotopes)
• 16O, 17O, 18O
• We are more concerned with average atomic
masses, rather than exact ones
• Based on abundance of each isotope found in nature
• We can’t use grams as the unit of measure
because the numbers would be too small
• Instead we use Atomic Mass Units (amu)
• Standard amu is 1/12 the mass of a carbon-12 atom
• Each isotope has its own atomic mass

12. Calculating Averages
Average = (%) x (mass1) + (%) x (mass2) + ….
100
Problem:
Silver has two naturally occurring isotopes, 107Ag with a
mass of 106.90509 amu and abundance of 51.84
%,and 109Ag with a mass of 108.90476 amu and
abundance of 48.16 % What is the average atomic
mass?
Average = (0.5184)(106.90509) + (0.4816)(108.90476)
= 107.87 amu

13. • If not told otherwise, the mass of the isotope is
the mass number in amu
• The average atomic masses are not whole
numbers because they are an average mass
value
• Remember, the atomic masses are the decimal
numbers on the periodic table
Average Atomic Masses

14. Properties of Isotopes
• Chemical properties are primarily determined by
the number of electrons
• All isotopes has the same number of electrons,
so they have nearly identical chemical
properties even though they have different
masses.
• Physical properties often depend on the
mass of the particle, so among isotopes they will
have slightly different physical properties such
as density, rate of diffusion, boiling point…

15. • Calculate the atomic mass of copper if copper has
two isotopes
• 69.1% has a mass of 62.93 amu
• The rest (30.9%) has a mass of 64.93 amu
• Magnesium has three isotopes
• 78.99% magnesium 24 with a mass of 23.9850 amu
• 10.00% magnesium 25 with a mass of 24.9858 amu
• The rest magnesium 26 with a mass of 25.9826 amu
• What is the atomic mass of magnesium?
More Practice Calculating Averages

16. More examples
• 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?

17. 17
Assign X and Y values:
X = % 10B Y = % 11B
Determine Y in terms of X
X + Y = 100
Y = 100 - X
Solve for X:
X (10.0) + (100 - X )(11.0) = 10.8
100 100
Multiply through by 100
10.0 X + 1100 - 11.0X = 1080

18. 18
Collect X terms
10.0 X - 11.0 X = 1080 - 1100
- 1.0 X = -20
X = -20 = 20 % 10B
- 1.0
Y = 100 - X
% 11B = 100 - 20% = 80% 11B

19. More Examples
Copper has two isotopes 63Cu (62.9
amu) and 65Cu (64.9 amu). What is
the % abundance of each isotope?
(Hint: Check periodic table for atomic
mass)
19

20. Mass Spectrometer
You are not responsible for
knowing the steps, but have
a general idea of what a
mass spectrometer is and
what data it outputs!