1. Atoms and the Periodic Table
The Atom
Ch. 4

2. Democritus (400 BC) Greek philosopher.
All things are composed of tiny,
indivisible particles: called:
Atomos means
“something that
cannot be cut”.

3. Lavoisier (1789) French scientist. Came up with the law
of conservation of
Mass.
“Matter is neither
created nor destroyed.”

4. Dalton (1803) Came up with the first “official” atomic Theory:
Elements are made of tiny particles called atoms.
All atoms of a given element are the same.

5. Dalton Atoms of different elements are different.
Atoms of different elements combine to form compounds.
Atoms cannot be subdivided, created, or destroyed.

6. Thomson (1906) Experimented with cathode rays Discovered that
cathode rays are
negatively charged
Developed the
“plum pudding”
model of the atom

7. Cathode Ray Experiment
Thomson passed negatively charged Cathode rays through a tube of Hydrogen: the rays bent toward the positively charged plate.
He tried it again with different gases and always got the same result.
He realized that all matter must contain these negatively charged particles (electrons).

8. “Plum Pudding”
Thomson said atoms were probably composed of negatively charged particles mixed with a positively charged substance.

9. History Summary: Democritus – Atomos
Lavoisier – Law of Conservation of Mass
Dalton – First official Atomic Theory
Thomson – Plum Pudding Model, theory of electrons

10. Subatomic Particles
Subatomic Particles
Charge
Symbol
Mass (kg)
Electron 1- e-
9.02x10-31
Proton 1+ p+
1.673x10-27
Neutron no
1.675x10-27
Electrons are about 1837 times smaller than protons, so they do not significantly contribute to an atom’s mass.
1 neutron has about the same mass as 1 proton, so we calculate the mass of an atom to be protons + neutrons.
Ex. Hydrogen – AMU
Hydrogen’s mass is essentially that of 1 proton
H has 1 proton, 0 neutrons and 1 electron

11. Atomic Number An element is defined by its number of protons
Ex. All examples of Nitrogen (number 7 on the periodic table) have 7 protons. If you change this number, you change the element!
Ex. If we change to 8 protons, what do we now have?
Oxygen

12. Calculating Number of Neutrons
All the atoms of one element have the same number of protons in their nucleus, but electrons & neutrons differ
From the Periodic Table:
Atomic Number (Z) = # of protons
Mass Number (A) = # of protons + # of neutrons
Atomic Symbol = X

13. Calculating Number of Neutron
Example: For Ca (Calcium)
Z = 20 = (20 protons)
A = 40 = (20 protons + neutrons)
Number of Neutrons = A - Z
A – Z = 40 – 20 = 20 neutrons
 Ca has 20 protons and neutrons

14. The Nucleus & Standard Atomic Notation
Z = # of protons
A = # of protons + # of neutrons
X = Ca A Z 40 20

15. Inferring the Number of Electrons
You can infer the number of electrons from Z
Protons have a positive charge = p+
 But, the atom is neutral on the periodic table
 Therefore, there must be enough electrons to BALANCE the number of protons
If the atom has p+ = 5  and NO CHARGE
 e- = 5  BALANCE

16. Atomic Mass Number always a whole number
mass # = protons + neutrons
always a whole number
ROUND from atomic mass on the Periodic Table!
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17. Isotopes Mass # (P + N) Atomic # (P) Nuclear symbol:
Isotopes are atoms of the same element with different mass numbers/different number of neutrons.
Nuclear symbol:
Mass # (P + N)
Atomic # (P)
Hyphen notation: carbon-12 (C-12)

18. Isotopes
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19. Isotopes Chlorine-37 atomic #: 17 mass #: 37 # of protons:
# of electrons:
# of neutrons: 17 37 20

20. Relative Atomic Mass
12C atom = × g – that’s small!
atomic mass unit (amu) – used by scientists to compare the size of really small particles
1 amu = 1/12 the mass of a 12C atom
© Addison-Wesley Publishing Company, Inc.
1 p = amu 1 n = amu 1 e- = amu
I’m so small nobody notices me

21. Average Atomic Mass Weighted average of all isotopes.
This is the crazy number on the Periodic Table.
You should round to the nearest tenth.
Avg.
Atomic
Mass OR
Avg.
Atomic
Mass
= (mass)(% in nature) + (mass)(% in nature)

22. 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
16.0
g/mol
This is the same number you get when you round the atomic mass to the nearest tenth on the Periodic Table! OR
Avg.
Atomic
Mass
= (16)(.9976) + (17)(.0004) + (18)(.020) = 16.0
g/mol

23. Average Atomic Mass
EX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37.
Avg.
Atomic
Mass
35.4 g/mol OR
Avg.
Atomic
Mass
= (35)(.8) + (37)(.2) = 35.4 g/mol

24. Electrostatic forces
These are the forces of attraction between electrons and protons

25. Nuclear force
Different from electrostatic forces, this is the force that keeps nucleons (particles found in the nucleus of an atom) together.
Fun fact: the total mass of the nucleus is actually slightly greater than the mass of the protons and the neutrons together- when you break a nucleus down, some of that mass is released as energy, which we use in nuclear power plants and nuclear weapons