1. History of the Atom & Atomic Structure
Chemistry
Ms. Piela

2. History of the Atom Democritus (460 BC – 360 BC)
Ancient Greek philosopher
No experiments performed!
Major Contribution: The Atom
He proposed that everything was made of these atoms and they were all indivisible
Was his theory correct?
No! There are subatomic particles!

3. History of the Atom John Dalton (1766-1844)
Major Contribution: Atomic Theory (1808)
This began the modern era of chemistry
Four Principles:
Elements are composed of indivisible particles called atoms.
All atoms of a given element are identical.

4. History of the Atom John Dalton
Compounds are composed of atoms of one or more elements, and will form only in whole-number ratios.
This is called the Law of Multiple Proportions
i.e. H2O exists, while H2.35O does not
A chemical reaction involves the combination, separation, or rearrangement of atoms, not their creation or destruction
This is called the Law of Conservation of Mass
Was his theory correct?
Mostly! Parts 1 & 2 have problems!

5. John Dalton
“When we attempt to conceive the number of particles in an atmosphere [gas], it is somewhat like attempting to conceive the number of stars in the universe; we are confounded with the thought. But if we limit the subject, by taking a given volume of any gas, we seem persuaded that… the number of particles must be finite”
- John Dalton on his approach to the theory of atoms, 1808

6. History of the Atom J.J. Thomson (1856-1940)
Major Contribution: The Electron
Cathode Ray Tube Experiment (1897)
Nobel prize (1906)

7. History of the Atom
J.J. Thomson

8. History of the Atom Thomson’s Atomic Model
Also known as the Plum Pudding Model
Was his theory correct?
No! Missing parts of atom!

9. History of the Atom Ernest Rutherford (1871-1937)
Two Major Contributions:
The nucleus
The atom is mostly empty space
Gold Foil Experiment (1910)
Nobel prize in Chemistry (1908)

10. History of the Atom
Ernest Rutherford’s Gold Foil Experiment

11. History of the Atom Rutherford’s Atomic Model Was his theory correct?
Mostly! Missing neutrons and location of electrons!

12. History of the Atom Niels Bohr (1885-1962)
Major Contribution: Planetary Model
of the Atom
Nobel Prize in Physics (1922) for spectrum of hydrogen
Atomic Line Spectra
Bohr observed that when light was given off from an atom, there were only single lines visible
Bohr proposed that each line represented an electron in a different orbit

13. History of the Atom
Atomic Line Spectra

14. History of the Atom
Bohr’s Atomic Model
Electrons
Nucleus

15. History of the Atom Current Theory of the Atom
Many scientists contributed to developing quantum mechanics, which is the current model of the atom
Known as the electron cloud model
The cloud is an area of probability where the electron is found
These electrons, moving at extremely high speeds, effectively occupy the entire area of the cloud, the same way that moving fan blades effectively occupy the entire area through which they pass.

16. History of the Atom Current Model of the Atom:
Probability cloud where electrons found
Nucleus

17. Atomic Structure Parts of the Atom Proton Neutron Electron Positive
Nucleus
Neutron
Neutral
Electron
Negative
Orbitting Nucleus

18. Atoms and the Periodic Table
Atomic Number
Atoms are identified by their number of protons
This is referred to as their atomic number
Think of atomic # like a social security card for each element

19. Atoms & Charges
In atoms that have a neutral charge, the numbers of electrons equals the number of protons
When an atom gains a charge, it is called an ion
Sodium Atom (Na)
11 Protons
11 Electrons
Sodium Ion (Na+)
10 Electrons

20. Atoms & Charges Chlorine Ion (Cl-) Chlorine Atom (Cl) 17 Protons
17 Electron
Chlorine Ion (Cl-)
18 Electrons

21. Atoms and the Periodic Table
Mass Number
The mass of an atom is the number of protons plus the number of neutrons
This is referred to as mass number
The mass of protons and neutrons are equal

22. Electrons and Mass of Atom
Why is the electron not part of the mass?
It takes roughly 1800 electrons to equal the mass of 1 proton, so it is left out.
If we wanted the mass of an elephant, would we weigh the flies buzzing around it? Doesn’t count!

23. Mass Number and Periodic Table
The periodic table does not give the mass number, but always the atomic number
For simplicity, we round the number on the periodic table to get the mass number
Round this number to whole number

24. Isotopes
Isotopes are elements that have the same number of protons, but contains a different number of neutrons
Compare: carbon-12 vs. carbon-14
The number indicates the mass number
Both contain the same number of protons (6), so carbon-14 must have two extra neutrons

25. Isotopes & Periodic Table
The masses given on the periodic table are an average of all the isotopes on the planet
We refer to the masses on the periodic table as the average atomic mass of an element
This explains why the atomic masses are not whole numbers – it is an average!