1. Atomic Theory As Seen Through History
ChEMISTRY Notes
Atomic Theory As Seen Through History

2. Democritus—The First Model
Democritus of Abdera (~400 B.C.) is the first person known to come up with the theory of the existence of particles called atoms.
He said atoms were indivisible and indestructible particles that made up everything
His theories were unable to be tested at the time.
Statue of Democritus

3. John Dalton-the atom
Englishman John Dalton is generally given credit for the first experiments to prove the nature of atoms
The experiments he did resulted in Dalton’s atomic theory.
John.Dalton,

4. Dalton’s atomic theory
All elements are composed of tiny indivisible particles called atoms.
Atoms of the same element are identical. The atoms of any one element are different than those of any other element.
Atoms of different elements can physically mix together or chemically combine in simple, whole number ratios.
Chemical reactions occur when atoms are separated, joined or rearranged, but atoms of one element are never changed into another element as a result of chemical reaction.

5. After Dalton
Much of Dalton’s theory still stands, but some has been changed.
One change is that atoms are now known to be divisible.
We will now learn about the three subatomic particles the atom can be broken down into.

6. J. J. Thompson-Electrons
English physicist J.J. Thompson first discovered the electron in 1897.
His experiments with cathode ray tubes led to the theory that particles much smaller than atoms existed, and that their charge was negative.
Today the electron carries exactly one unit of negative charge, and its mass is 1/1840th of the mass of a hydrogen atom.
J.J.Thompson,

7. Goldstein and chadwick
After the discovery of the electron, the race was on to find other particles; they knew they were there because the atom is electrically neutral.
E. Goldstein is credited for discovery of the positively charged proton, which has a much greater mass than electrons.
James Chadwick found yet another subatomic particle—the neutron. It has no charge, but the same mass as a proton.
James Chadwick

8. Rutherford—The nucleus
At this point in history, the prevalent thought was that the atom was mostly a positive substance with negative electrons throughout.
Rutherford sought to prove this by shooting particles through gold foil onto photo paper.
He expected to see the particles go through with slight deflection.
What he saw was that some particles went straight through, but some bounced back!

9. Rutherford, cont’d.
This gave rise to the theory that atoms mostly consist of empty space, with the majority of the mass in the center of the atom—called the nucleus.
The protons and neutrons are located in the nucleus with the electrons orbiting around it.

10. Synopsis
The atom is the smallest part of an element that retains its properties.
It is made of mostly empty space,with the majority of the mass concentrated in the middle (the nucleus).
The nucleus contains the positively charged protons and the chargeless neutrons.
The electrons are situated outside of the nucleus in the empty space.

11. Atomic number, mass number, atomic mass & isotopes

12. Atomic Number
The atomic number of an element tells you the number of protons in the nucleus.
Because atoms are electrically neutral, the number of protons will be the same as the number of electrons.
Each element has a different atomic number.
Atomic number

13. Answer the following What element has atomic number 32?
How many protons does helium have?
What element has 79 protons?
What is uranium’s atomic number?
How many electrons does calcium have?

14. Mass number
Most of the mass of an atom is in the nucleus; so much that the electrons can be neglected.
The mass number is the number of protons and neutrons in the nucleus.
If you know the mass number and the atomic number, you can determine the composition of an atom.
# of neutrons = mass number – atomic number

15. C 12 6 Carbon-12 Mass number, cont’d.
Composition can be written in shorthand by putting the symbol for the element with the mass number and atomic number to the left. C 12 6
Or, the mass number and chemical element can designate atoms, such as in:
Carbon-12

16. Answer the following How many neutrons are in potassium-39?
How many protons are in lead-208?
If an element has 22 protons and 26 neutrons, what is its mass number?
What element is in the previous question?
How many neutrons are in ? Hg

17. Isotopes
Atoms of the same element always have the same number of protons, but can have different numbers of neutrons.
An atom with the same number of protons but different number of neutrons are called isotopes.
Isotopes are chemically alike, because it is the protons which are responsible for the chemical behavior.

18. Answer the following
What is the difference between iodine-127 and iodine-125?
Is an element with 25 protons and 30 neutrons an isotope of one with 26 protons and 30 neutrons? Why or why not?