1. The Structure of an Atom
Ch.4 Notes

2. Section 1: Early Theories of Matter
Atom- the smallest particle of an element that retains the properties of the element.
Size? The world’s population is 6,840,000,000
The number of atoms in a penny is 29,000,000,000,000,000,000,000

3. Early Theories of Matter
Ancient Greek philosopher Democritus speculated matter was made up of atomos
The theory was rejected for 2000 years due to being challenged by Aristotle

4. Discovery of the Electron
John Dalton- proposed his atomic theory in based on his research that was not completely accurate
Atoms can be subdivided
Atoms of the same element can have slightly different masses

5. Discovering the Electron
Sir William Crookes - used a cathode ray tube to do experiments on the atom based on the relationship between mass and charge.
Electrons are negatively charged

6. Discovering the Atom
JJ Thomson ( )- in the 1890’s found a cathode ray could be deflected by magnetic and electric fields. He concluded the ray was a stream of negatively charged particles dislodged from an atom with a mass much less than a hydrogen atom.

7. Discovering the Atom
Robert Millikan ( ) – in 1909 he accurately determined the mass of an electron through the Oil Drop Experiment, it had a charge of -1
The actual mass of an electron is 9.1 x g

8. Nucleus Plum Pudding Theory-( aka the chocolate chip cookie model)
Matter is neutral, so Thomson proposed: spherical atoms have uniform (+) charge with (-) electrons embedded throughout.

9. Nucleus Ernest Rutherford, in 1911
Ernest Rutherford, in 1911
To test the theory, he aimed a thin beam of (+) charged Alpha particles at a thin sheet of gold.
He expected the alpha particles to pass through with little deflection
Instead, most passed through, but few bounced back and deflected at big angles.
He concluded atom is mostly empty space with large (+) charged nucleus in center

10. Atomic Summary *Nucleus -protons + neutrons -contains all the mass
-positive charge
*Electrons
-located in the cloud
-fast moving
-very small

11. Properties of Subatomic Particles
Relative
Mass
Actual
Mass
Particle
Symbol
Location
Charge
Electron
Cloud e-
9.11 x 10-28
Electron -1
1/1840 p+
Nucleus +1 1
Proton
1.673 x 10-24 n0
Nucleus
none 1
1.675 x 10-24
Neutron

12. How atoms differ:
Atomic number is how an atom is identified, it is equal to the number of protons in an atom, & determines the location of the atom in the periodic table.
The periodic table is arranged left to right & top to bottom by increasing atomic number.

13. atomic # 1 H
Chemical Symbol
1.008
Average atomic mass

15. Atomic Number Number of protons in nucleus of an element
This number identifies the element
Periodic table is arranged in sequence of increasing atomic numbers
Atoms are neutral: (+) charges must equal (-) charges

16. We know: atomic # = #protons = # electrons
How many protons and electrons are there in:
Be: p= 4 e= 4
F: p= 9 e= 9

17. Ions Be2+ has lost 2 electrons: F1- has gained one electron: p= 4 e= 2
Charged particles: Number of electrons has changed- proton # stays the same
Be2+ has lost 2 electrons:
F1- has gained one electron:
p= 4 e= 2
p=9 e=10

18. Isotopes
To identify an isotope a number is added to the end of the name, it is called a mass number.
Mass Number = # of protons + # of neutrons
Ex: Carbon-14, Neon-22, Potassium-41

19. Isotopes All atoms of same element have some number of protons
Most have different numbers of neutrons

20. Mass Number An isotope is identified by its mass number
Mass number= number of protons + number of neutrons
Number of neutrons = mass number – atomic number

22. Atomic Mass Atomic mass units (amu) = ½ the mass of a Carbon-12 atom
Atomic mass given in Periodic Table is not given in whole numbers
Atomic mass is the weighted average of the atomic masses of all naturally occurring isotopes. This weighted average takes into account the mass and abundance of each isotope

23. Atomic Mass
Average atomic mass= mass * abundance for each isotope: then add
Copper-63
Copper-65
Number of protons 29
Number of neutrons 34 36
Atomic mass
amu
amu
Abundance
69.17%
30.83%

24. Mass Contribution = mass x abundance
Cu-63= (62.930)(.6917)= 43.53
Cu-65= ( )(.3083) =20.02
= 63.55