1. The Atom

2. Two major parts of an atom
Nucleus (not to scale)
Electron Cloud

3. Three Major Sub-Atomic Particles
Protons
Neutrons
Electrons

4. THE PROTON p+
Heavy = 1 amu
Positive + 1 charge
Location: Nucleus

5. THE NEUTRON N° 
Heavy = 1 amu
No charge, neutral
Location: Nucleus

6. e- THE ELECTRON Very light = 1/2000 amu Negative  -1 charge
Location: electron cloud

7. Nucleus: Protons and Neutrons Electron Cloud: Only Electrons
Where are they located?
Nucleus: Protons and Neutrons
Electron Cloud: Only Electrons

8. ATOMIC NUMBER (Z) The # of protons in an atom
Can not change for an element
All atoms are neutral, so Z equals the # of electrons
For an ion – the number of electrons may differ

9. Example: Sodium
Atomic # = # of protons 11 Na

10. MASS NUMBER (A) The mass of an atom A = protons + neutrons
To determine # of neutrons
Neutrons = A - Z

11. Example
An atom of sodium has a mass of 24 amu, how many protons, electrons and neutrons does it have?
11 p+
11 e-
= 13 N° 11 Na

12. ISOTOPES
Atoms of the same element that differ in mass. (They have the same # of p+, but different # of N°)

13. Isotope Notation
Mass #
Atomic #

14. Isotope Notation Can also be written as ELEMENT – MASS #
Example: Carbon - 12

15. So, why do the elements on the PT have masses with decimals???

16. AVERAGE ATOMIC MASS
The weighted average mass of all naturally occurring isotopes of an element.

17. Example using exam scores!
Exam Scores can be weighted higher than homework or quiz grades:
What if… And you received:
Exam 50% Exam 93
Quizzes 20% Quizzes 82.5
Homework 30% Homework 85
Your grade =
(93 x .5) + (82.5 x .2) + (85 x .3) = 88.5

18. Example
Example: The element hydrogen has three isotopes. It exists in nature 99.41% of the time as Hydrogen-1, 0.4% of the time as Hydrogen-2 and 0.19% of the time as Hydrogen – 3. What is the average atomic mass of hydrogen?
0.9995* * *3 = u

19. Famous Scientists in Atomic Theory

20. Democritus (400 B.C.)
First to develop the idea of an atom.

21. DALTON Dalton created an atomic theory that stated:
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 from those of any other element

22. J.J. Thomson “Plum Pudding Model” Solid positively charged sphere
Electrons embedded within

23. Rutherford’s Gold Foil Experiment

24. Rutherford
Proved the existence of a tiny, dense, positively charged nucleus.
The rest of the atom is mostly empty space.