1. Useful Element Notations

2. Atomic Number (Z) The number of protons in the nucleus
Determines the identity of the atom (which element it is)
If you change the number of protons, the atom changes to another element.

3. Mass Number (A)
The total number of particles in the nucleus—protons and neutrons
The total number of particles that make up the mass of the atom

4. Isotopes: Count Those Neutrons
Atoms of an element which have various amounts of neutrons but the same number of protons
Since the number of neutrons changes, the mass of the isotopes is different.

5. Isotopic Notation
It shows the mass number, atomic number, and symbol for the element. X A Z

6. Therefore, there must be 5 neutrons.
Isotopic Notation
Example: 9 9 Be 4 4
9 protons and neutrons
4 protons
Therefore, there must be 5 neutrons.

7. Question
If isotopic notation for an element is , how many neutrons are in the nucleus? Ar 40 18 18 40 22 58

8. Atomic Mass
Most elements consist of a mixture of different isotopes, which have different masses.
Therefore, the mass of the element is the average of the weights of the isotopes.

9. Unified Atomic Mass Units
Dalton tried to find the masses of the atom but could only find the relative masses.
We now know the exact masses.

10. Unified Atomic Mass Units
A unified atomic mass unit (u) is approximately the mass of a proton.

11. Example Calculate the average atomic mass of a chlorine atom.
In nature, 75.77% of chlorine atoms have a mass of u per atom (Cl-35).
24.23% of chlorine atoms have a mass of u per atom (Cl-37).

12. Example Convert the percents to decimals. 75.77% = 0.7577
24.23% =

13. Example
Multiply the decimal percentage by the mass of each isotope to find the total mass of each isotope in the sample.
Cl-35: x u = u
Cl-37: x u = u

14. The weighted average mass of a typical chlorine atom: 35.46 u
Example
Add the results to get the weighted average mass of the sample. 
26.50 u u u
The weighted average mass of a typical chlorine atom: u 

15. Valence Electrons: Last But Not Least
Outer level electrons
In the s and p sublevels
Maximum # in each atom: 8
2 in the s
6 in the p

16. Valence Electrons: Last But Not Least
Emit spectra
Participate in chemical bonds
Give elements physical properties

17. A shorthand method of showing the valence electrons
Electron Dot Diagrams
A shorthand method of showing the valence electrons

18. Electron Dot Diagrams Na
valence electrons: 1

19. Electron Dot Diagrams Mg
valence electrons: 2

20. Electron Dot Diagrams Al
valence electrons: 3

21. Electron Dot Diagrams Si
valence electrons: 4

22. Electron Dot Diagrams P
valence electrons: 5

23. Electron Dot Diagrams S
valence electrons: 6

24. Electron Dot Diagrams Cl
valence electrons: 7

25. Electron Dot Diagrams Ar
valence electrons: 8

26. Ions: Charged Atoms

27. Changing Numbers
Changing the number of protons gives a different element.
Changing the number of neutrons gives a different isotope of the same element.

28. Changing Numbers
Changing the number of electrons gives an ion of that element.

29. Ions Are atoms which have gained or lost electrons
Occur when an electron gains enough energy to “jump” off an atom

30. Ions
Are charged, rather than neutral like atoms, because the number of electrons and protons are no longer equal

31. Anions Are ions that have gained extra electrons
Are negatively charged
Have gained negative charges

32. Cations Are ions that have lost some of their electrons
Are positively charged
Have lost negative charges