1. The Structure of the Atom

2. Atomic Theory Democritus & John Dalton J.J. Thomson
Atomos: All matter is made of tiny indivisible particles
J.J. Thomson
Discovered electrons
Plum Pudding Theory
Atoms were positive areas
Negatively charged electrons were randomly scattered in the atom

3. Rutherford’s Atomic Model
Ernest Rutherford
Discovered Nucleus
Nuclear Model
Nucleus: small, heavy, positive center of atom
Electron cloud: area outside nucleus where electrons are located
Mostly empty space e- +

4. Subatomic Particles
Electron – negatively charged particle, smallest, light, in electron cloud
Proton - positively charged pieces, heavy, in nucleus
Neutron - no charge, same mass as protons, in nucleus

5. Subatomic particles Relative mass Name Symbol Charge Electron e- -1
1/1840
Proton p+ +1 1
Neutron n0 1
***Relative Mass means compared to. *** A proton is 1840 times the mass of an electron.

6. How Atoms Differ

7. Counting the Pieces Atomic Number = p+ Mass Number = p+ + n0
determines kind of atom (fingerprint)
Mass Number = p+ + n0
Number of e- = # of p+
when it’s a neutral atom (no charge)

8. Na 11 Sodium 22.99 From periodic table Using the example 
11 is the atomic number
There are 11 p+ in Sodium
Na is the element symbol
Sodium is the element name
22.99 is the avg. atomic mass
This is NOT the mass number!!! 11 Na
Sodium
22.99

9. Symbols
Contain the symbol of the element, the mass number and the atomic number.
Element Symbol X
Mass
number
Charge
Atomic
number

10. F 19 9 Symbols Find the Mass # 19 Atomic # p+ 9 e- n0 9 9 10
Always the same
Same b/c no charge 9 9 9 10
= mass # - p+ =
19 – 9 =

11. Br 80 35 Symbols Find the Mass # Atomic # 80 p+ e- 35 n0 35 35 45
= mass # - p+ = =

12. Symbols
A neutral element has an atomic number of 34 and a mass number of 78. What is the
number of p+?
number of n0?
number of e-?
Complete symbol. 34 78 Se
Same as atomic # 44 = 34 34
Same as p+

13. Symbols
A neutral element has 91 protons and 140 neutrons what is the
Atomic #?
Mass #?
# of electrons?
Complete symbol. 91
Same as p+
231
p+ + n0 =
231 91 Pa
Same as p+ 91

14. Name-# Isotopic Notation
Contain the symbol of the element, the mass number and the atomic number. All are neutral (no charge)
Example: Sodium-24
Mass #
Element Name
Name-#

15. Sodium -24 Isotopic Notation Find the Mass # Atomic # 24 p+ e- 11 n013
= mass # - p+
= 24 – 11 =

16. Nickel -56 Isotopic Notation Find the Mass # Atomic # 56 p+ e- 28 n0
= mass # - p+
= 56 – 28 =

17. Charges If Neutral: Electrons tell you charge of the atom:
Atomic # = p+ = e-
Electrons tell you charge of the atom:
Lose/Gain e-: changes charge, creates ions
# e- < # p+: cation, lose e-, positive  ions
# e- > # p+: anion, gain e-, negative  ions

18. Cl 36 - 17 Symbols Find the Mass # Atomic # 36 p+ n0 17 e- 17 19 18
= 36 – 17 = 18
Charge is -1
Gained 1 e-

19. Ca 42 +2 20 Symbols Find the Mass # Atomic # 42 p+ n0 20 e- 20 22 18
= 42 – 20 = 18
p+ - 2 =
20 - 2=
Charge is +2
Lost 2 e-

20. Symbols
An element has 76 electrons and 117 neutrons, and a charge of +2, what is the
Atomic #?
Mass #?
# of protons?
Complete symbol. 78 +2
195 Pt
Charge is +2, lost 2 e-,
e- + 2 = =
195
p+ + n0 = 78 78
same as atomic #

21. Build an Atom Only 2 e- on first ring 8 e- on each ring after that
Pick 2 atoms out of cup
Cut out p+, n0 and e-
Glue
p+ & n0 in nucleus
e- on rings around nucleus
Add a dot of each in the legend
Determine p+, n0, e-, mass # and atomic # for your atoms