1. Atomic Structure
Modern Atomic Theory

2. Modern Atomic Theory
The atom consists of positive protons, negative electrons, and neutral neutrons.
Protons and neutrons are located in the nucleus of the atom, which is small and massive.
Electrons are located outside of the nucleus, which creates the volume of the atom.

3. The Modern View of Atomic Structure

4. Subatomic particles Actual Electron e- -1 9.11 x 10-28 Proton p+ +1 1
mass (g)
Relative
mass (amu)
Name
Symbol
Charge
Electron e- -1
1/1840
9.11 x 10-28
≈ 0
Proton p+ +1 1
1.67 x 10-24
Neutron n0 1
1.67 x 10-24
amu = atomic mass unit

5. Atomic Number
Henry Moseley discovered that each element has a unique positive charge.
Atomic Number or Z
Number of Protons
For neutral atoms # protons = # of electrons
How many protons and electrons are in Si?
14 p and 14 e
What element contains 8 protons?
oxygen

6. Ions Ion charge is written as a superscript.
Most atoms are not neutral.
An ion forms when an atom loses or gains electrons.
Lose electrons = positive charge
Gain electrons = negative charge
Ion charge is written as a superscript.
Charge = # protons - # electrons
What is the element with 16 protons and 18 electrons?
S-2
What is the element with 20 protons and 18 electrons?
Ca+2

7. Atomic Mass
Atomic mass or A is the number of protons and neutrons in an atom.
Atomic Mass Unit or amu
based on carbon-12, exactly 12 amu
We always round atomic mass to the nearest whole number.
Why?
Atomic mass is the weighted average of all of isotopes for that element based on abundance.

8. X Isotopic Notation A Z We write symbols for elements as follows:
Mass number X A Z
Element symbol
Atomic number

9. Isotopes
Atoms with the same number of protons different number of neutrons, therefore different atomic masses. 1 H 2 1 H 3 1 H

10. Isotopes

11. Naming Isotopes
We can also put the mass number after the name of the element.
carbon- 12
carbon -14
uranium-235

12. Calculating averages
Multiply the atomic mass of each isotope by it’s abundance (expressed as a decimal), then add the results.
Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of amu and the rest has a mass of amu.
62.93 amu x .691 = amu
64.93 amu x .309 = amu
Total = = 63.6 amu

13. Na Atomic Mass 23 11 What is the atomic mass of Na?
What is the atomic number of Na?
Write this in isotopic notation:
To find the neutrons subtract the two numbers.
23 amu
11 amu 23 Na 11
= 12 amu

14. Br Symbols 80 35 Find the number of protons number of neutrons 35 p
number of electrons
Atomic number
Mass Number
35 p
45 n 80
35 e Br 35 35
80 amu

15. Br Symbols 80 -1 35 Find the number of protons number of neutrons 35 p
number of electrons
Atomic number
Mass Number
35 p
45 n 80 -1
36 e Br 35 35
80 amu

16. Symbols
if an element has an atomic number of 34 and a mass number of 78 what is the
number of protons
number of neutrons
number of electrons
Complete symbol
34 p
44 n
34 e 78 Se 34

17. Symbols
if an element has 91 protons and 140 neutrons what is the
Atomic number
Mass number
number of electrons
Complete symbol 91
231 amu 91
231 Pa 91

18. Summary
Mass number
Element charge +1 H 1
Atomic number
Element symbol

19. Ba Symbols 135 +2 56 Find the number of protons number of neutrons
number of electrons
Atomic number
Mass Number
Is this an ion?
Is this an isotope?
56 p
79 n
135 +2
54 e Ba 56 56
135 amu
Yes
Yes