1. Section 4.2—Atomic Structure

2. What are atoms?
Atom - smallest piece of matter that still has the chemical properties of the element.
One Atom in a solid pure elemental sample. Note how it represents or lookes like everyone else!

3. THE ATOM’s Nucleus DID YOU KNOW??????
The nucleus holds nearly all the mass of the atom! YET – it is sooo tiny! Look below and consider that IF a nucleus were the size of a pea – then the rest of the atom would be the size of a football stadium!!
A nucleus is so dense that a nucleus the size of a pea has a mass of 250 million tons!!!!
SO, if it is SO small – How do the protons stay together!! Don’t they repel! The STRONG force of the nucleus holds it together!!!!!!
Small dense nucleus
Electron Cloud – mostly empty space

4. What’s in an atom? An atom is made of three sub-atomic particles
Location
Mass
Charge
Proton
Nucleus
1 amu = 1.6710-27 kg +1
Neutron
Nucleus
1 amu = 1.6710-27 kg
Electron
Outside the nucleus
~0 amu =
9.1010-31 kg -1
1 amu (“atomic mass unit”) = 1.66  kg

5. What gives an atom its identity?
What makes an atom “carbon” as opposed to “oxygen”?
Every atom has a different number of protons.
The number of protons determines the identity of the atom
The atomic number shows the number of protons.
Atomic number = protons

6. The Nucleus & Mass
Since the nucleus has protons & neutrons, and the mass of each one is 1 amu…
The mass of the nucleus (in amu’s) is the number of protons + neutrons
Since electrons have relatively no mass (0.054% of one proton or neutron), we don’t need to worry about them when determining mass of an atom
Mass # = protons + neutrons

7. Charges Protons have a positive charge
Electrons have a negative charge
Neutrons have no charge
Overall charge is therefore only due to the protons and electrons.
Overall Charge = #protons - #electrons

8. How do we show information about an element?

9. X A C Z Element symbols Element Symbol Charge Mass number
1 or 2 letters, found on the periodic table X A C Z
Charge
# protons - # electrons
(assumed to be “0” if blank)
Mass number
# protons + # neutrons
Atomic number # of protons

10. Example: Element symbols
O = Oxygen O 16 -2 8
Charge -2
Mass number 16
Atomic number 8

11. Fill in the missing values
Let’s Practice
Example:
Fill in the missing values
Symbol
Name
Atomic #
Mass #
Charge
Proton
Neutron
Electron
Magnesium-25 +2 82
126

12. Fill in the missing values
Let’s Practice
Remember:
Atomic number is the identity
Atomic number = protons
Charge = proton - electrons
Mass # = protons + neutrons
Example:
Fill in the missing values
Symbol
Name
Atomic #
Mass #
Charge
Proton
Neutron
Electron
Magnesium-25 +2 82
126 12 25 12 13 10
Lead-208
208 82

13. Isotopes

14. What are isotopes?
Isotopes – Atoms of the same element with a different number of neutrons
Some isotopes are radioactive or UNSTABLE —but not all…many are quite stable!

15. Isotopes Example
Hydrogen-2
Hydrogen-1
Mass # = 1 amu
Mass # = 2 amu
If they have a different number of neutrons, and neutrons have a mass of 1 amu…
Then isotopes of the same element will have different masses!
But because their protons are the same, they are the same element!
How is the Density of Water Effected by the isotope?

16. C C Carbon-12 Carbon-13 Identifying Isotopes
Isotopes can be differentiated by the different mass numbers in the element symbol 12 C 13 C
Carbon-12
Carbon-13
Or by the mass number following their name.

17. Mass Number versus Atomic Mass
Average Atomic Mass
# of protons + # of neutrons
Average of actual masses
Always a whole number
Not a whole number
For one specific isotope only
Weighted average of all isotopes
Is not found on the periodic table
Is found on the periodic table

18. Calculating Average Atomic Mass
Average atomic mass is a weighted average (it takes into account how often each isotope occurs).
Actual mass (not mass number)
“Sum of”
Average
atomic
mass  ( )  =
Abundance of isotope
Mass of isotope
What fraction of the time is that isotope present?

19. Example of Finding Avg Atomic Mass
Find the atomic mass of chlorine if
Chlorine-35 has a mass of amu
and Chlorine-37 has a mass of amu and is present 24.22% of the time.
Remember that percents add up to 100.
So they said the second isotope is present 24.22% of the time.
This means that the first isotope is present = 75.78% of the time
Isotope
Mass
Percent
Decimal 1
amu
75.78
0.7578 2
amu
24.22
0.2422
This chart summarizes the information in the problem:
= amu
(this is what’s on the periodic table for Cl!)

20. Ions

21. What are ions?
Ions – Atoms of with a different number of protons (+charges) than electrons (- charges)
Ions want to do stuff. They want to get/lose/share electrons to balance their charge

22. Ion Example If they have a different number of protons and electrons
Then ions will have a charge
But because their protons + and electrons are -