1. Atoms and their structure
Unit 3
Atoms and their structure

2. Modern View The atom is mostly empty space Two regions
Nucleus - protons and neutrons
Electron cloud - region where you might find an electron

3. Density and the Atom
Since most of the particles went straight through the gold foil, the atom was mostly empty space
Because the alpha particles turned so much, the positive particles must have been heavy
Small volume and big mass = big density.
This small dense positive area is the nucleus

4. Subatomic particles
Electron – located outside of nucleus, has negative charge
Proton - positively charged particles inside of nucleus that are many times heavier than the electron
Neutron - no charge but about the same mass as a proton, located inside of nucleus

5. Subatomic particles Relative mass (amu) Particle Name Actual mass (g)
Symbol
Charge
Electron e- -1
1/1840
9.11 x 10-28
Proton p+ +1 1
1.67 x 10-24
Neutron n0 1
1.67 x 10-24

6. Structure of the Atom There are two regions The nucleus
With protons and neutrons
Positive charge
Almost all the mass
Electron cloud - most of the volume of an atom
The region where the electron can be found

7. Size of an atom Atoms are small Measured in picometers, 10-12 meters
Hydrogen atom, 32 pm radius
Nucleus is very tiny compared to atom
If the atom was the size of a stadium, the nucleus would be the size of a marble
Radius of the nucleus is near m
Density near 1014 g/cm3

8. Counting the Pieces Atomic Number = number of protons
# of protons determines kind of atom
the same as the number of electrons in the neutral atom
Mass Number = the number of protons + neutrons
Includes all the particles with mass
NOT found on the periodic table

9. What about when Electrons ≠ Protons
Electrons may be gained or lost
Gaining electrons gives a negatively charged ion called an anion
Losing electrons gives a positively charged ion called a cation

10. What about when Electrons ≠ Protons
Practice with ions…
Magnesium makes ions with a 2+ charge. Are electrons lost or gained? How many electrons are moved?
Fluorine makes ions with a 1- charge. Are electrons lost or gained? How many electrons are moved?
An ion has 13 p+ and 10 e-. Give the symbol and charge for the ion.
An ion has 34 p+ and 36 e-. Give the symbol and charge for the ion.

11. Isotopes Dalton was wrong
Atoms of the same element can have different numbers of neutrons
different mass numbers (will have the same atomic number
called isotopes

12. X Symbols for Isotopes Mass number Atomic number Nuclear Notation
Contains the symbol of the element, the mass number and the atomic number X
Mass
number
Atomic
number

13. Symbols for Isotopes carbon- 12 carbon -14 uranium-235 Hyphen Notation
Contains the symbol (or name) of the element and the mass number.
carbon- 12
carbon -14
uranium-235

14. Na Symbols for Isotopes 24 11 Find the number of protons
number of neutrons
number of electrons
Atomic number
Mass Number
Name 24 Na 11

15. Br Symbols for Isotopes 80 35 Find the number of protons
number of neutrons
number of electrons
Atomic number
Mass Number
Name 80 Br 35

16. Symbols for Isotopes
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
Symbol – Nuclear & Hyphen notation
Name

17. Symbols for Isotopes
if an element has 91 protons and 140 neutrons what is the
Atomic number
Mass number
number of electrons
Symbol – Nuclear & Hyphen notation
Name

18. Symbols for Isotopes
if an element has 78 electrons and 117 neutrons what is the
Atomic number
Mass number
number of electrons
Symbol – Nuclear & Hyphen notation
Name

19. Atomic Mass How heavy is an atom of oxygen?
There are different kinds of oxygen atoms
More concerned with average atomic mass
Based on abundance of each element in nature
Don’t use grams because the numbers would be too small

20. Atomic Mass Is not a whole number because it is an average
are the decimal numbers on the periodic table

21. Measuring Atomic Mass Unit is the Atomic Mass Unit (amu)
One twelfth the mass of a carbon-12 atom
6 p+ and 6 n0
Each isotope of an element has its own atomic mass
we get the average atomic mass of an element using weighted averages (need mass & percent abundance)

22. Calculating averages
You have five rocks, four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks?
Total mass = (4 x 50) + (1 x 60) = 260 g
Average mass = (4 x 50) + (1 x 60) = 260 g

23. Calculating averages (% as decimal x mass) + (% as decimal x mass) + …
If 80% of the rocks were 50 grams and 20% of the rocks were 60 grams what is the weighted average mass of the rocks?
Weighted Average =
(% as decimal x mass) +
(% as decimal x mass) + …
(0.8 x 50 g) + (0.2 x 60 g) = 52 g

24. Homework – due 10/18 Pg 76 (#3) Pg 87 (#2-3)
Qs with * need to have answers in the form of complete sentences. Please show your math work as well.