1. Counting Atoms
Section 3.3

2. Atomic Number Atomic number: the number of protons of each atom
Represented by “Z”
The atomic number identifies the element

3. Isotopes
Isotopes: atoms of the same element that have different masses
Isotopes have the same number of electrons and protons, but a different number of neutrons
Most elements consist of a mixture of isotopes

4. Mass Number
Mass number: the total number of protons and neutrons that make up the nucleus of an isotope
Mass number = p+ + n0
Isotopes are usually identified by specifying their mass number

6. Designating Isotopes There are two methods to specify isotopes
Method 1 is called hyphen notation: write the name of the element, then a hyphen, then the mass number
Method 2 is called the nuclear symbol: in front of the symbol, superscript the mass number and subscript the atomic number

7. Hydrogen
Hydrogen is special in that it also has specific names for each isotope
Hydrogen with only 1 p+ and no n0 is called protium, hydrogen-1,
Hydrogen with 1 p+ and 1n0 is called deuterium, hydrogen-2,
Hydrogen with 1 p+ and 2 n0 is called tritium, hydrogen-3,

8. Isotopes of Hydrogen

9. Practice: name the isotope both ways
helium with 1 neutron
carbon with 7 neutrons
oxygen with 8 neutrons
uranium with 142 neutrons
helium-3,
carbon-13,
oxygen-16,
uranium-234,

10. More Practice
How many protons, electrons, and neutrons are there in an atom of chlorine-37?
17 electrons, 17 protons, 20 neutrons
How many protons, electrons, and neutrons make up an atom of bromine-80
35 protons, 35 electrons, 45 neutrons
Complete worksheet here on atomic notation

11. Relative Atomic Masses
If we express the mass of atom in grams, it would be extremely small
Relative mass is used instead
Carbon-12 is the standard atom and has been assigned a mass value of exactly 12 atomic mass units (amu)

12. More about Relative Atomic Mass
All other atomic masses are determined by comparing it with the mass of the carbon- 12 atom
See the chart on page 80 for some typical atomic masses
The masses could also be expressed by adding up the mass of the e-s, p+s, and n0s

13. Average Atomic Mass
Average atomic mass: the weighted average of the atomic masses of the naturally occurring isotopes of an element
The average atomic mass depends on both the mass and the relative abundance of each of the element’s isotopes

14. Calculating Average Atomic Mass
Copper consists of 2 naturally occurring isotopes
A sample of copper contains 69.15% of copper-63 ( amu) and 30.85% of copper-64 ( amu)
Change the percentages to decimals

15. Continued Solve like this:
(0.6915)( amu ) + (0.3085)( amu)= amu
The average atomic mass of copper is amu
Always round to 2 decimal places

16. The Mole The SI unit for the amount of a substance
Mole: the amount of a substance that contains as many particles as there are atoms in exactly 12 g of carbon-12
It is a counting unit

18. Avogadro’s Number
Avogadro’s number: the number of particles in exactly one mole of a pure substance; x 1023 particles

19. Molar Mass Molar mass: the mass of one mole of a pure substance
Usually written in units of g/mol
The molar mass of an element is numerically equal to the atomic mass of the element in atomic mass units (found on the periodic table)

21. More about Molar Mass
Molar mass is usually rounded to two decimal places
What is the molar mass of carbon?
12.01 g/mol
What is the molar mass of chlorine?
35.45 g/mol

22. Gram/Mole Conversions
The molar mass can be used as a conversion factor
see the board and the book for sample problems
What is the mass in grams of 3.50 mol of copper? 222g Cu
A chemist produced 11.9 g of Al. How many mol of Al were produced? mol Al

23. Conversions with Avogadro’s Number
Since x 1023 is the number of particles in a mole, that can be used as a conversion factor
See the board and your book for sample problems