1. Counting Atoms
CHAPTER 3.3

2. ATOMIC NUMBER
The atomic number (Z) of an element is the number of protons of each atom of that element.
Identifies an element

3. Isotopes
Isotopes are atoms of the same element that have different masses.
same number of protons & electrons
Different number of neutrons.
Three types of hydrogen atoms are known.
Protium (most common type) % of the hydrogen atoms found on Earth.
Deuterium accounts for % of Earth’s hydrogen.
Tritium is radioactive.
It exist in small amount in nature , but it can be prepared artificially .

5. MASS NUMBER
Mass number: is the total number of protons and neutrons that make up the nucleus of an isotope.
Mass number of hydrogen isotopes
Protium
Deuterium
tritium

6. DESIGNING ISOTOPES There are two methods for specifying isotopes:
The mass number is written with a hyphen after the name of the element. (Hyphen notation)
Helium-4
2. Nuclear symbol : show the composition of a nucleus

7. PRACTICE
How many protons, electrons, and neutrons, make up an atom of bromine-80?
Write the nuclear formula for carbon-13.
Write the hyphen notation for the isotope with 15 electrons and 15 neutrons.

8. RELATIVE ATOMIC MASS
One atomic mass unit or 1 amu is exactly1/12 the mass of a carbon-12 atom.
Ex:
The hydrogen-1 atom has an atomic mass of about 1/12 that of the carbon- 12 atom or about 1 amu. ~ amu.
An Oxygen-16 atom has about 16/12 or 4/3 the mass of a carbon-12 atom. ~ amu.

9. AVERAGE ATOMIC MASS OF ELEMENTS
Average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element.
For example naturally occurring copper consist of % copper- 63 which has an atomic mass of amu and % copper-65 which has an atomic mass of amu. What is the average atomic mass?

10. RELATING MASS to NUMBER of ATOMS
The relative atomic mass scale makes it possible to know how many atoms of an element are present in a sample of the element with a measurable mass.
3 very important concepts
The mole
Avogadro’s number
Molar mass

11. THE MOLE The mole is the SI unit for amount of substance.
A mole (mol) is the amount of substance that contains as many particles as there are atoms in exactly 12 g of carbon-12.
The mole is a counting unit just like a dozen is.

12. AVOGADRO’S NUMBER
Avogadro’s number : is the number of particles in exactly one mole of a pure substance x1023
If every person living on earth (~6 billion people) worked to count the atoms in one mole of an element , and if each person counted continuously at a rate of one atom per second , it would take about 3 million years for all the atoms to be counted.

13. MOLAR MASS
Molar mass: the mass of one mole of a pure substance (g/mol)
Ex: what is the mass in grams of 3.50 mol of the element copper, Cu?
Moles Cu x
Grams Cu
Moles Cu

14. PRACTICE
What is the mass in grams of 2.25 mol of the element iron, Fe?
what is the mass in grams of mol of the element K?
What is the mass in grams of mol of Na?
What is the mass in grams of 16.3 mol of Ni?
How many moles of calcium, ca are in 5.00 g of calcium?
How many moles of Gold, Au are in 3.60x10^-5 g of gold?
How many moles of zinc, Zn, are in g of Zinc?
How many moles of silver, Ag, are in 3.01x10^23 atoms of silver?
What is the mass in grams of 1.20 x 10^8 atoms of copper, Cu?
What is the mass in grams of 7.5x10^15 atoms of nickel,Ni?