**Isotope notes** Dalton was proved incorrect and his theory was modified

**Protons define the element** • Atoms that have the same number of protonsare always atoms of a specific element. • Example: Carbon

**Neutrons can vary** BUT • atoms can have different numbers of neutrons and still be an atom of a specific element.

**Isotopes** • This is because elements can have isotopes (basically atoms of the same element with a different number of neutrons in their nuclei).

**Dalton was wrong** • When Dalton stated his atomic theory in the early 1800’s, he assumed that all of the atoms of a given element were identical.

**James Chadwick** • Over 100 years after Dalton, James Chadwick discovered that the nuclei of most atoms contains neutrons as well as protons.

**Dalton’s Theory Changes** • Dalton’s theory now states: • All atoms of the same element contain the same number of protons and electrons, but atoms of a given element may have different numbers of neutrons.

**The Isotopes of Hydrogen** • Hydrogen – 1 • Also written H-1 • Also known as protium • Hydrogen has an atomic number of 1, so it has 1 proton • The hyphen notation above tells us that the mass number of H-1 is 1 • Number of neutrons = mass number – atomic number • So H-1 must have 0 neutrons

**The Isotopes of Hydrogen** • Hydrogen – 2 • Also written H-2 • Also known as Deuterium • Hydrogen has an atomic number of 1, so it has 1 proton • The hyphen notation above tells us that the mass number of H-2 is 2 • Number of neutrons = mass number – atomic number • So H-2 must have 1 neutron

**The Isotopes of Hydrogen** • Hydrogen – 3 • Also written H-3 • Also known as Tritium • Hydrogen has an atomic number of 1, so it has 1 proton • The hyphen notation above tells us that the mass number of H-3 is 3 • Number of neutrons = mass number – atomic number • So H-3 must have 2 neutron

**Calculate the number of neutrons** • For chlorine found on the periodic table (the most common form of chlorine that is found in nature) Chlorine-35 • For Chlorine-37 • Chlorine-35 = 18 neutrons, Chlorine-37 = 20 neutrons • For Cobalt found on the periodic table Cobalt-59 • For Cobalt-60 • Cobalt-59 = 32 neutrons, Cobalt-60 = 33 neutrons

**Calculating Average Atomic Mass** Average atomic mass is the atomic mass that appears on the periodic table. For example – Copper has an average atomic mass of 63.55 amu.

**Calculating Average Atomic Mass** Yet, in nature, most elements are found as mixtures of two or more isotopes. For example, copper consists of • 69.17% copper-63 which has a relative atomic mass of 62.94 amu AND • 30.83% copper-65which has a relative atomic mass of 64.93 amu

**Calculating Average Atomic Mass** To find the averageatomic mass, multiply the decimal equivalent of the percent (for example 69.17% = 0.6917) of each isotope by the respective relative atomic mass and add the results. (0.6917 X 62.94 amu) + (0.3083 X 64.93 amu) = 63.55 amu

**Practice Calculating Average Atomic Mass** • Boron – 10 is found 19.9% of the time in nature and has a relative atomic mass of 10.013 amu • Boron – 11 is found 80.1% of the time in nature and has a relative atomic mass of 11.009 amu • Calculate the average atomic mass of Boron

**Practice Calculating Average Atomic Mass** Boron (0.199 X 10.013) + (0.801 X 11.009) = 10.81 amu

**Practice Calculating Average Atomic Mass** • Magnesium – 24 is found 78.99% of the time in nature and has a relative atomic mass of 23.985042 amu • Magnesium – 25 is found 10.00% of the time in nature and has a relative atomic mass of 24.985837 amu • Magnesium – 26 is found 11.01% of the time in nature and has a relative atomic mass of 25.982593 amu • Calculate the average atomic mass of Magnesium

**Practice Calculating Average Atomic Mass** Magnesium (0.7899 X 23.985042) + (0.1000 X 24.985837) + (0.1101 X 25.982593) = 24.306 amu