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

Protons define the element Atoms that have the same number of protons are 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 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 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 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-65 which has a relative atomic mass of 64.93 amu

Calculating Average Atomic Mass To find the average atomic 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 Isotope Copper – 63 Copper – 65 Relative abundance in nature 69.17% 30.83% Relative atomic mass 62.94 amu 64.93 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 Isotope Boron – 10 Boron – 11 Relative abundance in nature 19.9% 80.1% Relative atomic mass 10.013 amu 11.009 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 Isotope Magnesium – 24 Magnesium – 25 Magnesium – 26 Relative abundance in nature 78.99% 10.00% 11.01% Relative atomic mass 23.985042 amu 24.985837 amu 25.982593 amu