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Published byEmerald Nicholson Modified over 5 years ago

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

Isotope notes Dalton was proved incorrect and his theory was modified

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**Protons define the element**

Atoms that have the same number of protons are always atoms of a specific element. Example: Carbon

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

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

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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.

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

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**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.

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**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

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**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

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**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

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**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

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**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 amu.

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**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 amu AND 30.83% copper-65 which has a relative atomic mass of amu

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**Calculating Average Atomic Mass**

To find the average atomic mass, multiply the decimal equivalent of the percent (for example 69.17% = ) of each isotope by the respective relative atomic mass and add the results. ( X amu) + ( X amu) = amu Isotope Copper – 63 Copper – 65 Relative abundance in nature 69.17% 30.83% Relative atomic mass 62.94 amu 64.93 amu

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**Practice Calculating Average Atomic Mass**

Boron – 10 is found 19.9% of the time in nature and has a relative atomic mass of amu Boron – 11 is found 80.1% of the time in nature and has a relative atomic mass of amu Calculate the average atomic mass of Boron

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**Practice Calculating Average Atomic Mass**

Boron (0.199 X ) + (0.801 X ) = amu Isotope Boron – 10 Boron – 11 Relative abundance in nature 19.9% 80.1% Relative atomic mass amu amu

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**Practice Calculating Average Atomic Mass**

Magnesium – 24 is found 78.99% of the time in nature and has a relative atomic mass of amu Magnesium – 25 is found 10.00% of the time in nature and has a relative atomic mass of amu Magnesium – 26 is found 11.01% of the time in nature and has a relative atomic mass of amu Calculate the average atomic mass of Magnesium

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**Practice Calculating Average Atomic Mass**

Magnesium ( X ) + ( X ) + ( X ) = amu Isotope Magnesium – 24 Magnesium – 25 Magnesium – 26 Relative abundance in nature 78.99% 10.00% 11.01% Relative atomic mass amu amu amu

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