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2. The mass of an atom is measured in atomic mass units
The mass of an atom is measured in atomic mass units. This is a tiny unit of mass, which cannot usefully be expressed in grams. One gram of hydrogen atoms will contain more than 600,000 million, million, million (6 x 1023) atoms.
Atoms are incredibly small. A speck of dust contains billions of atoms.
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3. This unit is known as the mole.
In order that we can measure chemical substance in sensible units such as grams and kilograms, we need to devise a unit of mass which is relevant to chemical calculations.
This unit is known as the mole.
To help us find the amount of substance in a mole we use a value known as...
Relative Atomic Mass
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4. Relative Atomic Mass (Ar)
In 1961 the carbon-12 atom was chosen as the standard relative to which all other atomic masses were to be measured. Each atom has a RAM (Ar) that describes how many times heavier it is than the mass of 1/12 of a 12C atom.
The relative atomic mass of an atom is measured using a piece of apparatus known as a mass spectrometer.
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5. The Mass Spectrometer Atoms or molecules vaporised here
Ionisation chamber
Magnetic field
Vapour
Ions
Path of heavier ion
Path of lighter ion
Path of ions of intermediate mass
Detector
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6. The relative atomic mass of an element can be found from most versions of the periodic table,
or from reference tables.
There is a standard way of writing the symbol, relative atomic mass, and atomic number of the elements:
Write the Ar (high left) 7 Li
Write the symbol 3
Write the Atomic Number (low left)
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7. Relative Formula Mass (Mr)
The relative formula mass of a compound is the sum of the relative atomic masses of the atoms in a molecule.
The molecule of propane (C3H8) has three Carbon atoms (Ar = 12) and eight Hydrogen atoms (Ar = 1).
Its relative formula mass is therefore (3 x 12) + (8 x 1) = 44
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8. The Mole
We use relative atomic and relative formula mass to measure amounts of chemical reactants.
The relative atomic or formula mass of a substance expressed in grams is the mass of one mole of a substance, e.g...
1. The relative atomic mass of Helium is 4.0 and therefore one mole of Helium atoms has a mass of 4.0g
2. The relative atomic mass of Carbon is 12.0 and therefore one mole of Carbon atoms has a mass of 12.0g
3. The relative atomic mass of Oxygen is 16.0 and therefore one mole of Oxygen atoms has a mass of 16.0g
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9. A Mole of an Element (Molecular Elements)
When you calculate the mass of a mole of an element remember that some elements exist in a molecular form, e.g...
Oxygen (O2), Nitrogen (N2), Hydrogen (H2), Fluorine (F2), Chlorine (Cl2), Bromine (Br2), and Iodine (I2)
N.B. These are all non-metals.
Nitrogen
Oxygen
Fluorine
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10. Oxygen gas exists as a diatomic molecule, O2
The relative atomic mass of Oxygen is 16.0
One mole of Oxygen atoms has a mass of 16.0g
Therefore one mole of Oxygen gas has a mass of 32.0g
The Noble gases (including Helium) all exist as individual atoms and therefore one mole of Helium atoms has the same mass as one mole of Helium gas.
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11. A Mole of a Compound
We use relative formula mass to measure amounts of a compound, e.g...
1. The relative formula mass of methane (CH4) is (12.0 x 1) + (1.0 x 4) = 16.0 and therefore one mole of methane has a mass of 16.0g
2. The relative formula mass of propane (C3H8) is (12.0 x 3) + (1.0 x 8) = 44.0 and therefore one mole of propane has a mass of 44.0g
3. The relative formula mass of carbon monoxide (CO) is (12.0 x 1) + (16.0 x 1) = 28.0 and therefore one mole of carbon monoxide has a mass of 28.0g
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12. Avogadro’s Constant (L)
Experimentation has led to the discovery of the number of atoms in one mole of an element.
This number can be accurately calculated from X-ray diffraction studies of crystals which give us an accurate measurement of the spacing of ions or atoms in crystals.
The number of particles in one mole is called Avogadro’s Constant in honour of the famous Italian scientist Amadeo Avogadro.
Avogadro’s Constant is x 1023
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13. This image shows only about 600 atoms!
Avogadro’s Constant (sometimes called Avogadro’s number) is x 1023.
or……602,000,000,000,000,000,000,000.
or……six hundred and two thousand, million, million, million.
This image shows only about 600 atoms!
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14. Isotopes
The mass of an atom is mainly in the nucleus of the atom. Protons and neutrons have an approximately equal mass of one atomic mass unit. Therefore the mass of an atom is equal to the sum of the number of protons and neutrons. This number is called the mass number of the atom.
Some Carbon atoms have six protons and six neutrons, and therefore have a total mass of twelve atomic mass units. The mass number of these Carbon atoms is 12.
Some Carbon atoms have six protons and eight neutrons, and therefore have a total mass of fourteen atomic mass units. The mass number of these Carbon atoms is 14.
These two different types of carbon atoms are called isotopes of Carbon and are known as Carbon-12 (12C) and Carbon-14 (14C).
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15. The two common isotopes of Carbon can be represented as:12 6 C 14 6
Natural Carbon is more than 99% 12C, and less than 1% 14C.
N.B. Natural Carbon also contains small amounts of 13C.
How many neutrons are there in one atom of each of the Carbon isotopes?
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16. The two common isotopes of Chlorine can be represented as:Cl 35 17 Cl 37 17
How many neutrons are there in one atom of each of the Chlorine isotopes?
Natural Chlorine is approximately 75% 35Cl, and 25% 37Cl.
We can use these figures to calculate the relative atomic mass (Ar) of a sample of natural Chlorine.
The two isotopes of Chlorine have identical chemical properties. The major factor which determines the chemical properties of an element is the electronic structure of the atom.
Each Chlorine isotope has seventeen electrons, and therefore the two isotopes behave chemically in the same way.
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17. Natural Chlorine is approximately 75% 35Cl, and 25% 37Cl.
(This ratio is found from mass spectroscopy.)
The ratio of 35Cl to 37Cl is therefore 3:1
Hence Ar = ((3 x 35) + (1 x 37)) / 4 = 35.5
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18. Relative Formula Masses
Use the following information to find the relative formula masses of water, carbon dioxide and ammonia.
Ar Nitrogen is 14.0
Ar Carbon is 12.0
Ar Oxygen is 16.0
Ar Hydrogen is 1.0
(Click for answers)
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19. A Mole of an Element
Use the following information to find the mass of 3 moles of Bromine liquid and the mass of 3 moles of Bromine atoms.
1. Bromine liquid exists as diatomic molecules with the formula Br2
2. The relative atomic mass of Bromine is 79.9
(Click for answers)
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