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ParticleChargeMass (u) Neutron Proton Electron
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Atoms
-Basics on atomic structure-Isotopes
-Isotopic Mass & Relative Atomic Mass
Revision from Year 11
• The Bohr model of the atom.
• It’s not perfect, but it is a simple model.
Particle Charge Mass (u)
Neutron
Proton
Electron
Mass and charge
• With your partner, try to describe the distribution of mass in an atom (i.e where is it and why?)
• Do the same for charge, where is the charge distributed within an atom?
– A diagram may help.
The Atomic number.
• Each element has a unique atomic number. This number tells us two things:
1. How many protons the atom has2. How many electrons the atom has.
Why are these numbers the same?
Atomic Mass.
• Try not to confuse the atomic MASS and the atomic NUMBER.
• The Atomic Mass tells us how much stuff is in the atom. How many PROTONS and NEUTRONS it has.
• Why might we ignore electrons when we talk about atomic mass?
Atomic number & atomic mass
• Atomic number– Found in the
periodic table– Table arranged by
increasing atomic number
– Tells us the number of protons in the nucleus.
• Atomic Mass– Found in the
periodic table– Tells us the of
protons AND neutrons in the nucleus
– A larger number than the atomic number.
We’re pretty lazy
• We have a short way of writing the atomic number and the mass number for an element:
• Which number is atomic number, which is the mass number?
Isotopes.
• Isotopes are elements of the same type but with different mass.
• Chlorine has two stable isotopes.• How many protons and neutrons does each
have? Try the others.35Cl 131I 12C
37Cl 127I 13C
Carbon 12. It’s important.
• 12C is used as a standard for the measurement of relative mass.
• Atomic masses are measured using a unit called unified atomic mass unit.
• This is a tiny number- 1 u is 1.66 x 10-27
• Mass of carbon atom- 12 u• So one twelfth of an atom of carbon is 1 u
Relative isotopic mass
• The Relative Isotopic Mass is the mass of an atom of an isotope compared to one twelfth of the mass of an atom of Carbon-12.
Relative isotopic mass
• The Relative Isotopic Mass is the mass of an atom of an isotope compared to one twelfth of the mass of an atom of Carbon-12.
• The Relative Isotopic Mass is the mass of an atom of an isotope compared to one twelfth of the mass of an atom of Carbon-12.
Relative atomic mass. Ar
• In a lump of carbon we have 12C and 13C.– These weigh a different amount.– We use a ‘weighted mean mass’ to account for
this.– This depends on:
• The percentage abundance of the isotope• The relative mass of the isotope.
Ar Definition
• The relative atomic mass is the weighted mean mass of an atom of an element compared to one-twelfth of the mass of an atom of carbon-12
Ar Definition
• The relative atomic mass is the weighted mean mass of an atom of an element compared to one-twelfth of the mass of an atom of carbon-12
• The relative atomic mass is the weighted mean mass of an atom of an element compared to one-twelfth of the mass of an atom of carbon-12
Ar
• For example:
• Boron contains 19.77% 10B and 80.23% 11B
• Ar(B)= (19.77/100 x 10) + (80.23/100 x 11) = 1.977 + 8.8253 = 10.8023 u
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