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