Topic 1 - Key Concepts in Chemistry
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John Dalton’s theory about the atom
• Atoms cannot be created, divided or destroyed. • Atoms of the same element are exactly the same and atoms of different elements are different. • Atoms join with other atoms to make new substances
What discovery caused Dalton’s model to change?
The discovery of sub-atomic particles
Who discovered the electron?
JJ Thomson
What did JJ Thomson discover?
The electron
How was the electron discovered?
• Thomson experimented with a cathode ray tube. • The beam moved towards the positively charged plate so he knew that the particles must have a negative charge.
What model was proposed by JJ Thomson?
• Plum pudding model • Negatively charged electrons scattered through a positively charged metal
Who discovered the Plum Pudding Model?
JJ Thomson
What was the plum pudding model?
Negatively charged electrons scattered through a positively charged metal
Who conducted the gold foil experiment?
JJ Thomson
What was the gold foil experiment?
• JJ Thomson shot a beam of positively charged particles at sheet of gold foil. • Most particles passed straight through suggesting atoms are mostly empty space. • A few particles were deflected and a few bounced back showing there must be a tiny, dense, positively charged nucleus.
Rutherford’s model of the atom:
• Mass is concentrated in the central nucleus • The atom is mostly empty space. • Electrons travel in random paths around the nucleus
Structure of the atom
• Small central nucleus made up of protons and neutrons. • Electrons orbit the nucleus in shells.
The radius of the nucleus
1x10^-14 metres. This is 1/10,000 of the atomic radius.
Relative masses of sub-atomic particles
Proton - 1 Neutron - 1 Electron - 1/1840
Relative charges -
Proton: +1 Neutron: 0 Electron: -1
Why do atoms contain equal numbers of protons and electrons?
• Atoms are stable with no overall charge. • Protons are positively charged and electrons are negatively charged so for charges to balance the number of protons and electrons must be equal.
Where is the mass of an atom concentrated?
In the nucleus
Atomic number represents
Number of protons.
Mass number -
Number of protons + number of neutrons
Fill in the blank: ‘Atoms of the same element have the same number of ______ in the nucleus and this is unique to that element’
Protons
What’s an isotope?
• Isotopes are atoms with the same number of protons (so they are the same element) but a different number of neutrons. • Isotopes of an element have the same atomic number but different mass numbers.
Exam Question - Boron has the atomic number 5 and the mass number 11. How many protons, electrons and neutrons does Boron have?
5 Protons 6 Neutrons 5 Electrons
Why is relative atomic mass not always whole?
• Different isotopes of the same element have different mass numbers. • The relative atomic mass is an average of the masses of all these isotopes.
What two values would be required to calculate the relative atomic mass of chlorine?
Mass numbers and relative abundances of all the isotopes of chlorine.