Atomic Structure and Radioactivity Flashcards
Paper 1 - P3
What is the radius of an atom?
1 x 10-¹⁰
Why are some energy levels have higher energy than others?
Energy levels which are further from the nucleus are at a higher energy than those which are closer to the nucleus.
How can atoms move between energy levels?
-If an atom absorbs electromagnetic radiation an election can move from a lower energy level to a higher energy level.
-Now the atom can emit electromagnetic radiation and the electron returns back to the lower energy level.
What is an isotope?
They are atoms of the same element that have different numbers of neutrons.
What did the Ancient Greeks think of atoms?
-That everything is made of atoms
-Atoms are tiny spheres which cannot be divided
What did Scientists discover about atoms in 1897?
-They discovered that atoms contain tiny negative particles called electrons.
-This showed that atoms are not sphere that cannot be divided.
-Atoms have an internal structure.
What did the plum-pudding model show for the structure of atoms?
An atom is a ball of positive charge with negative electrons embedded in it.
What happened in the Alpha-Scattering experiment?
-Scientists took a piece of gold foil as it is a few atoms thick.
-Then they fired tiny alpha particles at the gold foil.
-They first saw that most alpha particles passed straight through the foil without changing direction.
-And sometimes an alpha particle was deflected and it changed direction as it passed through the gold foil.
-And other times an alpha particle would simply bounce of the gold foil.
What did scientists find out during the Alpha-Scattering experiment?
-That most of the alpha particles went straight through the gold foil therefore meaning atoms are mainly empty space.
-Some of the alpha particles were deflected therefore the centre of an atoms must have a positive charge and that repelled the alpha particles.
-Sometimes an alpha particle bounced straight back therefore the mass of the atoms must be concentrated in the centre called the nucleus.
What did the Plum-pudding model get replaced by?
The nuclear model
What did the scientist Niels Bohr propose?
-That electrons orbit the nucleus at specific distances.
-Bohr’s work agreed with the results of experiments by other scientists.
-We now call the ‘orbits’ energy levels or shells.
Who discovered that the nucleus contained neutrons?
James Chadwick
What is radioactive decay?
When some isotopes have an unstable nucleus and to become stable the nucleus gives out radiation.
What is it meant by the activity?
The rate at which a source of unstable nuclei decay
How do we measure the activity of a radioactivity source?
We can use a Geiger-Muller tube
What is the count-rate?
The number of decays recorded each second by a detector
What is an Alpha particle?
2 neutrons and 2 protons with a helium nucleus
What is a Beta particle?
An electron which is ejected from the nucleus at very high speed.
How is a Beta particle formed?
Inside a nucleus when a neutron changes into a proton and an electron.
What is Gamma radiation?
A type of electromagnetic radiation from the nucleus
What are the properties of alpha particles?
-Alpha particles are large
-They can travel around 5cm in air before they collide with air particles and stop
-Stopped by a single sheet of paper
-Very strongly ionising
What are the properties of beta particles?
-Beta particles can travel around 15cm in air before stopping
-Stopped by a few millimetres of aluminium
-Quite strongly ionising
What are the properties of gamma radiation?
-Gamma radiation travels several meters in air before stopping
-Several centimetres of lead
-Weakly ionising
What happens to the atomic number in alpha decay?
It decreases by 2
