What is the approximate radius of an atom?
1 x 10⁻¹⁰ m.
Describe the basic structure of an atom
A positively charged nucleus (protons+neutrons) surrounded by negatively charged electrons in energy levels (shells)
How does the size of a nucleus compare to the size of an atom?
The nucleus radius is less than 1/10,000 of an atoms radius
Where is the mass of an atom most concentrated?
In the nucleus
How can electrons arrangements in an atom change?
Absorbing EM radiation - electrons gain energy. move to higher levels. (further from nucleus)
Emitting EM radiation - electrons (lose energy) move to lower energy levels (closer to the nucleus)
EM = electromagnetic radiation
Why does the nucleus make the atom positively charged overall?
The nucleus contains protons (positive charge), while neutrons are neutral.
What does the atomic number of an atom tell you?
Number of protons in the nucleus
What does the mass number of an atom tell you?
Total number of protons + neutrons in the nucleus.
What are isotopes?
Atoms of the same element with different amount of neutrons.
How do atoms become positive ions?
Losing one or more outer electrons, leaving more protons than electrons.
What was the plum pudding model of the atom?
A positive sphere of positive charge with negative electrons embedded.
What experiment overturned the plum pudding ?
Rutherfords alpha particle scattering experiment.
(Gold foil experiment)
What were the results of the alpha scattering experiment?
Most alpha particles passed straight through- Atom mostly empty space.
Some deflected at small angles - Nuclues positively charged
Few bounced back - nucleus is tiny and contains most of atoms mass
Why did the scattering. experiment change the atomic model?
Showed that the atom was small, dense and positively charged nucleus with electrons around it. The nuclear model.
What was Bohrs contribution to the nuclear model?
Suggested electrons orbit the nucleus at fixed distances ( energy levels), explaining why they don’t spiral.
What was James Chadwick’s contribution to the atomic model?
Discovered the neutrons, explains missing mass in nuclei not accounted for by protons alone.
What happens when a nucleus is unstable and what is this process called?
Radiation is emitted randomly to become more stable. This process is called radioactive decay
What is activity in radioactivity, and what is its unit?
Activity is the rate at which unstable nuclei decay.
Measured in becquerels (bq)
1 Bq = 1 decay per second
What is count-rate and how is it measured?
Count-Rate is the number of decays detected per second.
Usually measured by a Geiger-Muller connected to a counter
What is an alpha particle and what is its symbol?
An alpha particle (α) is a helium nucleus consisting of 2 protons and 2 neutrons.
What is a beta particle and how is it formed?
A beta particle (β⁻) is a fast moving electron ejected from the nucleus.
It is formed when a neutron changes into a proton and an electron
What is gamma ray and how does it differ from alpha and beta ?
Gamma radiation (γ) is electromagnetic radiation from the nucleus.
It has no charge or mass but can still transfer energy.
What is a neutron emission?
The release of a neutron from an unstable nucleus often after nuclear reaction or fission.
Compare alpha, beta and gamma in terms od penetration, range in air and ionising power.
Alpha ; Weak penetration (stopped by paper), range - few cm in air, very strongly ionising.
Beta ; Medium penetration (stopped by aluminium - 5mm), range ~ 1–2 m in air . Moderately ionising
Gamma : Very strong penetration (needs thick lead/concrete) , Range is virtually infinite in air, weakly ionising.
