atomic structure & radioactivity Flashcards
What is the radius of an atom?
- 1 x 10⁻¹⁰
Describe the basic structure of an atom
- a positively charged nucleus composed of both protons and neutrons surrounded by negatively charged electrons
Where is most of the mass of an atom concentrated?
- in the nucleus
What is the radius of a nucleus?
- less than 1/10000 of the radius of an atom
How are electrons arranged?
- the electrons are arranged at different distances from the nucleus (different energy levels)
How can electron arrangements change?
- the absorption of electromagnetic radiation; they move further from the nucleus (a higher energy level)
- the emission of electromagnetic radiation; they move closer to the nucleus (a lower energy level)
- if an electron gains enough energy, it can leave the atom to form a positive ion
Why do atoms have no overall electric charge?
- the number of electrons is equal to the number of protons in the nucleus
- therefore they are neutral atoms
Define atomic number
- the number of protons in an atom of an element
What do all atoms of a particular element have in common?
- they have the same number of protons
Define mass number
- the total number of protons and neutrons in an atom
Define isotopes
- atoms of the same element that have the same number of protons but a different number of neutrons
When do atoms turn into positive ions?
- if they lose one or more outer electrons
What happens when new experimental evidence of the atom is released?
- this may lead to a scientific model being changed or replaced
What did scientists believe about the atom before the discovery of the electron?
- atoms were thought to be tiny spheres that could not be divided
Who discovered the electron and what did this lead to?
- JJ Thomson - the plum pudding model of the atom
Describe the plum pudding model
- it suggested that the atom is a ball of positive charge with negative electrons embedded in it
Describe what happened during the alpha particle scattering experiment
- Most particles when straight through
- Some particles were slightly deflected
- Few particles were deflected by >90 degrees
What did the results from the alpha particle scattering experiment lead to?
- the conclusion that the mass of an atom was concentrated at the centre (nucleus), with most of the atom being empty space
- and that the nucleus was charged.
- this nuclear model replaced the plum pudding model
What was wrong with Rutherford’s nuclear model?
- the electrons in the cloud close to the nucleus would get attracted and cause the atoms to collapse
How did Niels Bohr adapt the nuclear model?
- he adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances.
- the theoretical calculations of Bohr agreed with experimental observations
What did later experiments of the atom lead to?
- the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles,
- each particle having the same amount of positive charge;
- these particles are known as protons
What did James Chadwick discover?
- he provided the evidence to show the existence of neutrons within the nucleus.
- this was about 20 years after the nucleus became an accepted scientific idea
Describe radioactive decay
- some atomic nuclei are unstable
- the nuclei gives out radiation as it changes to become more stable
- this is a random process called radioactive decay
Define activity (give its units)
- the rate at which a source of unstable nuclei decays
- measured in becquerel (Bq)