4. Atomic Structure Flashcards
What is the radius of an atom?
The radius of an atom is about 1 × 10-10 metres
Describe the basic structure of an atom
The basic structure of an atom is 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?
Most of the mass of an atom is concentrated in the nucleus
Compare the radius of a nucleus to the radius of the atom
The radius of a nucleus is less than 1/10 000 (10,000 times smaller) of the radius of an atom
Describe the arrangement of electrons
Electrons are arranged at different distances from the nucleus (different energy levels)
Give 2 facts about energy levels
- Each energy level is at a fixed distance from the nucleus
- Energy levels that are further away from the nucleus are at a higher energy than those which are closer to the nucleus
Explain the 2 ways an atom’s electron arrangement can be changed
An atom’s electron arrangement can be changed through:
- Absorbing electromagnetic radiation - where electrons move further away from the nucleus to a higher energy level
- Emmitting electromagnetic radiation - where electrons move closer to the nucleus to a lower energy level
Describe and explain the overall charge of atoms
Atoms have no overall charge as the number of electrons is equal to the number of protons in the nucleus
What is the atomic number?
The atomic number is the number of protons in an atom
What is the mass number?
The mass number is the total number of protons and neutrons in an atom
What are isotopes?
Isotopes are atoms of the same element, which have the same number of protons but different number of neutrons, so they have different masses
How do atoms become positive ions?
Atoms become positive ions when they lose one or more of their outer electrons
What may lead to a scientific model being changed or replaced?
New expermental evidence that doesn’t agree with the existing theory
What were atoms thought to be before the discovery of the electron?
Before the discovery of the electron, Dalton thought that were atoms tiny spheres that could not be divided
What happened in 1897?
In 1897, J.J Thomson discovered the electron which led to his proposal of the plum pudding model in 1904
What did the plum pudding model suggest?
The plum pudding model suggested the the atom was a ball of positive charge with negative electrons embedded in it
How was the plum pudding model disproved?
The plum pudding model was disproved by the results from Rutherford’s alpha particle scattering experiment
Describe what happened in the alpha particle scattering experiment?
- A beam of positively charged alpha particles were aimed at a thin gold foil
- Most of the alpha particles passed straight through the gold foil without changing direction
- Some were slightly changed direction but passed through the foil
- Very few particles bounced back off the gold foil
What were the results of the alpha particle scattering experiment?
Rutherford concluded that:
- Most of the atom consisted of empty space as most alpha particles went straight through the gold foil
- The nucleus must have a positive charge as some alpha particles were deflected
- The mass of the atom must be concentrated at the centre as few numbers of alpha particles rebounded straight back
This became known as Rutherford’s nuclear model
What did Rutherford do after the alpha particle scattering experiment?
Rutherford created the nuclear model after the alpha particle scattering experiment, which replaced the Plum Pudding Model
What did Niels Bohr do?
Niel Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus in shells that are at specific distances
Why was Bohr’s model accepted?
The theoretical calculations of Bohr agreed with experimental observations.
What did James Chadwick do?
20 years after the nucleus became an accepted scientific
idea and the nuclear model, James Chadwick demonstrated the existence of neutrons
Explain the process of radioactive decay
- Some nuclei of atoms are unstable becaue they have too many or too few neutrons
- The nucleus gives out radiation as it changes to become more stable
- This is a random process
What are the 4 ways nuclear radiation can be emitted?
Nuclear radiation can be emitted by:
- An Alpha particle (α)
- A Beta particle (β)
- A Gamma ray (γ)
- A neutron (n)
Describe an alpha particle
An alpha particle consists of two neutrons and two protons, it is the same as a helium nucleus
Describe a beta particle
A beta particle is a high speed electron ejected from the nucleus as a neutron turns into a proton
What is a gamma ray?
A gamma ray is electromagnetic radiation from the nucleus
What is ionising power?
Ionising power is the ability of radiation to change an uncharged atom into a charged ion
What is activity?
Activity is the rate at which a source of unstable nuclei decays
What is activity measured in?
Activity is measured in becquerel (Bq)
What is count rate?
Count-rate is the number of decays recorded each second by a
detector (eg Geiger-Muller tube)
Complete it and state whether it is alpha or beta decay
Alpha decay
Complete it and state whether it is alpha or beta decay
What effect does alpha decay have on the mass and charge of the nucleus?
Alpha decay causes both the mass and charge of the nucleus to decrease
What effect does beta decay have on the mass and charge of the nucleus?
Beta decay does not cause the mass of the nucleus to change but does cause the charge of the nucleus to increase
What effect do gamma rays have on the mass and charge of the nucleus?
The emission of a gamma ray does not cause the mass or the
charge of the nucleus to change
What is half-life?
The half-life of a radioactive isotope is the time it takes for the number of nuclei of the isotope in a sample to halve, or the time it
takes for the count rate (or activity) from a sample containing the
isotope to fall to half its initial level.
Net decline
NOT SURE
What is radioactive contamination?
Radioactive contamination is the unwanted presence of materials
containing radioactive atoms on other materials
What is the hazard caused by radioactive contamination?
What is the level of the hazard caused by radioactive contamination?
The decay of radioactive atoms, which emit radiation
The level of danger / hazard is determined by the type of radiation emitted
What is irradiation?
Irradiation is the process of exposing an object to nuclear radiation.
The irradiated object does not become radioactive
What precautions should be taken to protect from alpha radiation
Gloves can protect against alpha radiation
Lead clothing can protect from beta and gamma radiation
Lead shielding can also be used
Why it is important for the findings of studies into the effects of radiation on humans to be published and shared with other scientists?
It is important for the findings of studies into the effects of radiation on humans to be published and shared with other scientists so that the findings can be checked by peer review
State one fact about background radiation
Background radiation is around us all of the time
Where does background radiation come from?
Background radiation comes from:
- Natural sources such as rocks and cosmic rays from space
- Man-made sources such as the fallout from nuclear weapons
testing and nuclear accidents
What factors affect the level of background radiation and radiation dose?
The level of background radiation and radiation dose may be
affected by occupation and/or location
