Atomic Structure Flashcards
Define an atom
An atom is the smallest unit quantity of an element that can take part in a chemical reaction.
State and explain the observations and conclusions of Rutherford’s experiment on the helium atom
Observations
- Most of the alpha particle passed through the foil undeflected.
- A few alpha particles were deflected through various angles.
- Extremely very few alpha particles were deflected back through 180o or bounced back.
Conclusions
- Most of the space inside the atom is empty and this explains why most of the α-particles passed through the gold foil without getting deflected.
- Atoms contain electrons that are distributed around the nucleus, and occupy most of the volume.
- The positive charge of the atom is concentrated in a small fraction of the atom’s nucleus and this explains the very small fraction of alpha particles that bounced back.
Define Atomic number (symbol Z) of an element
This is the number of protons contained in the nucleus of the atom.
What is Mass number or Nucleon number (symbol A)?
This is the sum of protons and neutrons in the nucleus of an atom.
What is a nuclide?
This is any nuclear species with specified atomic number and mass number.
What are Isotopes?
Nuclides of the same element having the same atomic number but different mass numbers due to their difference in the number of neutrons.
Define Isotopy
This is the existence of nuclides of the same element having the same atomic number but different mass numbers due to their difference in the number of neutrons.
What are Isobars?
These are atoms of different elements which have the same mass number but different atomic numbers.
What is Relative isotopic mass?
This is the ratio of the mass of a single isotope of an element to one twelfth of the mass of a nuclide of C-12 isotope.
Define Nuclear stability
This is the measure of the extent to which the nucleus of an atom remains un-dissociated.
What are the factors affecting nuclear stability?
a) Binding energy per nucleon
- Binding energy is the minimum quantity of energy required to completely separate the nucleus of an atom into its free isolated protons and neutrons.
- The nuclear stability increases as the binding energy per nucleon increases.
- For lighter isotopes, the binding energy per nucleon increases with increase in the atomic number whereas for heavy isotopes, the binding energy per nucleon decreases with increasing atomic number.
- Therefore very light isotopes and very heavy isotopes tend to be unstable because their low binding energy per nucleon.
b) Neutron to proton ratio
- Stability of nucleus also depends on the exchange of energy between neutrons and protons in the nucleus which in turn depends on the neutrons to proton ratio.
- For lighter atoms, the n/p ratio is averagely equal to 1 and they are relatively stable.
- As the atomic number increases, the n/p ratio has to increase for the nucleus to be stable due to increased repulsive forces as the number of protons increases. The stability band ends with 209Bi.
c) Even or odd number of protons or neutrons
- The highest number of stable nuclides contains even numbers of both protons and neutrons, followed by those in which either the protons or neutrons are even or odd numbers and least number of stable nuclides involves odd numbers of both protons and neutrons.
How do unstable nuclides gain stability ?
Alpha, α – emission
- Alpha emission occurs predominantly when the atomic number of the unstable isotope is above 82.
- During the process the mass number reduces by 4 while the number protons reduces by 2.
Beta, β - emission
- Isotopes that occur above the stability band decay towards the stability band by β-emission.
- During the process a neutron breaks down into a proton and an electron. The electron is emitted while the proton is retained by the nucleus.
- So the number of protons increases by 1
Electron capture or positron emission (β+- emission)
- Radio isotopes below the stability band decay towards the stability band by either electron capture or positron emission (β+- emission)
a) Positron (𝟎𝟏𝒆) emission (for n/p too large)
- During the process a proton breaks down into a neutron and a positron. The positron is emitted while the neutron is retained by the nucleus.
- So the number of neutrons increases by 1.
b) Electron capture (for n/p too small)
- During the process a proton captures an electron forming a neutron which is retained by the nucleus. So the number of neutrons increases by 1.
Define Radioactivity
This is the spontaneous disintegration (decay) of unstable nuclei by emission of alpha, beta or gamma rays to form stable nuclei.
Describe an experiment to separate of radiation from radioactive material by an electric field (Experiment to show the effect of an electric field on radiation)
- α - particles are deflected away from the positive plate but towards the negative plate. This shows that α – particles are positively charged (α – particles are nuclei of Helium ions, 42He2+)
- β – particles are deflected away from the negative plate but towards the positive plate. This shows that β–particles are negatively charged (β – particles are electrons).
- The β – particles are deflected to a much greater extent than α – particles. This indicates that β – particles are much lighter than α – particles.
- γ – rays are not affected by electric and magnetic fields. This indicates they are uncharged. Gamma rays are electromagnetic waves similar to light rays and X-rays but with much shorter wavelength and extremely high penetrating power.
What are the differences between nuclear reactions and chemical reactions?
- Nuclear reactions occur in the nucleus of an atom and they involve protons and/or neutrons unlike chemical reactions that involve valence electrons.
- Energy changes in nuclear reactions are usually much greater than those in chemical reactions.
- Nuclear reactions are not affected by physical factors such as pressure, temperature while chemical reactions are affected by physical factors.
