Simple nature of atoms
(A)Models of atom
The models of atoms include Thomson, Rutherford, Bohr and electron-cloud models. The accepted model of the atom is that the atom consists of a small nucleus with radius 1.2×10-15m and composed of protons and neutrons with a number of electrons located in orbits some distance away.
Thomson’s model of the atom
Source: www.pixabay.com
Electron-cloud model
Source: www.en.wikipedia.org
(B)Atomic structure and properties
Values of A,Z and N for a new nuclides
| Nuclide | Nucleon number A | Proton number Z | Neutron number N=A-Z |
| Hydrogen 11H Carbon 126C Sodium 2311Na Copper 6529Cu Uranium 23892U | 1 12 23 65 238 | 1 6 11 29 92 | 0 6 12 36 146 |
Nuclides and isotopes
Nuclides of an element with the same proton number, Z, but different nucleon number, A, or neutron number, N, are called isotopes of that element.
Uses of radioactive isotopes
Technological uses
- Testing for leaks in pipes.
- Checking blood circulation and blood volume
- Dating of geological specimens using uranium
- Sterilization of foodstuffs
- Treatment of tumors
- Dating archeological specimens using carbon-14
- Checking silver contents in coins
- Liquid flow measurement
Medical uses
- Iodine- 131 with half life of 8days and activity of 8u-curie taken as liquid or capsule
- Technetium- 99 with half life of 6 hours yield gamma-rays of 140eV energy
- Cobalt- 60 sources of up to 104C
- Iodine- 123 is suitable for medical studies
(C) Radioactivity
Radioactivity is the spontaneous decay or disintegration of unstable nuclei of the atom of an element by the emission of alpha particles, beta particles and gamma radiation with the production of energy.
Properties of the emitted particles
| Radiation | Alpha particles | Beta particles | Gamma-rays |
| Nature Velocity Charge Mass Effect of magnetic field Ionizing power Penetrating power | Helium4Nuclei2 He (0.15-0.21) ×108ms-1 +Ze(3.2×10-19C) 6.65×10-27kg Slight deflection Heavy ionization Little penetrating power, it can be stopped by paper | High speed electrons 2.9×108ms-1 -e(-1.6×10-19C) 9.1×10-31kg Strongly deflected Medium ionization Medium penetrating power in several mm of aluminium | Electromagnetic waves of very short wavelength Speed of light 3.0×108ms-1 No charge Negligible Little or no effect Little ionization High penetrating power in several cm of lead. |
Half life a radioactive source
The half life is defined as the time it takes for the activity of the sample to reduce to half of its original value or the time it takes for the number of radioactive nuclei in the sample to reduce to half. T1/2 = 0.693/∆
(D) Nuclear reactions
Nuclear fission
Is a decay process in which an unstable nucleus splits into two parts of comparable mass.
Nuclear fusion
Nuclear fusion is the joining of two or more light nuclei to form a heavy nucleus with the release of enormous energy.
Relative advantage of fusion over fission
- Fusion is easily achieved with hydrogen
- The raw materials required for fusion are more readily available.
- Fusion reaction produces much larger release of nuclear energy
- Fusion processes produce less dangerous by-products.
Past questions
1.when a radioactive substance undergoes a beta decay, its? (Wassce 2008)
A. Mass number decreases by 1
B. Atomic number decreases by 1
C. Mass number increases by 1
D. Atomic number increases by 1
Answer: A
2. Nuclear fission is preferred to nuclear fusion in the generation of energy because? (Wassce 2008)
A. Very high temperatures are required for fusion
B. The raw materials for fusion are easily obtained
C. Energy obtained from fusion is relatively smaller
D. The by-products of fusion are very dangerous
Answer: A
3. Neutrons are used to induce artificial Radioactivity because they? (Wassce 2010)
A. Are energetic
B. Have to charge
C. Have no mass
D. Are ionizing
Answer: A
4. Which of the following particles are termed nucleons in a neutral atom? (Wassce 2013)
A. Protons and neutrons
B. Neutrons and electrons
C. Protons and electrons
D. Protons, electrons and neutrons
Answer: A
5. When an atom loses or gains a charge, it becomes? (Jamb 1991)
- An electron
- An ion
- A neutron
- A proton
Answer: B
6. The difference between X-rays and gamma-rays is that ? (Jamb 2004)
A. X-rays arise from energy changes in the electronic structure of atoms while gamma rays come from the nucleus
B. X-rays are Electromagnetic radiations while gamma-rays are negatively charged radiations
C. X-rays have higher frequencies than gamma-rays
D. X-rays are more penetrating than gamma-rays
Answer: A
7. A radioactive element has a half life of 4 days. The fraction that has decayed in 16Days is? (Jamb 2006)
A. ¾
B. I/16
C. 15/16
D. ¼
Answer: B
Solution: 1>½ >¼ >⅛ >1/16
8. The activity of a radioactive substance depends on? (Jamb 2017)
A. Temperature and purity
B. Purity and age
C. Temperature and age
D. Age, purity and temperature
Answer: C
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