Atoms are made up of protons, neutrons, and electrons, with the number of protons defining the element. Different isotopes of an element have the same number of protons but different numbers of neutrons. Radioactive decay occurs spontaneously as nuclei emit alpha, beta, or gamma radiation to become more stable, conserving nuclear particles but decreasing mass.

1. Atomic Theory
 Atoms are building blocks of elements
 Similar atoms in each element
 Different from atoms of other elements
 Two or more different atoms bond in simple ratios to
form compounds
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2. Subatomic Particles
Particle Symbol Charge
Mass
Electron e- 1- 1/1840
Proton p+ + 1
Neutron n 0 1
Charge measure in terms of electronic charge: e=1.6x10^-19
Mass is measure in unified atomic mass: units(u)
1 u is 1/12 of the mass of a carbon-12 atom.
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3. Location of Subatomic Particles
10-13 cm
electrons
protons
neutrons
nucleus
10-8 cm
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4. Atomic Number
Counts the number
of
protons
in an atom
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5. Atomic Symbols
 Show the mass number and atomic number
 Give the symbol of the element
mass number
23 Na sodium-23
atomic number 11
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6. Number of Electrons
 An atom is neutral
 The net charge is zero
 Number of protons = Number of electrons
 Atomic number = Number of electrons
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7. Subatomic Particles in Some Atoms
16 31 65
O P Zn
8 15 30
8 p+ 15 p+ 30 p+
8n 16 n 35 n
8 e- 15 e- 30 e-
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8. Isotopes
 Atoms with the same number of protons, but
different numbers of neutrons.
 Atoms of the same element (same atomic
number) with different mass numbers
Isotopes of chlorine
35Cl 37Cl
17 17
chlorine - 35 chlorine - 37
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9. Learning Check
Naturally occurring carbon consists of three isotopes,
12C, 13C, and 14C. State the number of protons,
neutrons, and electrons in each of these carbon atoms.
12C 13C 14C
6 6 6
#p _______ _______ _______
#n _______ _______ _______
#e _______ _______ _______
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10. Nuclear Density
• Mass of proton m(p)=1.67 x 10^-27 kg
• Radius of proton r = 0.80 x 10^-15 m
• Find the density=

11. Isotopes
Atoms made of protons, neutrons and electrons.
Neutrons+ Protons: Nucleus
• Electron + Proton # influences-> Chemical Properties
• Protons+ Neutron # influences-> Physical Properties
Isotopes
Atoms with same number of protons but different
number of protons are Isotopes.

12. Radioactivity
• A stable nuclei emits radiations. Radiation was
discovered by Henri Becquerel in 1896. These
radiations are not influenced by any outside
conditions such as temperature or pressure.
• It is a spontaneous process, but it occurs at
fixed intervals. Hence a proportion of sample
decay will occur in that time interval.

13. Three particles present in these
Radiations:
• Alpha, Beta And Gamma
Alpha
Beta
Gamma

14. • We know that nuclei consists of protons and
neutrons, if the balance between these two
types of particles is too far to one side, the
nucleus may emit alpha or B radiations.
• Gamma emitted after alpha or B radiation to
release excess energy.

15. A third conserved quantity
• In radioactive decay, both nucleon number A
and proton number Z are conserved.
• On comparison b/w mass before and after
decay, we witness a decrease in mass.
• This is because energy lost caries mass- recall
E=mc^2, so we witness a net decrease in
energy.

16. Properties of Ionizing radiation
• On moving past atoms, alpha or B collide/rub with
them and this as a result causes to knock or drag away
electrons from an atom.
• This process is called Ionization.
• Now, Energy is lost during ionization.
Size: Motion: Ionization Ability:
• Alpha: more mass &charge | slow | Most
• Beta: lighter | faster | Moderate
• Gamma: no charge | Least

17. Lesson Contents
1. Physical properties of a, b and g
2. Penetrating power of a, b and g
3. N v Z graphs
4. Decay laws

18. Alpha Radiation
Alpha particles contain two protons
and two neutrons

19. Alpha Radiation
a has the same constitution as a
helium nucleus
Alpha particles may be written as
They have a double positive charge
and a mass of 4 u

20. Alpha
• Since most effective ionization therefore,
most energy is lost during travel, hence cover
least distance and in a cloud chamber, most
prominent tracks are formed.

21. Detection of all three:

22. Beta-minus Radiation
Beta-minus particles are electrons

23. Beta-minus Radiation
b- is produced when a neutron decays
Beta-minus particles may be written
as
They have a negative charge and a
mass of 1/1800 u

24. Beta-minus Radiation
b- is produced when a neutron decays
+ Energy
The surplus mass is released as
kinetic energy in the b- and as an
antineutrino

25. Gamma Radiation
Gamma rays are a form of electro-
magnetic radiation

26. Gamma Radiation
g release is often associated with a
or b decay
Gamma rays remove energy from
an unstable nucleus

27. Type of Nature of the Nuclear Penetrating power, and what will block it Ionising power - the ability to remove electrons
radiation radiation Symbol (more dense material, more radiation is from atoms to form positive ions
emitted & (higher only) (higher only) absorbed BUT smaller mass or charge of
symbol particle, more penetrating)
a helium nucleus of 2 Low penetration, biggest mass and Very high ionising power, the biggest mass and
protons and 2 charge, stopped by a few cm of air or charge of the three radiation's, the biggest
neutrons, mass = 4, thin sheet of paper 'punch'!
charge = +2
Alpha
high kinetic energy Moderate penetration, 'middle' values of Moderate ionising power, with a smaller mass
electrons, mass = charge and mass, most stopped by a few and charge than the alpha particle
1/1850, charge = -1 mm of metals like aluminium
Beta
very high frequency Very highly penetrating, smallest mass The lowest ionising power of the three, gamma
electromagnetic and charge, most stopped by a thick radiation carries no electric charge and has
radiation, mass = 0, layer of steel or concrete, but even a few virtually no mass, so not much of a 'punch'
charge = 0 cm of dense lead doesn't stop all of it! when colliding with an atom
Gamma

28. Penetrating power

31. Effect of Magnetic Fields

33. Decay laws - alpha
When an isotope emits an a particle
 Its nucleon number decreases by 4
 Its proton number decreases by 2
For example:

34. Decay laws – beta-minus
When an isotope emits a b- particle
 Its nucleon number is unchanged
 Its proton number increases by 1
For example:

35. Decay laws – beta-plus
When an isotope emits a b+ particle
 Its nucleon number is unchanged
 Its proton number decreases by 1
For example:

36. Decay laws
Try writing the nuclear equations for
the decay of these isotopes

37. Decay laws
Answers

38. Decays

39. Randomness and Decay:
Spontaneous:
• Decays not effected by the presence of other
nuclei.
• No chemical reaction, external factors like
pressure and temperature effect the decay.
• Random:
• Impossible to predict the time
• Each nucleus has an equal probability to decay