The structure of the atom IB DP Physics Study Notes - 2025 Syllabus
The structure of the atom IB DP Physics Study Notes
The structure of the atom IB DP Physics Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Physics syllabus with Students should understand
that energy is released in spontaneous and neutron-induced fission
the role of chain reactions in nuclear fission reactions
the role of control rods, moderators, heat exchangers and shielding in a nuclear power plant
the properties of the products of nuclear fission and their management.
Standard level and higher level: 4 hours
Additional higher level: There is no additional higher level content.
- IB DP Physics 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Physics 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Physics 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
- IB DP Physics 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
Rutherford scattering
- In 1897 British physicist J.J. Thomson discovered the electron, and went on to propose a “plum pudding” model of the atom in which all of the electrons were embedded in a spherical positive charge the size of the atom.
- In the next slides we will disprove this model…
- ∙In 1911 British physicist Ernest Rutherford conducted experiments on the structure of the atom by sending alpha particles through gold leaf.
- ∙Gold leaf is like tin foil, but it can be made much thinner so that the alpha particles only travel through a thin layer of atoms.
FYI
- An alpha (α) particle is a double-positively charged particle emitted by radioactive materials such as uranium.
- Rutherford proposed that alpha particles would travel more or less straight through the atom without deflection if Thomson’s “plum pudding” model was correct:
FYI
∙Instead of observing minimal scattering as predicted by the “plum pudding” model, Rutherford observed the scattering as shown on the next slide:
Here we see that the deflections are much more scattered…
∙Rutherford proposed that all of the positive charge of the atom was located in the center, and he coined the term nucleus for this location.
FYI
∙IBO requires you to qualitatively understand the Geiger-Marsden scattering experiment.
∙Only by assuming a concentration of positive charge at the center of the atom, as opposed to “spread out” as in the plum pudding model, could Rutherford and his team explain the results of the experiment.
The nuclear radius
Now let’s calculate a ballpark figure for the nuclear radius by firing an alpha particle ($q = +2e$) at a nucleus ($Q = +Ze$). Assume the α begins far enough away that there is no $E_P$ between it and the nucleus.
$E_0 = E_{K0} + E_{P0} = E_{K\alpha}$
But as the α approaches the nucleus, repulsion will occur, and $E_P = \frac{kQq}{r}$ will increase, slowing it down.
In fact, at closest approach $R_0$, the α will momentarily stop before reversing direction.
Thus at the point of closest approach $E_K = 0$ and
$E = E_K + E_P = \frac{kQq}{R_0} = \frac{kZe(2e)}{R_0} = \frac{2Zke^2}{R_0}$.
∙Though its proof is beyond the scope of this course, the physical radius of the nucleus also depends on its neutrons, which contribute no charge. Thus the atomic mass number A is used, and here is the result:
Isotopes
∙ Recall the mass spectrometer, in which an atom is stripped of its electrons and accelerated through a voltage into a magnetic field.
∙ Scientists discovered that hydrogen nuclei had three different masses.
∙ Since the charge of the hydrogen nucleus is \( e \), scientists postulated the existence of a neutral particle called the neutron, which added mass without charge.
∙ The proton and neutron are called nucleons.
∙ For the element hydrogen, it was found that its nucleus exists in three forms.
∙ A set of nuclei for a single element having different numbers of neutrons is called isotopes.
∙ A particular isotope of an element is called a species or a nuclide.
∙ An element’s chemistry is determined by the number of electrons surrounding it.
∙ The number of electrons an element has is determined by the number of protons in that element’s nucleus.
∙ Therefore, it follows that isotopes of an element have the same chemical properties.
∙ For example, there is water, made of hydrogen \( H \) and oxygen \( O \), with the molecular structure \( H_2O \).
∙ But there is also heavy water, made of deuterium \( D \) and oxygen \( O \), with the molecular formula \( D_2O \).
∙ Both have exactly the same chemical properties.
∙ However, heavy water is slightly denser than regular water.
∙ A species or nuclide of an element is described by three integers:
– The nucleon number \( A \) is the total number of protons and neutrons in the nucleus.
– The proton number \( Z \) is the number of protons in the nucleus. It is also known as the atomic number.
– The neutron number \( N \) is the number of neutrons in the nucleus.
∙ It follows that the relationship between all three numbers is: 
In nuclear physics, you need to be able to distinguish between different isotopes.
Fundamental Forces and Their Properties
∙ Given that a nucleus is roughly \( 10^{-15} \, \text{m} \) in diameter, it should be clear that the Coulomb repulsion between protons within the nucleus must be enormous.
∙ Since most nuclei do NOT spew out their protons, there must be a nucleon force that acts within the confines of the nucleus to overcome the Coulomb force.
∙ We call this nucleon force the strong force.
In summary, the strong force:
1. Counters the Coulomb force to prevent nuclear decay and therefore must be very strong.
2. Is very short-range, since protons located far enough apart do indeed repel.
