Chapter 11: Atomic and nuclear structure (C2307838)

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1 Atomic spectra

The atomic spectrum includes either:

• Emission line, which are discrete/quantized [atomic] energy levels which cause a photon to emit. There are thus a few colored lines, overlaid on a black background
• Absorption line, which are wavelengths where photons are absorbed, are missing. This thus causes a colored background, overlaid by discrete lines in black

 Formative learning activity Maps to RK11.A What is the atomic spectra?

2 Atomic nucleus

The atomic nucleus is the dense region in the center of an atom, consisting of protons and neutrons. Protons are positively charged, and neutrons are neutrally charged. Protons and neutrons are essentially equal in mass.

Radioactive decay is the process by which an unstable atom spontaneously disintegrates, and emits with it ionizing particles. Half-life is the time taken for a quantity to fall to half its value. Half-life is probabilistic, in that it cannot determine whether a single atom will decay or not, but if analyzed globally, will show that half the substance has decayed (to something else) within that half-life. The half-life is given by $t_{\dfrac{1}{2}}=\dfrac{ln(2)}{\lambda}$, where $ln(2)\approx 0.693$, and $\lambda$ is the decay constant of a substance.

“For example, if the half-life of a substance if 5 years, if we begin with 200g, how much substance will there be in 20 years?” Jamie asked.

“After 5 years there will be 100g, after 10 years there will be 50 grams, after 15 years there will be 25 grams, and after 20 years there will be 12.5 grams,” Mandy answered.

“Another example,” Jamie continued, “if the initial quantity is 100g, the current quantity is 25g, and the time taken is 10 years, what is the half-life?”

“Going from 100 to 50 to 25, there have been 2 half-lives,” Mandy thought, “This means each half life has taken 5 years .”

The different types of radioactive decay include:

Mass-energy equivalence is $E=m.c^2$, where $m$ is the change in mass, and $c$ is the speed of light $3\times 10^8 m/s$, and shows energy content of a mass. It can determine how much energy is created when masses disappear, for example, in the case of electron-positron annihilation. Also, it can determine the nuclear binding energy, which is the energy required to split a nucleus into its components. In a nucleus, it is notable that the mass of the nucleus, is less than the mass of the constituent components. This is due to mass defect/deficit, which is that binding energy is required to keep the components together, thereby requiring some mass to be converted into this energy. This mass defect can be used to find the energy equivalence, which is the nuclear binding energy.

Nuclear fusion is the combining of two light nuclei to form a more stable heavier nucleus. Nuclear fission is the splitting of a heavier nucleus to form more stable lighter nuclei. Nuclei heavier than iron-56tend to undergo fission, and nuclei lighter than iron-56 tend to undergo fusion. Both fusion and fission release energy. Noting that creating bonds releases energy (see ), and breaking bonds requires energy, although it is evident why fusion releases energy, it becomes unclear why fission releases energy. Although fission initially requires energy, the energy released in subsequence is much larger, meaning fission net releases energy.

 Formative learning activity Maps to RK11.B What does the atomic nucleus consist of?

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