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In the fascinating world of nuclear chemistry, alpha decay is a significant process. It involves the emission of an alpha particle from an unstable nucleus. An alpha particle is a compact little package containing 2 protons and 2 neutrons. Together, these account for a total of 4 mass units. When a nucleus undergoes alpha decay, it loses these 4 units.
This means the atomic number decreases by 2, as it loses 2 protons, and the mass number decreases by 4. For example, when uranium-235 ( ^{235}_{92}U) decays via alpha emission, it transforms into a new element with an atomic number of 90 and a mass number of 231.

• Atomic Number Change: Decreases by 2
• Mass Number Change: Decreases by 4

This results in thorium-231 ( ^{231}_{90}Th), moving it down the periodic table and altering its chemical properties.

Beta decay is another common process encountered in nuclear chemistry, offering a different transformation for unstable nuclei. In beta decay, a neutron in the nucleus is converted into a proton, and a beta particle, which is essentially an electron, is emitted.
As a result, the atomic number increases by 1 because a neutron turns into a proton, but the mass number remains constant since the proton and neutron have approximately the same mass.

• Atomic Number Change: Increases by 1
• Mass Number Change: Remains the same

So, when thorium-231 ( ^{231}_{90}Th) undergoes beta decay, it becomes protactinium-231 ( ^{231}_{91}Pa). This transformation keeps the mass fixed but shifts the nucleus's identity to the next element on the periodic table.

The uranium-235 decay series is an intriguing chain of transformations that occur as this radioactive element seeks stability. Beginning with uranium-235, each step involves either alpha or beta decay, gradually transforming into different elements until a stable compound is formed.
The sequence typically begins with the emission of an alpha particle, leading to the creation of thorium-231. As discussed, thorium-231 then undergoes beta decay to transform into protactinium-231. This process repeats through multiple steps:

• Starts With: Uranium-235 ( ^{235}_{92}U)
• Alpha Emission: Forms Thorium-231 ( ^{231}_{90}Th)
• Beta Emission: Results in Protactinium-231 ( ^{231}_{91}Pa)

Each decay mechanism works systematically to gradually break down the original uranium nucleus, highlighting the complex pathways of nuclear transformations that ultimately stabilize an otherwise unstable nucleus.