(a) (i)
For the correct answer:
52
In nuclide notation, the upper number represents the nucleon number (mass number), which is the total sum of protons and neutrons in the nucleus. The lower number represents the proton number (atomic number), which defines the element itself. To find the exact number of neutrons, you must subtract the proton number from the nucleon number. For this strontium-90 nucleus, the calculation is $90 – 38 = 52$ neutrons.
(a) (ii)
For the correct answer:
38
A neutral atom always possesses an equal number of positively charged protons and negatively charged electrons to balance the overall electrical charge. The given nuclide notation explicitly shows a proton number of 38 at the bottom left, indicating there are 38 protons in the nucleus. Consequently, there must be exactly 38 electrons orbiting the nucleus in various electron shells to maintain the atom’s overall electrical neutrality.
(b)
For the correct answer:
fast-moving / negatively charged electrons
Beta particles ($\beta$-particles) are high-energy, extremely fast-moving electrons that are spontaneously ejected from the nucleus of an unstable radioactive atom during beta decay. This specific emission occurs when a neutron in the nucleus unstablely transforms into a proton and an electron. Because they are fundamentally electrons, beta particles carry a negative electrical charge (relative charge of $-1$) and have an almost negligible mass compared to nucleons.
(c)
For the correct calculated value:
87 years
To find the total decay time, you must first determine the exact number of half-lives required for the sample’s mass to reduce from 16 mg to 2.0 mg. Each successive half-life reduces the remaining radioactive mass by exactly half: decaying from 16 mg to 8.0 mg is one half-life, 8.0 mg to 4.0 mg is a second, and 4.0 mg to 2.0 mg is a third. This sequence represents a total of 3 half-lives. Since one half-life is given as 29 years, the total time taken is calculated as $3 \times 29 = 87$ years.
