Answer
Aluminum.
Work Step by Step
To solve this problem, we can use the principle of conservation of energy. Initially, the alpha particle has only kinetic energy, and as it moves closer to the nucleus, some of this kinetic energy is converted into electric potential energy due to the work done by the electric force between the proton and the charged bead.
$$U_i+K_i=U_f+K_f$$
$$\dfrac{kQq}{r_i}+K_i=\dfrac{kQq}{r_f}+K_f$$
where $Q=Ze$ is the charge of the nucleus, $2e$ is the charge of the alpha particle, $r_i=\infty$, and $r_f=6.0\;\rm fm$, $K_f=0$
$$0+K_i=\dfrac{k(2e)(Ze)}{r_f}+0$$
Solving for $Z$ to find the element.
$$Z=\dfrac{r_f K_i}{2ke^2}$$
Plug the known; and remember to unify the unit of the energy.
$$Z=\dfrac{(6\times 10^{-15})(6.24\times 10^6\times 1.6\times 10^{-19})}{2(9\times 10^9)(1.6\times 10^{-19})^2}$$
$$Z=\color{red}{\bf 13}\tag{It is Aluminum}$$
