What are quark particles and why are they not used as elementary unit

Q. It is now known that a proton and a neutron consist of three elementary units called 'quarks'. Two types of quarks are involved the 'up' quark (u) of charge $+\frac{2}{3}e$ and the 'down' quark (d) of charge $-\frac{1}{3}e$, where e is the magnitude of electron's charge. $\overline{u}$ and $\overline{d}$ are their antiparticles. An antiparticles has the same magnitude of charge as the particle but of opposite sign. The possible compositions of a proton and a neutron are

Q. In a quark model of elementary particles, a neutron is made of one up quark of charge $\frac{2}{3}\epsilon$ and two down quark of charges $\left(\frac{-1}{3}e\right)$ . if they have a triangle configuration with a side length of the order of ${10}^{-15}$ m. the electrostatic potential energy of the neutron is MeV is:

Q. Particles which are not composite and hence truly elementary are.

Q.

Neutrons are made up of one up quark and two down quarks and the quarks have chare.....

So why neutrons do not have charge ?

Q. In a quark model of elementary particles, a neutron is made of one up quarks [charge $\left(2/3\right)e$] and two down quarks [charges $-\left(1/3\right)e$]. Assume that they have a triangle configuration with side length of the order of ${10}^{–15}m$.

A) Calculate electrostatic potential energy of neutron and compare it with its mass $939MeV$.
B) Calculate electrostatic potential energy of proton and compare it with its mass $939MeV$.