Why is proton called 质子 and charge called 电荷? For instance, in medical terminology, we can tease out the meaning from the words. But I cannot tell why we use 质 for proton and 荷 for charge.
质 in Chinese mean inseparable (不可分割) such as 质数 （prime number）

Another explaination, 质：事物的根本特性.

https://www.zhihu.com/question/38322079