Fill in the Blank Questions 27.The number of protons in the nucleus of an atom is called the __________________. 28.Atomic mass refers to the numbers of ___________ and __________ of an atom. 29.Atoms that have the same number of protons but differ in their number of neutrons are _________________. 30.Nonpolar compounds are said to be _________________ because they shrink away from contact with water. 31.When water ionizes, the negatively charged OH fragment is the ____________ ion. Â Â