The atomic number is the number of protons in the nucleus of an atom of an element. each element has a unique number of protons. for example, an atom with 47 protons is always a silver atom and has an atomic number of 47. the atomic number is used to distinguish different elements from one another. the mass number a is the sum of the number of protons and neutrons in the nucleus of an atom of an element. atoms that have the same atomic number but different mass numbers are called isotopes. for example, all atoms with atomic number 6 are carbon atoms, but carbon atoms with mass numbers of 12 and 13 are isotopes of one another. each element is assigned a unique chemical symbol. for example, the symbol for hydrogen is h, and the symbol for oxygen is o. some elements have symbols based on their names in languages other than english. for example, the symbol for iron is fe, which is based on the latin ferrum. the atomic number, mass number, and chemical symbol are often combined to describe a given atom. for example, an atom of lithium (li) with atomic number 3 and a mass number of 7 could be written 7/3 li . since the atomic number and the symbol are both unique to a given element, the atomic number is often left off of this notation, leaving ^7 li. i) which subatomic particle(s) are found in the nucleus of the atom? enter the subatomic particle or particles separated by commas. ii) the atomic number of oxygen is 8. what is the mass number of an oxygen atom with 7 neutrons? mass number = ? iii) an atom has a mass number of 25 and 13 neutrons. what is the atomic number of this atom? atomic number = ?