Question 18 What is the ion that selenium forms? (write your answer with no spaces, like this: Na+1) QUESTION 11 Two isotopes of an element differ only in their symbol. atomic number. atomic mass. number of protons. number of electrons. Question 8 The elements in Group 1A are called the Question 18 What is the ion that selenium forms? (write your answer with no spaces, like this: Na+1) QUESTION 11 Two isotopes of an element differ only in their symbol. atomic number. atomic mass. number of protons. number of electrons. Question 8 The elements in Group 1A are called the