The chemical properties of an atom are only determined by its atomic number, which is the number of protons in the nucleus. The number of neutrons affects the mass of the atom, but that doesn't directly affect its chemistry. The number of electrons is variable; atoms can lose electrons, or gain additional electrons. This affects the atom's charge, but does not change its chemical nature.
a) The atom with 3 protons (forget the rest of the numbers) is always called Lithium, abbreviated Li. By convention, the mass of the atom is written as a subscript. The mass of this atom is 3 protons + 4 neutrons = 7 total nucleons. The mass of the electrons is very small and usually ignored.
The atom with 20 protons (forget the rest of the numbers) is always called Calcium. (Look on your periodic table for atomic number 20.) You already know where the mass is written -- as a subscript. The charge is conventionally written as a superscript. In this case, there are 20 protons and only 18 electrons, so there are two units of "extra" positive charge. Write +2 as a superscript.
c) Electrons are negatively charged and protons are positively charged. If you have one electron and one proton, there is no total charge -- the charges cancel. If an atom has 10 electrons, and an overall charge of -2, how many protons must it have?
d) An atom with 6 protons is always called...? (Look up atomic number 6 on your periodic table.)
- Warren
