Atoms cannot be divided using chemicals. They do consist of parts, which include protons, neutrons, and electrons, but an atom is a basic chemical building block of matter.

Not always. The reason being,  increasing atomic number doesn't always equate to increasing mass is because many atoms don't have a number of neutrons equal to the number of protons. In other words, several isotopes of an element may exist. If a sizeable portion of an element of lower atomic number exists in the form of heavy isotopes, then the mass of that element may (overall) be heavier than that of the next element. If there were no isotopes and all elements had a number of neutrons equal to the number of protons, then atomic mass would be approximately twice the atomic number (approximately because protons and neutrons don't have exactly the same mass... the mass of electrons is so small that it is negligible). Different periodic tables give differing atomic masses because the percentages of isotopes of an element may be considered changed from one publication to another.

The element symbol for oxygen is O and its atomic number is 8. The mass numbers for oxygen must be 8 + 8 = 16; 8 + 9 = 17; 8 + 10 = 18. respectively