All atoms of any element have the same number of protons and, if uncharged, they also have the same number of electrons.
Isotopes are atoms with the same number of protons (and are therefore the same element) but different numbers of neutrons.
Atoms that are isotopes of the same element have different mass numbers (and indeed different masses) and the same number of protons.
When referring to the atom of a particular isotope, the mass number and the atomic number are shown as well as the symbol for the element.
The mass number and atomic number are to the left of the element as shown for two isotopes of magnesium below.
The isotope with the smaller mass number is the lighter isotope (24Mg in this case).
The number of protons is referred to the atomic number and is given with the element symbol in the periodic table.
The first four periods (rows) of the periodic table are shown.
These contain many important elements.
Atoms of the same element may, however, have different numbers of neutrons.
| 1 | 2 | ||||||||||||||||
| 1H |  | 2He | |||||||||||||||
| 3Li | 4Be | 5B | 6C | 7N | 8O | 9F | 10Ne | ||||||||||
| 11Na | 12Mg | 13Al | 14Si | 15P | 16S | 17Cl | 18Ar | ||||||||||
| 19K | 20Ca | 21Sc | 22Ti | 23V | 24Cr | 25Mn | 26Fe | 27Co | 28Ni | 29Cu | 30Zn | 31Ga | 32Ge | 33As | 34Se | 35Br | 36Kr |
The first four periods (rows) of the periodic table are shown.
These contain many important elements.
Atoms of the same element may, however, have different numbers of neutrons.
Isotopes are atoms with the same number of protons (and are therefore the same element) but different numbers of neutrons.
Atoms that are isotopes of the same element have different mass numbers (and indeed different masses) and the same number of protons.
When referring to the atom of a particular isotope, the mass number and the atomic number are shown as well as the symbol for the element.
The mass number and atomic number are to the left of the element as shown for two isotopes of magnesium below.
| mass number | 25 | Mg | mass number = number of protons + number of neutrons atomic number = number of protons | 24 | Mg | ||
| atomic number | 12 | 12 |
The isotope with the smaller mass number is the lighter isotope (24Mg in this case).