What is the top number in an isotope?

Answer 1

An isotope is identified by atomic number (element number or name), by mass number, and by atomic weight. The mass numberfor an isotope is listed as a leading superscript such as 29Si, which in this example is silicon containing 14 protons and 15 neutrons (a rarer but stable isotope of the element). Similarly the atomic weight is indicated by a superscript such as U235 , an isotope of uranium having 92 protons and 143 neutrons.

The actual atomic weight is often expressed as a decimal number corresponding to the mass of the atom in "atomic units", where a proton is about 1.0072, a neutron 1.0086, and an electron about .00055 atomic units.

Answer 2

Usually the top number in each element's box is the atomic number. This number represents a count of how many protons are in that atom's nucleus. It is always a whole number (because it's a count) and it never changes for that particular element. Hydrogen (H) has an atomic number of 1. Uranium (U) has an atomic number of 92.

Answer 3

The top number in an isotope is the mass number. It represents the total number of protons and neutrons in the nucleus of an atom.

Answer 4

The Atomic Mass of the isotope; for example, for the isotope uranium 235: 235U. The atomic number is bellow the atomic mass.

Answer 5

Any no on upper side of atomic symbol in Periodic Table represents its Atomic mass, which is average value of mass numbers of all the isotopes of that element.,

Answer 6

The Mass Number

Answer 7

Atomic Number