Why do atoms of the same element have the same mass number?

Answer 1

Atomic Mass is determined by the total number of protons and neutrons. A certain element can have different numbers of neutrons (but not different numbers of protons) and still be the same element. So for example typical Carbon has 6 protons and 6 neutrons. But there is also Carbon (less typical) that has 6 protons and 8 neutrons. Typical Nitrogen has 7 protons and 7 neutrons. So the 14 nucleons in typical Nitrogen will have about the same mass as the not so typical Carbon, also with 14 nucleons. Elements with the same proton number but different neutron number are called "isotopes". So Atomic masses can be similar between isotopes of different elements.

Answer 2

They have different numbers of neutrons but the same number of protons and electrons. The number of protons in the nucleus (or the number of electrons) determines the chemical properties and the atomic number so they are isotopes but because of the different number of protons have different masses.

Answer 3

An element is differentiated from another by the number of protons in its nucleus. A nucleus with 17 protons will have 17 electrons orbiting it. It is the electrons that are responsible for the chemical properties of the element.

Most nuclei have one or more neutrons in the nucleus; this does not change the electronic structure, so doesn't change the chemical properties of the element. The effects of the neutrons are

1. they change the mass of the nucleus.
2. they may change the stability (or lack of it) of the nucleus.

The first of the elements is the easiest to use as an example. Hydrogen has one proton and no neutrons. Its mass is almost exactly 1 amu (atomic mass unit)

Hydrogen can also exist as 'heavy hydrogen' aka deuterium. This has one proton and one neutron, total mass ~ 2 amu.

Also, super-heavy hydrogen aka tritium. One proton, two neutrons, mass ~ 3 amu.

These nuclei are called isotopes. Some elements, as found in nature, consist of just one isotope; their atomic weight will be close to a whole number (p amu where p is the no. of protons+the number of neutrons). Many elements occur naturally as a mixture of two or more isotopes. Naturally occurring hydrogen contains a little deuterium. If we suppose there is 1% deuterium. then the atomic weight of the mixture will measure at ~1.02 amu, rather than ~ 1 amu.

Chlorine is a mixture of two isotopes, A.W.s 35 and 37. The resulting measured A.W. of the mixture is ~35.5 amu.

Answer 4

If the atoms of an element do have the same mass number, it is because the atoms contain equal numbers of protons and neutrons. For many elements, not all of the atoms contain the same number of neutrons, but all atoms of an element must have the same number of protons, which is the element's atomic number.

Answer 5

They have different numbers of neutrons. This is what MAKES them isotopes.

[You really need to know what isotopes are before you ask any questions about them.]

For example, neon-20 has 10 protons and 10 neutrons, but

neon-22 has 10 protons and 12 neutrons.

Answer 6

The atomic mass of any element is the weighted average of all of its naturally occurring isotopes. Therefore it is possible to have similar atomic masses to a certain degree. However, having exactly the same is impossible unless comparing isotopes.

Answer 7

it doesnt depend on the element it depends on the atomic number that's the way its been disovered.

i hope that's right

Answer 8

why do you think i looked on this thing to find the answer.

YOU TELL ME!