why does some elements dont have atomic mass number?

Numbers for atoms....

Using the phrase "atomic mass number" can be a bit confusing. Don't do that.

Atomic number - an ordinal value which is also equal to the relative charge on the nucleus and therefore, the number of protons in the nucleus. The atomic number of carbon is 6. Six protons in the nucleus.

Average atomic mass (atomic weight) - a weighted average of the isotopic masses of the naturally occurring (usually stable) isotopes of that element. Carbon has an average atomic weight of 12.011 based on the two stable isotopes of carbon: C-12 and C-13.

Isotopic mass - each isotope of an element has a specific atomic mass. For instance, C-12 is assigned a mass of exactly 12 and C-13 has a mass of 13.003355 amu.

Mass number - The mass number is not an atomic mass, or an isotopic mass. It's not a mass at all. The mass number is more correctly called the "nucleon number" and is the number of protons and neutrons in the nucleus. The mass number is always a whole number (counting number), whereas, the isotopic mass always includes a decimal fraction. (Except for carbon-12 which is assigned an isotopic mass of exactly 12.)

The periodic table usually shows the atomic number and the average atomic weight for an element. Some elements (Tc, Pm and the elements above bismuth, for instance) do not have stable isotopes and therefore, an average atomic weight would be meaningless, because it would always be changing. Therefore, for these unstable elements, the periodic table will include the mass number of the isotope with the longest half-life. Often the mass number will be include parentheses. So when you see (222) for radon, it's telling us that the Rn-222 is the isotope of radon with the longest half-life.

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A common misconception is that an element has isotopes and some kind of atom which is not an isotope, or that stable atoms are not isotopes, or some other such nonsense. Make sure you know that all atoms are isotopes. There are no atoms which are not isotopes. Even if there is only one kind of naturally occurring atom of an element, it is still an isotope of that element.

Atoms of the same element have the same number of protons (i.e. the same atomic number). However, isotopes are atoms which have the same number of protons but different numbers of neutrons.

As isotopes have the same number of protons, they are the same number. As mass number is the total number of protons and neutrons and thus isotopes have different mass numbers.

Conclusively, atoms of the same element but having different mass numbers are called isotopes.

"Why do some elements not have an atomic number?" is that your question?

they all do as far as I know.

Do you have an example of some that do not?

The atomic mass shown on a periodic table is usually a weighted average of the masses of the naturally-occurring isotopes. For elements that do not occur in nature, sometimes an atomic mass is shown in parentheses or brackets, because certain isotopes have been produced in a laboratory, but if the element does not occur in nature, there is no way of saying what the "natural" mix of isotopes would be.

Every element has an atomic number. There are no exceptions.
Its possible you're looking at an incomplete or incorrect chart.

They do. It's the number of protons. Number of protons is equal to number of electrons and so on..

Atomic number = number of protons.
This is the unique identifier of any element.
Iron atoms all have 26 protons, regardless of how many neutrons they have.
If an atom has 26 protons, then it is iron.
If an atom is iron, then it has 26 protons.
Period.

Atomic mass is the sum of the number of protons and the number of neutrons.

In chemistry, it is the weighted average of all isotopes that are "naturally found".
isotope = same element (same number of protons) but different number of neutrons.
The identity of the isotope is given by the atomic mass = proton + neutron.

For example, we find different isotopes of iron in nature:
iron-54 = 26 protons + 28 neutrons
iron-56 = 26 protons + 30 neutrons
iron-57 = 26 protons + 31 neutrons
iron-58 = 26 protons + 32 neutrons

Most of it (in nature) is Fe-56 (92%), with 6% being Fe-54, 2% being Fe-55 and a pinch of Fe-58.
Therefore, if you take a whole bunch of iron atoms, you expect their average atomic mass to be close to 56
(the exact measure is 55.845 when you have a "natural mix")

In physics, where calculations deal with individual atoms, knowing which isotope you are dealing with is important.
In chemistry, where calculations deal with large quantities involved in reactions, the average mass is important.

Some elements (especially those produced in labs) do not have any "natural" atoms. They are all artificially produced AND they are radioactive (they disappear, usually quite fast). Therefore, in chemistry, the concept of a "natural average mass" does not make sense (it does not fit the definition) and these elements will sometimes be listed without an official atomic mass.

In physics, in order to identify an element, you need to identify the number of protons and some of its properties. For example, an atom with 115 protons is an atom of moscovium (element number 115). However, so far, only about a hundred atoms have been produced, each one lasting less than a second.
Their masses range from Mc-287 to Mc-290, and there are never enough to have an idea of the distribution (percentage of each), therefore we cannot measure the average atomic mass. And even if we did, NONE of it would be "natural".
The most stable isotope is Mc-290, which has 115 protons and 175 neutrons.
THAT particular isotope (during its one second life-time) has an atomic mass of 290.
But in general, Moscovium (atomic number 115) would not be listed with an "atomic mass".

Just because