Nuclide
The word substance is used in chemistry to describe elements or compounds. It is used because when dealing with an unknown substance, it is important to not assign either name to it.
It is the neutron that makes changes in atomic nuclei to change them from one isotope to another. For any given element, that element will have a fixed number of protons. It is, after all, the number of protons that determine the elemental identity. But the number of neutrons in a given element can vary, and we use the term isotope to talk about which particular atom we're investigating. That is, we apply the term isotope to speak to an atom of a given element with a certain number of neutrons in its nucleus.
Because each isotope of an element has a mass different from any other isotope of the same element, and the atomic mass of an element is an average, weighted by the proportion of each isotope, in the naturally occurring element.
The mass of a mixture of isotopes for an element is a weighted average of the masses of each isotope, calculated based on the isotope abundances. The formula for calculating the average atomic mass is: (mass of isotope 1 x abundance of isotope 1) + (mass of isotope 2 x abundance of isotope 2) + ...
In an atom of any isotope of any element, there is a number of protons which is similar to the number of electrons.
The opposite of an isotope would be a pure element that does not contain any variations in the number of neutrons in its nucleus. This means that all atoms of that element have the same number of protons and neutrons.
The root is the core element of any medical term.
There is no specific number; each element has another number of natural and artificial isotopes.
A term used to describe any element or compound is called a substance or substances.
The number of neutrons in the nucleus of an atom of any isotope of an element is equal to the "atomic mass number", which is usually stated in a number immediately after a hyphen at the end of the element name in the name of the isotope, minus the atomic number. Because most elements have more than one stable isotope, the average number of neutrons in the mixture of isotopes of an element that occurs naturally is usually not an integer.
It isn't, as such. Isotopes of the same element have different numbers of neutrons, and neutrons have a mas of one atomic mass unit (amu). So isotopes have different atomic masses, but being told the number of neutrons any isotope has, will not enable you to say what element or atomic mass it had, unless you remembered the details for every single isotope. Even then different elements can have the same number of neutrons. Isotopes do get named after their atomic mass however - uranaium 235 has an atomic mass of 235, for instance.
Any time you see the atomic mass of an element in brackets on the periodic chart, that number represents the mass number of the longest-lived or best-known isotope of that element. In other words there is no stable isotope, i.e. all forms of it are radioactive.