The identity of the isotope is the neon atom because they have the same atomic number and different atomic mass.
Explanation:
An isotope is an atom that has the same atomic number but different atomic mass of the atom in the periodic table.
Atomic number is the amount of electrons (and also protons) that an atom has.
Atomic mass (or atom weight) is the sum of protons and neutrons of an atom.
Both atomic number and atomic mass are taken from the periodic table.
In this example the isotope contains:
protons= 11
electrons= 10
neutrons= 12
From this we can deduce that:
isotope atomic mass = protons + neutrons = 11 + 12 = 23
First it is necesary to find an atom with atomic number 11 or 10, because the atomic number doesn't change in the isotope. Those are sodium (Na) and neon (Ne).
sodium atomic number= 11
neon atomic number= 10
Also it is important to analyze the atomic mass:
sodium atomic mass= 23
neon atomic mass= 20
With these information we realize that sodium has:
protons: 11
electrons: 11
neutrons: atomic mass - atomic number= 23 - 11 = 12 neutrons
And neon has:
protons: 10
electrons: 10
neutrons: atomic mass - atomic number= 20 - 10 = 10 neutrons
According to the definition of atomic number, the particles that remains the same could be electrons or protons, so in this case both sodium and neon could be the isotope's identity.
Now it is necessary to determine which one is the correct element by analyzing the atomic mass, so according to the definition of isotope the identity of the isotope will be the one with a different atomic mass, that is the neon atom.
