Iodine 131, cesium 137, and strontium 90 are all considered isotopes. What are isotopes?

Isotopes are variants of an element, where the number of neutrons is different but its number of protons stay the same.

Explanation:

If the number of protons had changed then it would just be a different element.

Electron number changing simply adds charge to the atom, and doesn't change it or anything.

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Isotopes are one of two or more forms of the same chemical element.

Explanation:

Different isotopes of an element have the same number of protons in the nucleus, giving them the same atomic number,but a different number of neutrons giving each element isotope of a different atomic weight.

n has four pairs of electrons in the valence shell, electrons repel one another ,n has 7 protons in its nucleus, and since n has 7 protons, it must fill the second shell, giving it 4 pairs of electrons. the electrons form 3 bonds and 1 lone pair of electrons. each pair of electrons repels the other pairs, so they are equally far apart.

so all of the above