Tips on identifying and remembering Isotope, Isoelectronic, and Isomer. Now for reason, that's been noticed to me, students always forget, or never remember these three words. I will show you why.
So let's go through these three words, and really understand the difference and similarities between them. Now they all start with i-s-o-, and what does that mean? Iso- means the same, but different. It's a Greek prefix.
Look at the first one; Isotope. I wrote two Isotopes up here C14, and C12. We know that this is the mass number. They have the same number of protons, these two atoms. They both have six protons. However, they have different number of neutrons. This one has 8, and this one has 6. I know that, because these two make up the mass; protons and neutrons make up the mass.
Therefore they also have different masses, notice. Number of neutrons therefore different masses. They have the same number of protons, therefore the same atom. They're both Carbon. So same number of protons, different number of neutrons. Or same atom with different masses. That's an Isotope.
When something is Isoelectronic, same but different, they have the same number of electrons. So this Phosphorous or Phosphide P-3 or P3- is Isoelectronic to Cl- or Chloride, because they have the same number of electrons. Electronic sounds like electrons, they have the same number of Electrons. They both have 18 electrons. Phosphorous originally had 15, it added 3 to make 18. Chloride originally had 17, it added 1 to make 18. They're Isoelectronic to each other.
However, they have different number of protons. Phosphide has 15 protons, Chloride has 17 protons. Therefore, they're different ions, because of different number of protons. Therefore, different ions. However, same number of electrons means same electronic configuration. Notice, they both are also Isoelectronic to Argon which is the Noble Gas. Very stable.
Then the last one is Isomer. Iso- meaning same, but different. So how are these guys the same, but different. They have the same formula. They have four Carbons, and 10 Hydrogens; C4H10. There's also a C4H10, but different structures. Notice this structure is a branched structure of the first one. This one is the four Carbon chain. So therefore, they have different properties. The same formula means therefore they have the same percent composition.
Hopefully, this little explanation helped you understand the prefix iso-, what that means. Hopefully, that helped you identify what an Isotope is or when something is Isoelectronic, and what an Isomer is and how they're very similar to each other, but different. So hopefully that helped you.