Here's why that is the case.
The idea here is that in order for an atom to be an atom of boron, it must have
In other words, if an atom has an atomic number equal to
However, the number of neutrons that an atom can have and still be an atom of boron can vary. The problem tells you that boron has two isotopes, boron-10 and boron-11.
These two atoms are said to be isotopes of boron because
- they have the same number of protons inside the nucleus
- they have different numbers of neutrons inside the nucleus
The number added to the name of the isotope tells you the mass number of the isotope, i.e. the number of protons and neutrons located inside the nucleus.
So you can say that
#"boron-10: " "protons + neutrons" = 10#
#"boron-11: " "protons + neutrons" = 11#
Since both of them must have
#"boron-10: " "5 protons + 5 neutrons"#
#"boron-11: " "5 protons + 6 neutrons"#
So, to sum this up, boron-10 and boron-11 are isotopes because they have the same number of protons in the nucleus, i.e.