Video Transcript
An atom has three protons and five
neutrons in its nucleus. Which of the following is an
isotope of this atom? (A) An atom with five protons and
three neutrons in its nucleus. (B) An atom with four protons and
four neutrons in its nucleus. (C) An atom with three protons and
five neutrons in its nucleus. (D) An atom with three protons and
four neutrons in its nucleus. Or (E) an atom with no protons and
five neutrons in its nucleus.
This question asks us to consider
an atom with three protons and five neutrons in its nucleus. The nucleus is the center of the
atom where the protons and neutrons are found. The protons are the positively
charged particles that determine what element the atom is. For example, this atom has three
protons, which means it’s an atom of lithium, element number three on the periodic
table. Neutrons are the neutral particles
that contribute to the stability of the atom. Unlike protons, changing the number
of neutrons does not change the identity of the element.
The question asks us to find an
isotope of this atom. The definition of an isotope is a
version of an element with the same number of protons but a different number of
neutrons. The atom in question is lithium
with three protons. All lithium atoms will have three
protons. However, the different versions or
isotopes of lithium will have different numbers of neutrons. So, to find the answer that is an
isotope of this atom, we wanna find the answer that has the same number of protons,
three, and a different number of neutrons. Looking at the number of protons in
the various choices, we can eliminate choices (A), (B), and (E). If we wanna find an isotope of this
atom, it needs to have the same number of protons. Otherwise, it is not an atom of
lithium.
If we compare the number of
neutrons in the remaining responses, we can see that choice (C) has the same number
of neutrons, five, whereas choice (D) has a different number of neutrons, four. Since an isotope is a version of an
element with the same number of protons but a different number of neutrons, we want
the different number of neutrons found in choice (D), the correct answer, an atom
with three protons and four neutrons in its nucleus. While choice (C) is also a lithium
atom since it has the same number of neutrons, it’s not an isotope of the atom in
question. It is the same kind of atom as the
atom in question.
Isotopes of an element are named by
their mass numbers. The mass number is the number of
protons and neutrons added together, as each proton and each neutron weighs one
unified atomic mass unit. So the atom in the question with
three protons and five neutrons has a mass number of eight and is known as
lithium-8. Choice (D), an atom with three
protons and four neutrons in its nucleus, has a mass number of seven and is known as
lithium-7. An atom with five protons and three
neutrons in its nucleus would be known as boron-8, while an atom with four protons
and four neutrons in its nucleus would be known as beryllium-8.
The different element names of
these atoms comes from the fact that they have different number of protons. Five protons makes an atom of boron
atom, element five on the periodic table, while four protons makes an element of
beryllium atom, element four on the periodic table. Each of these two choices has a
mass number of eight, which shows up in the name of each choice, boron-8 and
beryllium-8. The number of protons and neutrons
added together for each of these two choices is eight. Choice (E) is a little bit strange
as it describes an atom with no protons in its nucleus. In fact, this is not possible. Even hydrogen, the lightest atom
with the fewest protons in its nucleus, has one proton in its nucleus. There is no element zero on the
periodic table as all atoms have at least one proton.
Lastly, in a question about protons
and neutrons, you may be wondering if the number of electrons has any effect on our
answer. The short answer to that question
is no. The number of electrons is not
included in the definition of this isotope. If it has the same number of
protons but a different number of neutrons, it’s an isotope of the atom regardless
of the number of electrons present. For a neutral atom, the number of
electrons is the same as the number of protons to balance out the charge. If there’s a different number of
electrons, the atom gains a charge and becomes an ion.
The lithium atoms shown have three
electrons surrounding their nucleus. If one of the electrons left, it
would form an Li+ ion. Since electrons are extremely
light, they do not contribute to the mass number of the isotope. So, even if an electron left, this
would still be known as lithium-7. So it is indeed possible for
something to be both an isotope and an ion. An isotope requires there to be a
change in the number of neutrons, while an ion requires there to be a change in the
number of electrons. While it’s valuable to know about
electrons and ion formation, it’s not required for this question. We simply need to know that
electrons are not involved in the identity of an isotope.
Since an isotope is a version of an
element with the same number of protons but a different number of neutrons, the
choice that is an isotope of this atom is choice (D), an atom with three protons and
four neutrons in its nucleus.
