Video Transcript
Why do atoms not have an overall
charge? (A) The number of electrons equals
the number of neutrons. (B) The number of electrons equals
the number of protons. (C) The number of neutrons equals
the number of protons. (D) The number of electrons and
neutrons equals the number of protons. (E) The number of electrons plus
protons equals the number of neutrons.
This question asks about overall
charge. Charge can be positive or negative
like the opposite ends of a magnet. If there’s no charge, we can say
that the charge is neutral. Overall charge refers to the charge
of the atom as a whole. And if we want to know about the
atom’s overall charge, we need to know about the charges of the subatomic particles
that make up the atom.
Protons, found in the nucleus, have
a positive charge. Neutrons, also found in the
nucleus, have no charge or a neutral charge. And electrons, found in the
electron cloud outside the nucleus, have a negative charge. So in other words, this question’s
asking, how do the numbers of protons, neutrons, and electrons result in an atom not
having an overall positive or negative charge? It’s important to know that the
charge of a positive proton and the charge of a negative electron have equal
strength. In other words, putting together
one proton with a positive charge and one electron with a negative charge will
balance out to be neutral.
In fact, the only way to have a
neutral atom is for the total positive charge to be equal in strength to the total
negative charge. This arrangement is described in
answer (B), the number of electrons equals the number of protons. If the number of electrons equals
the number of protons, then the positive and negative charges of the protons and
electrons will balance out and become neutral. When an atom gains or loses
electrons to become an ion, it no longer has an equal number of electrons and
protons. As a result, it will have an
overall positive or negative charge.
The number of neutrons has no
effect on the overall charge of the atom. Neutrons help give the nucleus of
the atom stability. Even though we have already
identified the correct answer, let’s thoroughly examine the other possible
answers.
As we look at these answers, let’s
consider an atom of the element fluorine with nine protons, 10 neutrons, and nine
electrons. We know that it has nine protons
because it has an atomic number of nine, which also tells us that, as a neutral
atom, it will have nine electrons. Most fluorine atoms have 10
neutrons because the atomic mass of fluorine is very close to 19. Nine protons plus 10 neutrons
equals an atomic mass of 19.
Choice (A), the number of electrons
equals the number of neutrons, is incorrect. For one, this answer tells us
nothing about the number of protons in the atom. So we don’t know if the negative
charge of the electrons is balanced out by the positive charge of the protons. And in fact, the number of
electrons doesn’t always equal the number of neutrons. For example, in our fluorine atom,
there are nine electrons and 10 neutrons. So (A) is incorrect.
Choice (C) is incorrect for
essentially the same reasons. Not only does this answer tell us
nothing about how much of a negative charge there is to balance out the positive
charge, there’s also not an equal number of neutrons and protons in many atoms. So choice (C) is also
incorrect.
Our next incorrect answer is choice
(D), the number of electrons and neutrons equals the number of protons. It does not make sense to combine
electrons and neutrons and compare them to protons. Neutrons do not contribute any
negative charge to the atom. So combining electrons and neutrons
before comparing them to the number of protons does not accurately give us a picture
of how the positive and negative charges of the constituent parts of the atom are
balanced. While this choice does factor in
both electrons and protons, the addition of neutrons makes it inaccurate. Also, we can compare the answer to
our example atom of fluorine and see that the number of electrons and neutrons does
not always equal the number of protons.
Our last incorrect answer is choice
(E), the number of electrons plus protons equals the number of neutrons. Again, this choice incorporates
both electrons and protons but in an inaccurate way. The total number of electrons and
protons and the number of neutrons are irrelevant to the overall charge of the
atom. Also, in our example atom of
fluorine, there are 18 protons and electrons combined and only 10 neutrons. So this statement is not true for
all atoms.
Overall, atoms have no overall
charge because their positive charge and their negative charge are of equal strength
because the positively charged particles and the negatively charged particles within
them have the same strength of charge. The charges only canceled out when
there’s the same number of particles of each type. So atoms do not have an overall
charge because, answer (B), the number of electrons equals the number of
protons.
