If you have a container filled with one pure
substance (such as a pure element), practically
all of those molecules must be neutral (they must
have no electric charge). Otherwise, the mixture
would be highly unstable. Why is that?
For electric charge, we say that opposites
attract and like repels like. In other words,
objects with opposite electric charges (+ and -)
will attract, while objects with the same electric
charge (+ and +, or - and -) will push each other
away. Because of this, if (for example) you could
somehow fill a container with only
positively-charged particles, they would
immediately shoot away from each other--maybe
explosively breaking their container.
On the other hand, if you put both
positively-charged and negatively-charged
particles into a container, they would immediately
bond together to form new molecules that have
neutral charge. Technically, this would be a
chemical reaction between charged elements, but
because it would happen so fast, it is almost
impossible to set up and observe. This is why
reactions that are useful to study are usually
Attraction and repulsion caused by electric
charge are both examples of electrostatic
forces. These electrostatic forces are usually
very strong compared to other forces like gravity,
so electrically-charged things tend to find
oppositely-charged things very quickly and become
charge neutral very quickly. This is why
scientists often assume electroneutrality.
Electroneutrality is an approximation that says
that the total charge in a system is zero (meaning
the total positive charge cancels the total
negative charge). It is not exactly true--you
can often have a slight imbalance in charge--but
systems are usually so close to neutral that we
might as well say that they are neutral.
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