|Why do atoms need neutrons? And also I like to
know why there are isotopes and why do some atoms
have many isotopes and others only one or two?
There are two forces involved in holding atoms
together. These are the electromagnetic force and
the strong nuclear force (there is also a weak
nuclear force that is involved with radioactive
decay, but it's a side-show). The electromagnetic
force causes the negatively charged electrons to
be attracted to the positively charged protons;
this is the force that holds the electrons onto
atoms. Neutrons, as you observe, have no electric
charge, and so the electromagnetic force has no
influence on them. They are glued to the protons
via the strong nuclear force, as they do possess
properties of the nuclear forces.
I do not
understand the nuclear forces well enough tobe
able to explain why atomic nuclei require the
numbers of protons and neutrons that they do, or
why some isotopes are stable and others are not.
All elements have many isotopes; those with
seemingly fewer have fewer *stable* isotopes. I
observe that generally the ratio of protons to
neutrons decreases as the atomic number goes up;
for instance, hydrogen has one proton and zero or
one neutrons for its stable isotopes, carbon has
six protons and six or seven neutrons, and calcium
has twenty protons and twenty-something neutrons.
I do not know why.
Not all atoms need neutrons --- the hydrogen atom
has no neutrons. As the atomic number (the number
of protons) increases, the number of neutrons also
increases. This is because protons have positive
charges and the only reason that they are willing
to stay together in the nucleus is because the
neutrons effectively act as "filters" for these
positive charges by interacting with the protons
themselves. Isotopes are found because for a given
number of protons in the nucleus (meaning for a
given element) the precise number of neutrons
required to make the element stable can vary.
While one particular number of neutrons might
correspond to greatest stability, other neutron
counts may also be acceptable.
I'm guessing from your question that you already
know atoms are made up of three things: protons
(which determine the element, or the atom's
properties), electrons (which determine the atom's
reactivity, to some extent) and neutrons (which
determine the atom's isotope). I don't know if
it's necessarily true that an atom "needs" any of
these things. That's just what atoms are made up
of. You are correct that certain atoms have lots
of isotopes (e.g. carbon, xenon) while some have
only three (hydrogen). Certain atoms have
radioactive (unstable) isotopes while other atoms
do not. I've never asked myself why this is the
case. That's a really good question. It probably
has to do with quantum mechanics, but the truth
is, I really don't know.
The reason that the ice cube is taking a longer
time to melt in the salt water is because the salt
needs heat to break it up into Na+ and Cl-.So
instead of the heat going into melting the ice
cube it goes into breaking up the salt into ions.
In other words the heat is used to dissolve the
salt in the water rather than melting the ice.
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