Chemistry Comes Alive
All protons are alike, regardless of the atom considered. Te
same is true of all neutrons and all electrons. So what deter-
mines the unique properties of each element? Te answer is that
atoms of diﬀerent elements are composed of
of protons, neutrons, and electrons.
Te simplest and smallest atom, hydrogen, has one proton,
one electron, and no neutrons
. Next in size is the
helium atom, with two protons, two neutrons, and two orbiting
electrons. Lithium follows with three protons, four neutrons,
and three electrons. If we continued this step-by-step progres-
sion, we would get a graded series of atoms containing from 1 to
112 protons, an equal number of electrons, and a slightly larger
number of neutrons at each step.
All we really need to know to identify a particular element,
however, are its atomic number, mass number, and atomic
weight. ±aken together, these provide a fairly complete picture
of each element.
of any atom is equal to the number of protons
in its nucleus and is written as a subscript to the leF of its atomic
symbol. Hydrogen, with one proton, has an atomic number of
H). Helium, with two protons, has an atomic number of 2
He), and so on. Te number of protons is always equal to the
number of electrons in an atom, so the atomic number
tells us the number of electrons in the atom as well. As we will see
shortly, this information is important indeed, because electrons
determine the chemical behavior of atoms.
Mass Number and Isotopes
of an atom is the sum of the masses of its
protons and neutrons. Te mass of the electrons is so small that
it is ignored. Recall that protons and neutrons have a mass of
1 amu. Hydrogen has only one proton in its nucleus, so its
atomic and mass numbers are the same: 1. Helium, with two
protons and two neutrons, has a mass number of 4.
Te mass number is usually indicated by a superscript
to the leF of the atomic symbol. ²or example, helium is
Tis simple notation allows us to deduce the total number and
kinds of subatomic particles in any atom because it indicates
the number of protons (the atomic number), the number of
electrons (equal to the atomic number), and the number of neu-
trons (mass number minus atomic number). In our example, we
can do the subtraction to ﬁnd that
He has two neutrons.
²rom what we have said so far, it may appear as if each
element has one, and only one, type of atom representing it.
Tis is not the case. Nearly all known elements have two or
more structural variations called
have the same number of protons (and electrons), but diﬀer
in the number of neutrons they contain. Earlier, when we said
that hydrogen has a mass number of 1, we were speaking of
H, its most abundant isotope. Some hydrogen atoms have a
mass of 2 or 3 amu (atomic mass units), which means that
they have one proton and, respectively, one or two neutrons
Atomic structure of the three smallest atoms.
Isotopes of hydrogen.