I. Protons
Protons are positively charged subatomic
particles that are found, along with neutrons, in the nucleus of the atom. Protons,
along with neutrons, make up most of the mass of the atom. The mass of a single
proton is about 1.67265 x 10-24 grams, or 1.0073 u (atomic mass units).
Although the positive charge of the proton is equal to the negative charge on the
electron, one proton has as much mass as around 1840 electrons. The elements on the
periodic table are arranged in order of increasing number of protons (see atomic number
below.) A hydrogen atom has one proton and a helium atom has two.
II. Atomic Number
The number of protons in the nucleus of
an atom is called its atomic number. The atomic number, which is given the symbol Z,
is what determines the identity of an element. All atoms of the same element have
the same number of protons and the same atomic number. Atoms of different elements,
by definition, will have a different number of protons and therefore, different atomic
numbers. Elements with the atomic numbers from 1 to 112 have been identified so far.
One of the numbers found in each elemental box on the periodic table will be the
atomic number. Unlike the mass number, the atomic number is always a whole number.
III. Neutrons
The neutron is a neutral particle that
is found in the nucleus of most atoms. Although the neutron has no charge, it does
contribute to the mass of the atom. Each neutron has a mass of about 1.67495 x 10-24
grams, or 1.0087 u. The most common type of hydrogen, called protium, has no
neutrons. Deuterium, another form of hydrogen, has one proton and one neutron in the
nucleus of each atom. Tritium, the third form of hydrogen, has two neutrons and one
proton in each nucleus.
IV. Mass Number
The vast majority of the mass of an atom
is found in the nucleus. The mass of a proton or a neutron is approximately 1 u
(atomic mass unit). It would take around 1840 electrons to equal the mass of one
proton. For this reason, the masses of the electrons are not considered when
calculating the mass number of an atom. The mass number, which is given the symbol A
in elemental notation, consists of the total number of protons and neutrons in the nucleus
of the atom.
V. Isotopes
Although all atoms of the same element
have the same number of protons, they can have a different number of neutrons. Atoms
of the same element with different numbers of neutrons are called isotopes. The
three forms of hydrogen discussed in the above section on neutrons represent different
isotopes of hydrogen. Isotopes are often identified by mass number. For
example, carbon-12 would be carbon with a mass number of 12, while carbon-14 has a mass
number of 14. As in the case of carbon-14, some isotopes of certain elements are
unstable, which means that they undergo radioactive decay.
VI. Atomic Mass
The atomic masses shown on the periodic
table represent a weighted average based on the relative abundance of each isotope of a
particular atom. Although some books and some teachers still refer to atomic mass as
"atomic weight", this is not considered correct.
V. Quantum Numbers
As you now know, scientists no longer think of electrons
following the fixed orbits described by Bohr's planetary model of the atom. Rather,
electrons are thought to effectively take up the entire space around the nucleus, out to a
certain distance. Quantum numbers are used to describe the allowable values of
certain physical quantities of an electron's behavior.
The first quantum number, also called
the principle quantum number, describes the radius of the electrons orbit. The
principle quantum number is designated by the letter n, and its
value corresponds to the numbered energy levels of the Bohr atom. So, an electron
with a n value of 4 will be found in the fourth energy level.
The second quantum number, called the
angular momentum quantum number, is given the letter l.
This quantum number may have a value ranging from zero to n-1,
and thus is limited by the value of the principle quantum number. The second quantum
number gives us the type of sublevel. A sublevel with l=0 is an s
sublevel. l=1 designates a p sublevel.
d sublevels have an l value of 2. f
sublevels have an l value of 3.
The third quantum number, designated by
the letter m, defines the spatial orientation of the orbital.
The value of m will range from +l
to -l. thus, an s orbital can have only a
value of 0 for m, but a p sublevel can have a
value of -1, 0 or +1.
The fourth quantum number, which is
given the letter s, describes the spin on the electron as either
clockwise or counterclockwise. The Pauli exclusion principle states that no two
electron in an atom can have the same set of four quantum numbers. Therefore, if two
electrons occupy the same orbital, they must have opposite spins.
