Download Medical Chemistry Lecture By : Asst. Lect. Tariq-H-Almgheer

Medical Chemistry Lecture By : Asst. Lect. Tariq-H-Almgheer- College of Medicine- Babylon University
of the helium atom contains two protons, whereas the nucleus of an oxygen atom contains
eight protons. The nucleus of an atom has a positive charge because the nucleus contains
protons. The charge of a nucleus is equal to the number or protons it contains. Thus, the
helium nucleus has a charge of +2, whereas the oxygen nucleus has a charge of +8.
An atom is electrically neutral. Therefore, the charge created by the protons in the
nucleus must be balanced exactly by an equal number of electrons outside the nucleus. For
example, the helium atom has two protons in its nucleus. To balance the charge of +2
created by these protons, it must have two electrons outside the nucleus. Similarly, an
oxygen atom has eight protons in its nucleus and it must have eight electrons outside the
nucleus.
Remember that neutrons, protons, and electrons are all very small particles. The
atom that contains these particles is also very small. A uranium atom, one of the largest,
has a diameter of 2.8 X 10'8 cm. This is so small that a length of 1 cm corresponds to 36
million uranium atoms placed side to side!
Atoms are small, but the nucleus is even smaller; it occupies only a small part of the
total volume of the atom. If one could magnify the size of an atom so that its nucleus was the
size of a baseball the atom would be a sphere approximately 16 km in diameter. Two
important points are made by this picture of an atom. First, the most massive particles of the
atom are concentrated in a very small volume of space. Second, most of the atom is empty
space.
The number of protons in the nucleus is an important property of an atom. This
number determines not only the number of electrons outside the nucleus, but also the atomic
number.
ATOMIC NUMBERS
At the turn of the century it was discovered that the number of positive charges on the
nuclei increases by one from atom to atom as one moves from one group to the next in any
period of the periodic table. For example, the positive charge on the hydrogen nucleus is +1;
on helium, +2; on lithium, +3. The positive charge is due to the protons in the nucleus.
Therefore, each element has one more proton in its nucleus than the element just before it in
the periodic table. This fact allows us to give numbers, called atomic numbers, to each
element. The atomic number of an element is equal to number of proton in its nucleus.
The atomic number of