Atoms having the same number of protons in their nucleus have the same chemical properties.
The number of protons in the nucleus is known as the
atomic number (
Z).
Elements are substances for which
all atoms have the
same atomic number. Examples are shown at the right.
Chemists refer to atoms of elements by their name or a symbol. The table shows that symbols for elements have one or two letters with the first letter always being capitalized. The symbol for many elements is closely related to their name in English.
| Element | Symbol | Atomic
Number | Mass
(nucleon)
Number |
| hydrogen | H | 1 | 1 |
|
| carbon | C | 6 | 12 |
| nitrogen | N | 7 | 14 |
| oxygen | O | 8 | 16 |
| chlorine | Cl | 17 | 35 |
have no electrical charge, thus the
number of protons equals the number of electrons.
As you will see later, electrons are particularly significant in determining chemical properties.
Example: The number of electrons for all atoms of Cl is 17.
The
number of neutrons in an atom can be deduced from the
mass (nucleon) number (
A) of the atom.
Remember that protons and neutrons are referred to as nucleons.
The mass (nucleon) number (A) equals the sum of the numbers of protons and neutrons in the nucleus.
The mass (nucleon) number for a particular atom is given as a superscript preceding the atom (see below).
This in combination with the atomic number can be used to calculate the number of neutrons present in that atom.
Example: The number of protons for all atoms of chlorine is 17 (as see above). The number of neutrons for 35Cl is 18 (35 - 17).