Atoms have no electrical charge, thus the
number of protons equals the number of electrons.
As you will see later, chemists are mostly interested in electrons because it is the electrons that are involved in chemical change.
All atoms of an element have the
same number of
protons.
As you will see later, some atoms of an element have different numbers of neutrons.
Also chemical change affects the number of electrons on an atom. Therefore some atoms of an element may have different numbers of electrons.
| Element | Symbol | Atomic
Number |
| hydrogen | H | 1 |
| carbon | C | 6 |
| nitrogen | N | 7 |
| oxygen | O | 8 |
| chlorine | Cl | 17 |
The
number of protons in the
nucleus of an atom is known as its
atomic number (
Z).
The atomic number of some common atoms is shown.
Chemists refer to atoms of elements by their name or symbol.
The table shows that symbols for elements have one or two letters with the first letter always being upper case.
Many elements have a symbol closely related to their English name.
Combining these two ideas to deduce the number of electrons or the number of protons in an atom from its atomic number:The atomic number for chlorine is 17.
Thus the number of protons in the nucleus of all chlorine atoms is 17.
Therefore the number of electrons for uncharged chlorine atoms is 17.