Left to their own devices, atoms are electrically neutral.
That means that they have an equal number of
protons and electrons.
During the course of most natural events,
protons are not gained or lost, but electrons may be.
Atoms with more or fewer electrons than protons are
electrically charged. They are called ions:
an atom that loses electrons takes on a positive charge (cation);
an atom that gains electrons takes on a negative charge (anion).
Complex cations and anions can also occur: (NH4)+1, (SO4)-2
We distinguish one element from another on the basis of the atomic number, which is the number of protons.
So, an atom of any element can have a variable number of electrons and neutrons, but given the number of protons, the fundamental properties of the element are unchanged.
This is the basis for Dmitri Mendeleevís organization of the
Periodic Table of the Elements.
The table is a way of organizing elements
on physical grounds,
but also serves to group elements with consistent chemical properties.
The periodic table is read from top to bottom, left to right, as atomic number increases: 1=H, 2=He, 3=Li, 4=Be, 5=B, 6=C, and so on.
Elements in columns (groups) have similar
and so tend to behave similarly.
The Periodic Table
Most atoms will form the same kinds of ions all the time.
For example, all the alkalis form +1 ions,
and the halogens form -1 ions.
A transition metal cation with a higher charge is more oxidized than
one of lower charge. That comes from the fact that materials with
high proportions of Fe+3/Fe+2 form in environments
where oxygen is abundant.
The opposite is also true, and we call Fe+2 reduced iron.
The transition metals are more electronically complex.
They may form ions of various charges.
For example, iron (Fe) is found as +2 and +3 ions.