posted by me .
can anyone explain to me this i don't understand it.
Certain groups of different atoms will join together and act
as a single unit when forming compounds. Such a group
of atoms is called a radical. In the compound calcium
carbonate, written CaCO3, the carbonate group CO3 is
a radical. The carbonate group has a negative valence of 2,
and it combines directly with one atom of calcium, which
has a positive valence of 2. Some other examples of radicals
are sulfate (SO4
–2), nitrate (NO3
–1), hydroxide (OH–1), and
If a subscript is needed with a radical, the symbol for the
radical is placed inside parentheses. Thus, the formula for
magnesium nitrate is written Mg(NO3)2.
The radical, as defined above, is the name of a group of elements that are grouped together, along with a negative or positive sign. I was taught this when I was in college and graduate school and I taught it to students through the 70s. However, the term has been changed and today the unit is called a polyatomic ion. Frankly, I think the new name stinks; unfortunately, it is used throughout chemistry now and I have no choice but to follow. The old term of radical is reserved for organic chemistry and it refers to "free radicals." Enough of the soap box.
The description you have sounds clear to me and it is well illustrated with numerous examples. If you will clarify what you don't understand about the term polyatomic ions, I shall be happy to help guide you to an understanding.