What Does a Superscript Number Mean?
A superscript number in chemistry most often indicates either the electrical charge of an ion when placed after the atomic symbol, or the mass number of an isotope when placed before the atomic symbol. This usage helps clarify important atomic details such as charge balance and isotope identity.
Superscript Number Denoting Charge
When a superscript number appears after an atomic symbol, it represents the electrical charge of that ion. This charge comes from an imbalance between protons and electrons:
- A positive superscript means the atom lost electrons (deficiency of electrons).
- A negative superscript means the atom gained electrons (excess of electrons).
For example:
- Fe3+: Iron has lost three electrons, acquiring a +3 charge.
- O2-: Oxygen has gained two electrons, showing a -2 charge.
- PuO2+: Though plutonium is at +6 oxidation, the entire ion carries a net +2 charge because each oxygen is -2, resulting in +6 – 2 – 2 = +2.
This notation indicates the ion’s net charge rather than specific charges on individual atoms in polyatomic ions.
Superscript Number Denoting Mass Number
If the superscript number is placed before the atomic symbol, it identifies the mass number of the atom or isotope. The mass number is the sum of protons and neutrons in the nucleus.
For instance, 14C represents a carbon isotope with 6 protons and 8 neutrons, totaling a mass number of 14. This notation helps distinguish isotopes that differ in neutron count but share the same atomic number.
| Position | Meaning | Example | Explanation |
|---|---|---|---|
| After atomic symbol | Charge (ion’s net electrical charge) | Fe3+, O2- | Indicates surplus or deficiency of electrons on ion |
| Before atomic symbol | Mass number (protons + neutrons) | 14C | Shows isotope’s mass number distinguishing isotopes |
Additional Considerations
- Superscript charges apply to entire ions, not just one atom within polyatomic ions.
- Oxidation states inside complex ions may differ from the net charge shown.
- Using superscripts correctly is vital for chemists to represent ionic species and nuclear isotopes clearly.
Key Takeaways
- Superscript after atomic symbols denotes ion charge.
- Superscript before atomic symbols indicates isotope mass number.
- Charge reflects gain or loss of electrons.
- Mass number equals total protons and neutrons.
- Superscripts clarify chemical and nuclear properties concisely.
What does a superscript number after a chemical symbol indicate?
It shows the electrical charge of an ion. A positive charge means electrons are lost; a negative charge means electrons are gained.
How can I tell the difference between a superscript before and after an atomic symbol?
A superscript before it shows the mass number (protons + neutrons). After it shows the ion’s charge.
Does a superscript charge apply to individual atoms or the whole ion?
The superscript charge refers to the net charge on the entire ion, not just one atom inside the ion.
Why is there a superscript +3 on Fe^(3+)?
It means iron has lost three electrons, resulting in a +3 charge.
Can the superscript charge be different from oxidation states inside a complex ion?
Yes. For example, in PuO^+2, plutonium has an oxidation state of +6, but the overall ion charge is +2.
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