The number in the top right of an element’s symbol is its ionic charge, showing how many electrons the atom has gained or lost. You’ll see notations like Na⁺, Mg²⁺, or Cl⁻ when elements form ions. This is different from the numbers printed on a standard periodic table, which sit above or below the symbol and represent the atomic number or atomic mass.
Because different chemistry contexts place numbers in different corners of an element’s symbol, the “top right” position can cause real confusion. Here’s how to tell what each number means.
The Superscript Charge: Ions
When you see a small number with a plus or minus sign in the upper right of a chemical symbol, it tells you the atom’s electrical charge. Sodium written as Na⁺ has lost one electron, giving it a single positive charge. Magnesium written as Mg²⁺ has lost two electrons. Chlorine written as Cl⁻ has gained one electron, giving it a negative charge. The number tells you how many electrons were gained or lost, and the sign tells you the direction.
Atoms are normally electrically neutral because they have equal numbers of protons (positive) and electrons (negative). When an atom loses electrons it becomes positively charged and is called a cation. When it gains electrons it becomes negatively charged and is called an anion. The superscript in the top right is shorthand for that charge.
A few formatting rules matter here. When the charge is 1, the number is usually dropped, so you write Na⁺ rather than Na¹⁺. For charges of 2 or higher, the number comes first and the sign comes second: Ca²⁺, Al³⁺, O²⁻. This convention applies to polyatomic ions too. Phosphate, for example, is written PO₄³⁻, meaning the entire group of atoms carries a charge of negative three.
How Ion Charges Differ From Oxidation Numbers
Oxidation numbers also appear as superscripts to the upper right, and they look almost identical to ion charges. In simple ionic compounds like table salt (NaCl), the oxidation number and the ion charge are the same: Na is +1 and Cl is −1. But in more complex molecules, oxidation numbers are a bookkeeping tool that assigns hypothetical charges to atoms even when no actual ion exists. Carbon in methane (CH₄) is assigned an oxidation number of −4, written C⁻⁴, even though carbon hasn’t truly gained four electrons.
If you’re in an introductory chemistry course, you’ll mostly encounter the superscript as a straightforward ion charge. Oxidation numbers become more relevant in later coursework involving redox reactions.
Numbers on the Periodic Table Itself
If you’re looking at a printed periodic table rather than a chemical equation, the number above the element symbol is almost always the atomic number. This counts the protons in the atom’s nucleus and defines what element it is. Hydrogen has an atomic number of 1 (one proton), helium has 2, lithium has 3, and so on. Every element has a unique atomic number, and the periodic table is arranged in order from lowest to highest.
The other number you’ll typically find in a periodic table cell, often below the symbol, is the atomic mass (sometimes called atomic weight). For carbon, this is approximately 12.01. This value reflects the weighted average mass of all naturally occurring isotopes of that element, measured in atomic mass units. It’s not a whole number because it accounts for the relative abundance of each isotope.
Periodic table layouts vary. Some place the atomic number at the top center, others at the top left. Some include additional information like electron configuration or electronegativity. If you’re unsure which number is which, the atomic number is always a whole number and always increases by one as you move across the table.
Nuclear Notation: Numbers on the Left
In nuclear chemistry, you’ll encounter a different system where numbers appear to the left of the element symbol. The mass number (total protons plus neutrons) sits as a superscript on the upper left, and the atomic number sits as a subscript on the lower left. Carbon-12, for example, is written with 12 in the upper left and 6 in the lower left. The atomic number is often omitted since it’s redundant with the element symbol itself.
This left-side notation describes isotopes, which are versions of an element with different numbers of neutrons. Oxygen, for instance, has three naturally occurring isotopes: oxygen-16, oxygen-17, and oxygen-18. All three have 8 protons, but they have 8, 9, and 10 neutrons respectively. The mass number in the upper left distinguishes them.
Quick Way to Tell Them Apart
- Upper right with a + or − sign: ionic charge (how many electrons gained or lost)
- Above the symbol on a periodic table: atomic number (number of protons)
- Below the symbol on a periodic table: atomic mass (average mass of all isotopes)
- Upper left in nuclear notation: mass number (protons + neutrons for a specific isotope)
- Lower right as a subscript: number of atoms in a molecule (like the 2 in H₂O)
The position and the presence of a plus or minus sign are your best clues. If the number has a charge sign, it’s telling you about electrons. If it doesn’t, it’s telling you about protons, neutrons, or atoms in a formula. Once you know what to look for, each number snaps into place.