Atom Calculator

Atomic number: The number of protons in the atom. This tells you which element it is.

Mass number: The total number of protons plus neutrons in one isotope. It is a whole number, not the decimal atomic weight shown on many periodic tables [1].

Ion charge: The net charge count found by protons minus electrons. Positive means fewer electrons than protons, and negative means more electrons than protons.

Isotope: Atoms of the same element with the same number of protons but different numbers of neutrons.

Neutral atom: An atom with equal numbers of protons and electrons, so the charge is 0.

Positive ion: A particle with more protons than electrons, so the charge is positive.

Negative ion: A particle with more electrons than protons, so the charge is negative.

Mistake: Typing a decimal into Protons , Neutrons , Electrons , or Ion charge .
Fix: Use whole numbers only, because this calculator treats each particle count and charge as an integer count.

Mistake: Entering a periodic-table decimal weight into Mass number (protons + neutrons) .
Fix: Put in a whole-number isotope mass number instead. Mass number must equal Protons plus Neutrons .

Mistake: Using the wrong sign in Ion charge .
Fix: Enter 0 for neutral, a positive number when Protons is greater than Electrons , and a negative number when Electrons is greater than Protons .

Mistake: Filling in fields that do not match the choice in What do you know? and expecting all of them to be used.
Fix: Select the correct known set first, then enter only the values required for that set and read the rest from the outputs.

Mistake: Entering a Mass number (protons + neutrons) smaller than Atomic number (protons) or Protons .
Fix: Increase Mass number or lower the proton count so Neutrons does not become negative.

Mistake: Thinking Element changes when only Neutrons changes.
Fix: The Element is set by Atomic number or Protons . Changing Neutrons changes the isotope, not the element.

• This calculator uses a beginner model with whole-number counts only. It rejects decimal values for particle counts and ion charge.
• Element lookup is only shown for atomic numbers 1 through 118. Above that range, the count math can still work, but the element name is not matched.
• The tool uses mass number, not standard atomic weight. If you enter a decimal atomic weight from a periodic table, the result will not represent a single isotope correctly.
• The charge model is based on simple count difference: protons minus electrons. It does not describe bonding, electron shells, or stability.
• If your selected input set would make neutrons or electrons negative, the combination is treated as invalid under this model.
• Real chemistry can include isotopes, ions, and nuclear behavior not covered here. This calculator focuses only on the basic count relationships among protons, neutrons, electrons, atomic number, mass number, and charge.

Core relationships

This calculator solves the missing values from the basic atom-count rules used in intro chemistry.

Z = p

A = p + n

q = p - e

e = p - q

n = A - p

Here, Z is atomic number, A is mass number, p is protons, n is neutrons, e is electrons, and q is ion charge.

How each input set is solved

If you choose Atomic number, mass number, and charge, the calculator sets protons equal to atomic number, finds neutrons from mass number minus protons, and finds electrons from protons minus charge.

If you choose Protons, neutrons, and electrons, the calculator sets atomic number equal to protons, mass number equal to protons plus neutrons, and charge equal to protons minus electrons.

If you choose Atomic number, neutrons, and charge, the calculator sets protons equal to atomic number, mass number equal to protons plus neutrons, and electrons equal to protons minus charge.

If you choose Protons, mass number, and charge, the calculator sets atomic number equal to protons, neutrons equal to mass number minus protons, and electrons equal to protons minus charge.

Consistency check

After solving, the calculator checks whether the counts describe a possible basic atom or ion in this model. It flags inputs that would make particle counts negative or break the rules for mass number or charge.

Valid if A = p + n and q = p - e, with p >= 1, n >= 0, e >= 0, and whole-number counts

Worked mini-example

Example: suppose you know Atomic number (protons) = 11, Mass number (protons + neutrons) = 23, and Ion charge = 0.

p = Z = 11

n = A - p = 23 - 11 = 12

e = p - q = 11 - 0 = 11

Type = neutral atom because p = e

That gives 11 protons, 12 neutrons, 11 electrons, atomic number 11, mass number 23, and charge 0. With element lookup turned on, atomic number 11 matches sodium.

Mass number versus atomic weight

Mass number is the whole-number count of protons plus neutrons for one isotope. Standard atomic weight is a different idea: it is a periodic-table value based on isotopic information and is often a decimal, so it should not be entered as mass number [1][3].

Assumptions used here

This page assumes simple count-based chemistry only. It does not calculate electron configuration, isotope abundance, or whether an ion is stable in real conditions. Element matching is limited to atomic numbers 1 through 118.