Atom Calculator

Protons set the element, neutrons the isotope, electrons the charge — three numbers, three different questions.

Protons17the atomic number, and the ONLY thing that decides which element this is. Change it and you have a different element

Neutrons1835 − 17. Change this and you have a different ISOTOPE of the same element

Electrons1817 − (-1). Change this and you have an ION — same element, same isotope, different charge

ElementChlorine (Cl)standard atomic weight 35.45

Isotope notationCl-35the mass number identifies the isotope. Cl-35 has 18 neutrons

Mass number against atomic weight35 against 35.45the mass number is a COUNT of nucleons and is always a whole number. The atomic weight is an average over isotopes and almost never is. They are different kinds of quantity

IonCl⁻an anion — it has GAINED electrons, so it is bigger than the neutral atom

The formula

protons = Z; neutrons = mass number − Z; electrons = Z − charge

Three numbers, three different questions

The proton count decides which element it is. The neutron count decides which isotope. The electron count decides the charge.

Change the first and you have transmuted an element, which needs a nuclear reaction. Change the third and you have made an ion, which happens whenever salt dissolves. They are not comparable operations.

Mass number and atomic weight are different quantities

A mass number counts nucleons and is always a whole number. Chlorine-35 has exactly 35.

An atomic weight averages over the isotopes as they occur naturally and is 35.45 for chlorine. One is a property of a single atom, the other of a sample, and they share a unit only by convention.

Anions are bigger, cations smaller

Adding an electron increases repulsion in the outer shell and the atom swells. Removing one lets the remaining electrons draw in towards the unchanged nuclear charge.

A sodium ion is markedly smaller than a sodium atom, and a chloride ion markedly larger than a chlorine atom — which is why ionic crystals pack the way they do.

Ideal behaviour is an approximation

The colligative and gas relations here assume dilute solutions and ideal gases, which real systems approach and do not reach.

At high concentration or high pressure the deviations become large, and the corrections are substance-specific. These figures are right where the assumptions hold and approximate where they do not.