How Subatomic Particles Determine Atomic Structure

Three particles answer different questions

Protons and neutrons occupy the nucleus, while electrons occupy orbitals around it. Their charges and approximate masses create a compact problem-solving table:

Atomic number identifies the element

The atomic number is the number of protons:

$$Z=p$$

Changing the proton count changes the element. Changing only neutrons produces another isotope of the same element; changing only electrons produces an ion.

Mass number counts nuclear particles

For a particular isotope, the mass number is

$$A=p+n.$$

Electron mass is tiny compared with proton and neutron mass, so introductory mass-number calculations ignore it. The neutron count follows from $n=A-Z$.

Net charge compares protons and electrons

Using elementary-charge units,

$$q=p-e.$$

Equal proton and electron counts make a neutral atom. More electrons give a negative ion; fewer electrons give a positive ion.

Example: chlorine-35 and chloride

Chlorine has atomic number 17. A chlorine-35 atom therefore contains 17 protons and $35-17=18$ neutrons. If it is neutral, it has 17 electrons. If it gains one electron, it still has 17 protons and 18 neutrons but now has 18 electrons:

$$q=17-18=-1.$$

The result is a chloride ion. The element and isotope have not changed; only the net charge has.

A reliable order of operations

Read the atomic number first to set the proton count. Use mass number minus atomic number for neutrons. Then use the stated charge to solve for electrons. This order keeps element identity, isotope identity, and ionic charge from being mixed together.