Chapter 4 · Atoms and Elements
4.4The Properties of Protons, Neutrons, and Electrons
10 min · two checks
Predict
What does the symbol A over Z next to an element stand for?
The idea
- Assign charge and relative mass to each subatomic particle.
- Read A and Z, and find the neutron count.
Charge is measured in units of the elementary charge. A proton is +1, an electron is −1, a neutron is 0. Protons and neutrons each have a relative mass of about 1 atomic mass unit. An electron’s mass is negligible when you are estimating the mass number of an atom. The atomic number Z is the number of protons and identifies the element. The mass number A is protons plus neutrons. It is a count, so it is a whole number, not the decimal atomic mass on the periodic table.
Neutrons = A − Z. In a neutral atom, electrons = Z. The full nuclear symbol writes A as a superscript and Z as a subscript to the left of the element symbol. If the atom is an ion, a charge at the upper right tells you the electrons differ from Z. You will practice ions in a few sections; this section’s job is to read a neutral atom without hesitating.
Keep these
- Z = protons = electrons in a neutral atom.
- A = protons + neutrons. Neutrons = A − Z.
- A is a whole-number count. The table’s atomic mass is a weighted average.
Worked path
A neutral atom of phosphorus-31 is written ³¹₁₅P. How many protons, neutrons, and electrons does it have?
Observe
Z = 15, A = 31, charge not shown so the atom is neutral.