Atomic Structure
Subatomic Particles
Atoms are made up of even smaller particles called subatomic particles. These subatomic
particles are: electrons, protons, and neutrons. [There are other subatomic particles, but
from a chemical perspective, they are rather unimportant.]
Symbol, charge, and approximate mass of subatomic particles
Particle
Symbol
Electrical Charge
Approximate Mass
Proton
P+
+1
1 amu
Neutron
N
0
1 amu
Electron
e-
-1
0 amu
amu=atomic mass unit 1 amu=1.66 x 10-24g
Arrangement of subatomic particles
The nucleus contains the protons and neutrons. The nucleus is very small in comparison to
the size of the atom, yet it is very heavy. Most of the mass of the atom is found inside
the nucleus. If the nucleus was the size of a marble, the atom would be about the size of
a football field.
The electrons are found outside the nucleus in certain energy levels.
Atomic Number (Z) and Mass Number (M)
The atomic number is what determines the atom’s identity.
Atomic number = number of protons in an atom
For atoms that are electrically neutral, the number of protons = the number of electrons
For ions, the number of protons ≠ the number of electrons
Cations have less electrons than an electrically neutral atom
Anions have more electrons than an electrically neutral atom
Mass number = number of protons + number of neutrons
So, number of neutrons = mass number – number of protons
