ATOMIC STRUCTURE

Fundamental Subatomic Particles

Particle	Symbol	Charge		Mass
electron	e-	-1		0.0005486 amu
proton	p+	+1		1.007276 amu
neutron	no	0		1.008665 amu

The number of protons, neutrons, and electrons in an atom can be determined from a set of simple rules.

• The number of protons in the nucleus of the atom is equal to the atomic number (Z).
• The number of electrons in a neutral atom is equal to the number of protons.
• The mass number of the atom (M) is equal to the sum of the number of protons and neutrons in the nucleus.
• The number of neutrons is equal to the difference between the mass number of the atom (M) and the atomic number (Z).

Examples: Let's determine the number of protons, neutrons, and electrons in the following isotopes.

12C

13C

14C

14N

The different isotopes of an element are identified by writing the mass number of the atom in the upper left corner of the symbol for the element. ¹²C, ¹³C, and ¹⁴C are isotopes of carbon (Z = 6) and therefore contain six protons. If the atoms are neutral, they also must contain six electrons. The only difference between these isotopes is the number of neutrons in the nucleus.

¹²C: 6 electrons, 6 protons, and 6 neutrons

¹³C: 6 electrons, 6 protons, and 7 neutrons

¹⁴C: 6 electrons, 6 protons, and 8 neutrons