Using a small set of rules, you can solve the number of particles in any isotope.

Rules: Use Periodic Table to find element symbol if you have atomic number and vice versa atomic number = protons mass number = protons + neutrons charge = protons – electrons

Example 1: fill in the blanks.

isotope	atomic #	p+	n	e–	mass #	charge
beryllium-9						0

Solution 1:
•atomic number = 4 (beryllium is element 4 on the Periodic Table)
•# of protons = 4 (same as atomic number)
•mass number = 9 (from the isotope name)
•# of neutrons = 5 (subtract atomic number from mass number)
•# of electrons = 4 (electrons = protons in a neutral element)

isotope	atomic #	p+	n	e–	mass #	charge
beryllium-9	4	4	5	4	9	0

Example 2

isotope name	atomic number	protons	neutrons	electrons	mass number	charge
		11	13	10

Solution:
•atomic number = 11 (same as # of protons)
•mass number = 24 (protons + neutrons)
•isotope name is “sodium-24” (element name and mass number)
•charge is +1 (protons – electrons)

isotope name	atomic number	protons	neutrons	electrons	mass number	charge
sodium-24	11	11	13	10	24	+1

 

Example 3:

isotope name	atomic number	protons	neutrons	electrons	mass number	charge
	23				49	+3

Solution:
•element is vanadium (element 23 on Periodic Table)
•isotope name is “vanadium-49” (element name and mass number)
•# of protons = 23
•# of neutrons = 26 (mass number minus protons)
•# of electrons = 20 (charge = protons – electrons; +3 = 23 – x)

isotope name	atomic number	protons	neutrons	electrons	mass number	charge
vanadium-49	23	23	26	20	49	+3

Extra comments about electrons and charge:
•if charge is negative, it will have more electrons than protons
•if charge is positive it will have less electrons than protons
•if charge is zero, it will have equal numbers of electrons and protons

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