Journey Inside the Atom · Lesson 8 of 14
Atomic Number and Mass Number
“Atomic number identifies an element while mass number counts its nuclear particles”
• Define atomic number and mass number. • Relate atomic number to proton count. • Use neutrality to find electron count. • Define nucleons and calculate neutron number. • Interpret standard atomic notation. • Solve multi-step atomic structure problems.
Two numbers allow us to describe an atom compactly. One identifies which element the atom belongs to; the other tells us the total number of heavy particles in its nucleus.
Atomic Number
The number of protons present in the nucleus of an atom. It is represented by Z and determines the identity of the element.
Hydrogen has one proton and atomic number one; helium has two protons and atomic number two. If the proton number changes, the identity of the element changes.
Atomic Number in a Neutral Atom
A neutral atom has no overall charge. Each proton contributes relative charge +1 and each electron −1, so the two numbers are equal in a neutral atom.
Mass Number
The total number of protons and neutrons in the nucleus. It is represented by A.
The protons and neutrons present in a nucleus.
Electrons are not included in mass number because their mass is negligible compared with protons and neutrons for the calculations in this chapter.
Standard Atomic Notation
Atomic notation places the mass number at the upper left of the symbol and atomic number at the lower left. Carbon with Z = 6 and A = 12 is written as ¹²₆C.
A Reliable Problem-Solving Method
First read Z and A. Set protons equal to Z. If the atom is neutral, set electrons equal to protons. Finally use A − Z to find neutrons. Keeping this order prevents the common mistake of adding electrons into mass number.
Problem
An atom has atomic number 26 and mass number 56. Find protons, electrons and neutrons.
- 1.Z = 26, so protons = 26.
- 2.The atom is neutral, so electrons = 26.
- 3.Neutrons = A − Z = 56 − 26 = 30.
- 4.The atom contains 26 protons, 26 electrons and 30 neutrons.
Problem
A nucleus contains 20 protons and has mass number 41. Find neutrons.
- 1.Protons = 20, so Z = 20.
- 2.Use neutrons = A − Z.
- 3.41 − 20 = 21.
- 4.The nucleus contains 21 neutrons.
Problem
An atom has atomic number 17 and 18 neutrons. Find mass number.
- 1.Z = 17, so protons = 17.
- 2.Mass number = protons + neutrons.
- 3.A = 17 + 18 = 35.
- 4.The mass number is 35.
Problem
A neutral atom with mass number 23 has 11 electrons. Find neutrons.
- 1.Neutrality gives protons = electrons = 11.
- 2.Therefore Z = 11.
- 3.Neutrons = 23 − 11 = 12.
- 4.The atom has 12 neutrons.
Atomic number counts only protons and identifies the element. Mass number counts protons plus neutrons. Electrons equal protons only for a neutral atom and are not included in mass number.
Quiz
Which description best matches Atomic Number?
Which description best matches Mass Number?
Which term matches this description: The number of protons present in the nucleus of an atom.
Which term matches this description: The total number of protons and neutrons in the nucleus.
Which statement is a key takeaway from this lesson?
Practice Problems
- A neutral atom has Z = 12 and A = 24. Find all three particle counts.
- An atom contains 15 protons and 16 neutrons. Find Z and A.
- A neutral atom has 17 electrons and A = 35. Find neutrons.
- Explain why different proton numbers mean different elements.
Key Takeaways
• Atomic number is the number of protons in the nucleus and uniquely identifies an element. • A neutral atom contains equal numbers of protons and electrons. • Mass number is the total number of protons and neutrons in the nucleus. • Number of neutrons can be calculated as mass number minus atomic number.