The nucleon number of an atom is the total number of

Step 1: Analyze the question The question asks what the nucleon number of an atom counts.

Step 2: Explain the concept A nucleon is any particle found in the nucleus. The two particles in the nucleus are:

• Protons (positively charged)
• Neutrons (no charge)

The nucleon number is also called the mass number, and it is written as A.

The relationship is:

Nucleon number (A) = Number of protons (Z) + Number of neutrons (N)

Step 3: Check each option

• A. electrons and protons: Wrong. Electrons are outside the nucleus, so they are not nucleons.
• B. electrons and neutrons: Wrong, for the same reason.
• C. neutrons minus electrons: Wrong. This is not a meaningful quantity, and electrons are not nucleons.
• D. protons and neutrons: Correct. These are the particles in the nucleus.

Step 4: Example Sodium-23 is written as ²³₁₁Na.

• Protons = 11
• Neutrons = 23 − 11 = 12
• Nucleon number = 11 + 12 = 23

Exam tip: "Nucleon" means "nucleus particle", so think "nucleus = protons + neutrons". Also remember that:

• Atomic number (Z) = protons only
• Nucleon (mass) number (A) = protons + neutrons