Atoms And Nuclei

Step 1: Understanding the Question:
The question asks for the mass number of a lithium atom, given the number of its subatomic particles.
Step 2: Key Formula or Approach:
The mass number ( ) of an atom is defined as the total number of protons ( ) and neutrons ( ) in the nucleus.
Step 3: Detailed Explanation:
From the problem statement, we have:
- Number of protons ( ) = 3
- Number of neutrons ( ) = 4
- Number of electrons = 3
The number of electrons does not contribute to the mass number, as the mass of an electron is negligible compared to that of a proton or a neutron.
Using the formula for the mass number:
Step 4: Final Answer:
The mass number of the lithium atom is the sum of its protons and neutrons, which is 7.