Atomic number

Question

A diagram of an atom is missing its periodic-table identifier, but the number of protons in its nucleus is known. Which property can be determined directly from that count?

1. AIts atomic number✓ Correct

   The proton count establishes the atomic number.

2. BIts number of neutrons✗ Incorrect

   Isotopes of the same element can have different neutron counts.

3. CIts sample's physical state✗ Incorrect

   The proton count gives atomic number, not the sample conditions or its state.

4. DIts bond type in every compound✗ Incorrect

   A proton count alone does not specify the bonding in a particular compound.

Answer

A. Its atomic number

Rationale

The periodic table uses atomic number to identify an element. That number comes from the protons in its nucleus, even when its isotopes differ in neutron count.