According to the periodic law of elements, the variation in properties of elements is related to their :
atomic masses
nuclear masses
atomic numbers
nuclear neutron-proton number ratios.

Options

A. atomic masses
B. nuclear masses
C. atomic numbers
D. nuclear neutron-proton number ratios.

Answer

To solve the question regarding the periodic law of elements and the variation in properties of elements, we can follow these steps:

### Step 1: Understand the Periodic Law
The periodic law states that the properties of elements are a periodic function of their atomic numbers. This means that when elements are arranged in order of increasing atomic number, elements with similar properties appear at regular intervals.

**Hint:** Recall that the periodic law is based on the arrangement of elements by atomic number, not atomic mass.

### Step 2: Analyze the Given Options
We have four options to consider:
- Atomic masses
- Nuclear masses
- Atomic numbers
- Nuclear neutron-proton number ratios

**Hint:** Think about which of these options is directly related to the arrangement of elements in the periodic table.

### Step 3: Identify the Correct Option
Since the modern periodic table is arranged according to atomic numbers, the variation in properties of elements is primarily related to their atomic numbers. Elements with the same number of valence electrons typically exhibit similar chemical properties and are grouped together.

**Hint:** Focus on the concept of groups and periods in the periodic table, which are defined by atomic number.

### Step 4: Conclusion
Based on the understanding of the periodic law and the analysis of the options, the correct answer is **atomic numbers**.

**Final Answer:** Atomic numbers