What is atomic number of an atom?  

number of protons 

The correct answer is option 4. number of protons.

The atomic number of an atom is a fundamental characteristic that defines its identity. It represents the number of protons present in the nucleus of the atom.

Here's a more detailed explanation:

1. Protons: Protons are positively charged subatomic particles found in the nucleus of an atom. Each proton has a relative mass of approximately 1 atomic mass unit (amu).

2. Atomic Number (\(Z\)): The atomic number (\(Z\)) of an atom is equal to the number of protons it contains. It is a unique identifier for each element on the periodic table. For example, hydrogen (H) has an atomic number of 1, indicating that it has 1 proton in its nucleus. Carbon (C) has an atomic number of 6, indicating that it has 6 protons in its nucleus.

3. Identity of the Element:  Since the atomic number represents the number of protons in the nucleus, it determines the identity of the element. All atoms of the same element have the same atomic number. For example, any atom with 6 protons in its nucleus is carbon, regardless of the number of neutrons or electrons it may have.

4. Periodic Table: The elements in the periodic table are arranged in order of increasing atomic number. This arrangement reflects the periodicity of elemental properties. Elements with similar chemical properties are grouped together in columns (groups) on the periodic table.

In summary, the atomic number of an atom is the number of protons in its nucleus. It is a fundamental property that defines the identity of the element and its place in the periodic table.