Atomic number of any element is the number of protons present in the nucleus of an atom of that element. The protons are present in the nucleus and they do not take part in the reactions. Therefore, the atomic number of an element which is nothing but the number of protons present in the nucleus of an atom of that element also remains same. Diatomic molecule is a molecule made up of 2 atoms, Example: Oxygen(O2). Triatomic molecule is a molecule made up of 3 atoms. Example: Water(H2O), Ozone(O3), etc.

Atomic number of an atom Is the no. of protons or electrons in the element. yes for eg . Na atom when loose one electron becomes Na+ it's atomic no. changesdiatomic molecule=O2triatomic molecule =O3

Atomic Number:
The atomic number of an element refers to the number of protons present in the nucleus of an atom of that element. It is denoted by the symbol "Z" and is unique for each element. The atomic number determines the identity of an element and its position in the periodic table.

Change in Atomic Number when Atoms Form Ions:
The atomic number of an element remains the same when its atoms form ions. This is because the atomic number is defined by the number of protons in the nucleus, which does not change during the ionization process. However, the number of electrons in an atom may change when it forms an ion. If an atom loses or gains electrons, it becomes an ion with a positive or negative charge, respectively. The number of electrons in an ion determines its charge, but it does not affect the atomic number.

Example of Diatomic and Triatomic Molecules:
- Diatomic Molecule: A diatomic molecule consists of two atoms of the same or different elements that are chemically bonded together. One example is oxygen (O2), which consists of two oxygen atoms bonded together by a double covalent bond. Another example is hydrogen (H2), where two hydrogen atoms are bonded by a single covalent bond.

- Triatomic Molecule: A triatomic molecule consists of three atoms of the same or different elements bonded together. One example is carbon dioxide (CO2), which consists of one carbon atom bonded to two oxygen atoms by double covalent bonds. Another example is water (H2O), where two hydrogen atoms are bonded to one oxygen atom by single covalent bonds.

In Summary:
- Atomic number refers to the number of protons in an atom's nucleus.
- The atomic number remains the same when atoms form ions, as it is determined by the number of protons.
- Diatomic molecules contain two atoms, such as O2 and H2.
- Triatomic molecules contain three atoms, such as CO2 and H2O.