Which of the following particles is not present in all atoms : electron, proton, neutron ?

Answer

To determine which of the following particles is not present in all atoms: electron, proton, or neutron, we can analyze the characteristics of these subatomic particles.

### Step-by-Step Solution:

1. **Understanding Subatomic Particles**:
- Atoms are made up of three main subatomic particles: electrons, protons, and neutrons.
- Electrons have a negative charge, protons have a positive charge, and neutrons are neutral.

2. **Defining Atomic Number and Mass**:
- The atomic number (Z) of an atom is defined as the number of protons present in the nucleus of the atom.
- The atomic mass (A) is the total number of protons and neutrons in the nucleus.

3. **Presence of Protons and Electrons**:
- In a neutral atom, the number of electrons equals the number of protons, ensuring that the atom has no overall charge.
- Therefore, all atoms must have at least one proton (except in the case of hydrogen, which has one proton) and at least one electron.

4. **Presence of Neutrons**:
- Neutrons are present in the nucleus of most atoms, but not all. For example, hydrogen (the simplest atom) has one proton and no neutrons.
- The atomic mass of hydrogen is 1 (1 proton and 0 neutrons), which confirms that neutrons are not present in all atoms.

5. **Conclusion**:
- Since all atoms must have electrons and protons, but not all atoms have neutrons (as seen in hydrogen), the particle that is not present in all atoms is the neutron.

### Final Answer:
The particle that is not present in all atoms is the **neutron**.

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