In a neutral atom, which pair of fundamental particles is always present in equal numbers so that the overall electric charge is zero?

Introduction / Context:
An atom is electrically neutral when its total positive charge balances its total negative charge. The three main subatomic particles are protons, neutrons, and electrons, each with characteristic charge and position in the atom. This question asks which pair of particles must be present in equal numbers to ensure that the net charge of an atom is zero. Understanding this balance is fundamental to atomic theory and basic chemistry.

Given Data / Assumptions:

• Protons carry a positive charge of plus one in elementary charge units.
• Electrons carry an equal magnitude of negative charge minus one.
• Neutrons have no electric charge.
• We consider a neutral atom, not an ion.

Concept / Approach:
Electric charge in atoms comes from protons and electrons. Protons reside in the nucleus, and electrons are located in regions around the nucleus. Neutrons add mass and contribute to nuclear stability but do not contribute to electric charge. In a neutral atom, the total positive charge from protons must equal the total negative charge from electrons. This means that the number of protons and electrons must be the same. If the numbers differ, the species becomes an ion, either positively or negatively charged.

Step-by-Step Solution:
Step 1: Recognise that protons provide positive charge and electrons provide negative charge, while neutrons are neutral. Step 2: For the net charge of an atom to be zero, total positive charge and total negative charge must cancel each other. Step 3: Let the number of protons be Z and the number of electrons also be Z in a neutral atom. Step 4: The total positive charge is then plus Z and the total negative charge is minus Z, so the sum is zero. Step 5: Conclude that in any neutral atom, the number of protons and the number of electrons must be equal.

Verification / Alternative check:
Consider a carbon atom as an example. Carbon has atomic number 6, meaning it has 6 protons in its nucleus. A neutral carbon atom also has 6 electrons around the nucleus. The total positive charge from 6 protons is plus 6, and the total negative charge from 6 electrons is minus 6. Together they give zero net charge. If you remove one electron, you get a positive ion with charge plus one; if you add one electron, you get a negative ion with charge minus one. These simple examples show that equality of proton and electron numbers defines neutrality.

Why Other Options Are Wrong:
Neutrons and electrons do not have equal charges in magnitude and neutrons are uncharged, so equal numbers of them do not guarantee neutrality. Protons and neutrons may be present in equal numbers in some nuclei, but this has no direct effect on electric charge because neutrons do not contribute to charge; equal numbers of protons and neutrons simply describe some stable isotopes. Protons and positrons both carry positive charge, so equal numbers of them would give a large positive charge and would not produce a neutral atom. Therefore, only protons and electrons must match in number for neutrality.

Common Pitfalls:
Learners may confuse atomic number and mass number, thinking that equal numbers of protons and neutrons are required for stability and neutrality. While equal proton and neutron counts can make some nuclei stable, neutrons do not affect charge. Another mistake is to overlook electrons when thinking about charge, focusing only on the nucleus. To avoid this, always remember that protons and electrons are the charged particles and that neutrality requires equal numbers of these two in an atom.

Final Answer:
In a neutral atom, the number of protons and electrons is equal, ensuring zero net electric charge.