In basic atomic theory, the formation of a negatively charged ion (anion) from a neutral atom usually occurs by which process?

Introduction / Context:
This question deals with the basic idea of ion formation in chemistry. It asks how a negatively charged ion, called an anion, is formed from a neutral atom. Understanding this process is fundamental for explaining ionic bonding, acid base reactions and the behaviour of salts in solution.

Given Data / Assumptions:

• We are considering a neutral atom in its ground state.
• The atom can gain or lose subatomic particles to form an ion.
• We focus specifically on the formation of an anion, which has an overall negative charge.
• The options mention changes in electrons, protons and neutrons.

Concept / Approach:
An ion is formed when an atom gains or loses electrons, not when it changes the number of protons in the nucleus. The number of protons determines the identity of the element and is not normally altered in chemical reactions. A neutral atom has an equal number of protons and electrons. If it gains extra electrons, the total negative charge exceeds the positive charge of the nucleus, and the atom becomes a negatively charged ion known as an anion. If it loses electrons, it becomes a positively charged cation.

Step-by-Step Solution:
1) Start with a neutral atom where the number of protons equals the number of electrons, giving net charge zero.2) An anion must carry an overall negative charge, which means the total negative charge from electrons is greater than the positive charge from protons.3) The simplest way to increase negative charge is for the atom to gain extra electrons, while the number of protons in the nucleus remains unchanged.4) Gaining one electron gives a charge of −1, gaining two electrons gives −2, and so on, depending on the valency of the element.5) Losing neutrons would change the mass number and isotope, but not the net charge, so it does not create an anion.6) Gaining protons would increase positive charge and move the atom to another element, which is not what happens in normal chemical processes forming anions.7) Therefore, formation of anions occurs by the gain of one or more electrons.

Verification / Alternative check:
Common examples illustrate this clearly. A neutral chlorine atom has 17 protons and 17 electrons. When it gains one electron, it has 17 protons and 18 electrons, giving a net charge of −1 and forming a chloride ion, Cl−. Similarly, an oxygen atom with 8 protons and 8 electrons can gain two electrons to form an oxide ion, O2−, with 8 protons and 10 electrons. In each case, the change in charge is due solely to electron gain. The nucleus, containing protons and neutrons, remains the same in ordinary chemical reactions.

Why Other Options Are Wrong:
Loss of neutrons from the nucleus: This changes isotope but not net electric charge, so it does not create a negative ion.
Gain of protons by the nucleus: Adding protons would increase positive charge and actually make it harder to form an anion; it also changes the element identity and is not a typical chemical process.
Simultaneous loss of protons and electrons: While this might leave the net charge similar, it is not a recognised mechanism for forming anions in basic chemistry.
No change in number of electrons or protons: This would leave the atom neutral, not converted into an ion.

Common Pitfalls:
Some learners mix up cations and anions and may incorrectly think that losing electrons leads to a negative charge. It helps to remember the phrase: cation is positive (think of the plus sign in the letter t), and anion is negative. Also, students sometimes forget that protons are fixed inside the nucleus in chemical reactions and that only electrons are exchanged or shared. Focusing on electrons as the mobile charge carriers in chemical processes makes it easier to recall that gaining electrons produces an anion.

Final Answer:
The formation of an anion occurs by the gain of one or more electrons by a neutral atom.