In chemistry, what do we call an electrically charged atom or group of atoms?

Introduction / Context:
Many chemical reactions, particularly those in aqueous solution, involve charged particles that move between reactants and products. Understanding the basic term for an electrically charged atom or group of atoms is essential for studying acids, bases, salts and electrochemistry. This question checks whether the learner recognises that such charged species are called ions and can be either positively or negatively charged.

Given Data / Assumptions:
- We are considering atoms or groups of atoms that carry a net positive or negative charge.
- The options include the terms ion, meson and electron, along with a none of the above choice.
- Standard school level definitions of these terms are assumed.
- The focus is on common chemical terminology rather than subatomic particle physics.

Concept / Approach:
An ion is defined as an atom or group of atoms that has gained or lost one or more electrons and therefore carries a net charge. A cation is positively charged and an anion is negatively charged. A meson is a type of subatomic particle studied in nuclear physics, not a chemical species formed by atoms in reactions. An electron is a fundamental negatively charged subatomic particle, not an atom or group of atoms as described in the question. Therefore the correct term for a charged atom or group of atoms is ion.

Step-by-Step Solution:
Step 1: Recall that when a neutral atom loses electrons, it becomes positively charged and is called a cation. Step 2: Recall that when a neutral atom gains electrons, it becomes negatively charged and is called an anion. Step 3: Recognise that both cations and anions are collectively referred to as ions. Step 4: Note that the question explicitly mentions an atom or group of atoms, which matches the definition of an ion that can be monatomic or polyatomic. Step 5: Conclude that the correct term among the options is ion.

Verification / Alternative check:
To verify, consider common examples. Na+ is a sodium ion formed when sodium loses one electron. Cl- is a chloride ion formed when chlorine gains one electron. SO4 2- is a sulfate ion, an example of a polyatomic ion, which is a group of atoms carrying a net charge. In each case, we refer to these charged species as ions. The consistent use of this term in chemical formulas, ionic equations and textbooks confirms that ion is the correct answer.

Why Other Options Are Wrong:
Option B, meson, refers to a category of particles such as pions and kaons that occur in high energy physics, not to charged atoms or molecular groups in chemistry. Option C, a electron, is grammatically incorrect and conceptually wrong in this context, because an electron is a single subatomic particle, not an atom or group of atoms. Option D, none of the above, cannot be correct because the term ion accurately describes the concept asked about in the question.

Common Pitfalls:
Learners may confuse the term ion with other words like atom, molecule or electron, especially when first encountering chemical terminology. Another pitfall is mixing up the words cation and anion without realising they are subtypes of ions. Paying attention to precise definitions and practising with many examples of ionic compounds and polyatomic ions can help solidify the understanding that any charged atom or group of atoms is called an ion.

Final Answer:
An electrically charged atom or group of atoms is called An ion.