In solid-state physics and basic electricity, metals are considered good conductors of electric current mainly because of which characteristic of their atomic structure and bonding?

Introduction / Context:
When we say that metals are good conductors of electricity, we really mean that they allow electric charge to flow through them very easily when a potential difference is applied. This question tests your understanding of the microscopic reason behind this property. You are expected to connect the idea of metallic bonding, the presence of free or delocalised electrons and the resulting high electrical conductivity, which is a key concept in solid-state physics and basic electrical engineering.

Given Data / Assumptions:

• We are considering pure metallic solids such as copper, aluminium and silver.
• Electric current in a metal is due to the motion of charge carriers inside the metal.
• Metals have a regular lattice of positive metal ions surrounded by electrons.
• The options mention packing of atoms, presence of free electrons and melting point.

Concept / Approach:
In metallic bonding, atoms lose some of their outer (valence) electrons, which become delocalised and form an 'electron sea' or 'electron gas' spread throughout the metal lattice. The remaining positively charged metal ions occupy fixed positions. When a potential difference is applied across the metal, these delocalised electrons experience a force and drift through the lattice, producing an electric current. The very existence of these mobile charge carriers is the main reason for high conductivity. Packing of atoms and melting point can influence other properties like strength and thermal stability, but they are not the fundamental cause of high electrical conductivity in metals.

Step-by-Step Solution:
Step 1: Recall that electric current in a conductor is defined as the flow of charges per unit time.Step 2: Identify that in metals the charge carriers are electrons, not ions or protons.Step 3: Remember that metallic bonding creates a lattice of positive ions immersed in a sea of delocalised electrons.Step 4: When a voltage is applied, these delocalised electrons drift in a preferred direction and carry current.Step 5: Recognise that the presence of many such free electrons with high mobility is what makes metals good conductors.Step 6: Conclude that the key reason is the presence of free or delocalised electrons, not simply high melting point or loose packing of atoms.

Verification / Alternative check:
Compare metals with insulators like wood or plastic. Insulators do not have a sea of free electrons; their electrons are tightly bound in covalent or ionic bonds and cannot move freely. As a result, even if their atoms are closely or loosely packed, they do not conduct well. Semiconductors have fewer free carriers and therefore moderate conductivity. This comparison highlights that the crucial difference is the number and mobility of free electrons, confirming the correct option.

Why Other Options Are Wrong:
The idea that atoms are lightly or loosely packed is not generally true for metals; many metals have closely packed crystal structures and still conduct well. High melting point is related to bond strength and thermal stability, not directly to electrical conductivity. Saying that all of the above are equally important overemphasises secondary factors and ignores the primary role of free electrons.

Common Pitfalls:
Students sometimes memorise that metals conduct electricity without understanding the microscopic picture and may incorrectly attribute conductivity to hardness, density or melting point. Others may think in terms of empty spaces between atoms. To avoid such errors, always connect electrical conductivity in metals to metallic bonding and the presence of mobile, delocalised electrons.

Final Answer:
Metals are good conductors of electricity mainly because they contain a large number of free or delocalised electrons that can move through the lattice.