Simple Atomic Structure

An atom consists of a central nucleus, containing protons and neutrons, surrounded by electrons. Nearly all of the atom's mass is concentrated in the nucleus, while the electrons occupy the surrounding space and govern the atom's chemical behaviour.

The dense central core of an atom, containing protons and neutrons and holding nearly all of the atomic mass, is called the nucleus.

A positively charged particle found inside the nucleus, composed of quarks, is called a proton.

An electrically neutral particle found inside the nucleus, comparable in mass to the proton, is called a neutron.

A negatively charged fundamental particle that orbits the nucleus in shells/orbitals and determines the atom's chemical behaviour is called an electron.

The number of protons present in the nucleus of an atom, which uniquely identifies a chemical element, is called the atomic number (Z).

The total number of protons and neutrons present in the nucleus of an atom is called the mass number (A).

Atoms of the same element that have the same atomic number but different numbers of neutrons (and hence different mass numbers) are called isotopes.

A charged atom, formed when a neutral atom gains or loses electrons, is called an ion.

A positively charged ion, formed when an atom loses one or more electrons, is called a cation.

A negatively charged ion, formed when an atom gains one or more electrons, is called an anion.

The force that binds protons and neutrons together in the nucleus, overcoming the electrostatic repulsion between protons, is called the strong nuclear force.

The process by which a free neutron (outside the nucleus) transforms, occurring with a defined half-life, is called beta decay.

\[\rho=\frac{\text{mass of nucleus}}{\text{volume of nucleus}}\]

Relationship between mass number, atomic number, and neutron number:

A = Z + N

where

• A = mass number,
• Z = atomic number (number of protons),
• N = number of neutrons.

• The atom is composed of a nucleus surrounded by electrons.
• The nucleus contains protons and neutrons, collectively called nucleons.
• Nearly all of the atomic mass is concentrated in the nucleus, since electrons have a very small mass in comparison.

Properties of the Proton

• Carries a positive elementary charge.
• Has a defined mass and radius.
• Composed of quarks.
• Possesses intrinsic spin.
• Stable under normal conditions and participates in nuclear interactions.

Properties of the Neutron

• Electrically neutral (no charge).
• Comparable in mass to the proton.
• A free neutron undergoes beta decay with a defined half-life.
• Contributes to nuclear stability when bound inside a nucleus.
• Plays a central role in nuclear fission and fusion reactions.
• Variation in neutron number (with the same proton number) gives rise to isotopes.

Properties of the Electron

• Carries a negative elementary charge.
• Has a very small mass compared to protons and neutrons.
• Treated as a fundamental particle (not made of smaller constituents).
• Arranged in shells/orbitals around the nucleus.
• Determines the atom's chemical behaviour and bonding.
• Gain or loss of electrons converts a neutral atom into an ion.

Atomic Number and Mass Number

• Atomic number (Z) = number of protons; defines the identity of the element.
• Mass number (A) = number of protons + number of neutrons.

Formation of Ions

• Loss of electron(s) by an atom results in a net positive charge, forming a cation.
• Gain of electron(s) by an atom results in a net negative charge, forming an anion.
• The number of protons (and hence atomic number) remains unchanged during ion formation; only the electron count changes.

1. Find the mass: M = A × m_n ​= 64 × 1.67 × 10⁻²⁷

2. Find the volume: V = \[\frac {4}{3}\]​πr³ ≈ 2.87 × 10⁻⁴⁴ m³

3. Find the density: d = \[\frac {M}{V}\] ​≈ 3.72 × 10¹⁸ kg m⁻³

Result: The nucleus has an extremely high density because a large amount of mass is packed into a very tiny volume.

• Atom = Nucleus (protons + neutrons) + Electrons (in shells/orbitals around the nucleus).
• Proton: positive charge, composed of quarks, stable.
• Neutron: neutral charge, comparable mass to proton; free neutron undergoes beta decay.
• Electron: negative charge, fundamental particle, governs chemical behaviour.
• Atomic number (Z) = number of protons.
• Mass number (A) = number of protons + number of neutrons (A = Z + N).
• Isotopes: same Z, different N (and hence different A).
• Ions: cation formed by electron loss; anion formed by electron gain; proton count unchanged.
• Nuclear density is extremely high, e.g., copper nucleus ≈ 3.72 × 10¹⁸ kg m⁻³.