An atom is the smallest unit of an element with a nucleus of protons and neutrons, surrounded by electrons in energy levels. It retains the element’s chemical identity and behaves according to the arrangement of its subatomic particles.

Physics FAQ by ECAICO

Q: What are the three basic particles in an atom?

The atom is made of protons (positive), neutrons (neutral), and electrons (negative). Protons and neutrons are in the nucleus; electrons occupy orbitals around it and define the atom’s reactivity and charge.

Q: What is the difference between atomic number and mass number?

The atomic number is the number of protons (Z) and defines the element. The mass number is the total of protons and neutrons (A = Z + N). Isotopes share atomic number but differ in mass number.

Q: What is the Bohr Model and how does it explain electron shells?

The Bohr model shows electrons revolving in fixed orbits or shells around the nucleus, with each shell holding a specific number of electrons defined by 2n². It works well for hydrogen but is limited for complex atoms.

Q: What is the Quantum Mechanical Model of the Atom?

The quantum mechanical model describes electrons as wave-like particles in orbitals—regions of probability—not fixed paths. It’s based on Schrödinger’s equation and accurately explains all atoms using quantum principles.

Q: What are quantum numbers and how do they define atomic orbitals?

Quantum numbers (n, ℓ, mℓ, ms) define each electron’s energy, shape, orientation, and spin. They determine the structure of orbitals and are essential to the modern atomic model from Schrödinger’s equation.

Q: What Are Orbits, Orbitals, and How Do the Bohr and Schrödinger Models Differ?

In the Bohr model, electrons revolve around the nucleus in fixed circular orbits. Each orbit corresponds to a distinct energy level. In contrast, the Schrödinger (quantum mechanical) model replaces orbits with orbitals—three-dimensional regions of space where electrons are most likely to be found.

What Is the Atom Made Of? Structure, Shells, and Behavior – ECAICO Physics FAQ

Atoms are the fundamental building blocks of all matter — from the air we breathe to the circuits in modern electronics. But what exactly is an atom made of? In this ECAICO Physics FAQ, we break down the structure of the atom, its subatomic particles, and how the arrangement of electrons in shells determines chemical behavior, bonding tendencies, and the electrical properties of materials like semiconductors.

Digital infographic showing atom structure, Bohr and quantum models, orbitals, and subatomic particles

Educational 16:9 graphic summarizing atomic structure, key models, subatomic particles, and electron behavior

Understanding atomic structure is not just for chemistry students — it’s essential knowledge for engineers, material scientists, and anyone working with energy, electronics, or advanced physics. Whether you’re exploring quantum mechanical models, comparing the Bohr model, or just trying to grasp how protons, neutrons, and electrons interact, this guide is your foundation.

Q1: What is an atom?

Answer:
An atom is the smallest unit of an element that retains its chemical identity. It consists of a dense central nucleus made of protons and neutrons, surrounded by electrons that occupy discrete energy levels (or shells). These subatomic particles interact through fundamental forces, and their arrangement determines the physical and chemical behavior of matter.

Diagram showing the basic structure of an atom with labeled protons, neutrons, and electrons arranged in shells around the nucleus.

The atom consists of a central nucleus containing protons and neutrons

Q2: What are the three basic particles in an atom?

Answer:
Atoms are composed of three fundamental subatomic particles:

Protons (p⁺): Positively charged particles located in the nucleus.
Neutrons (n⁰): Neutral particles (no charge), also located in the nucleus.
Electrons (e⁻): Negatively charged particles that orbit the nucleus in defined energy levels.

Each particle has unique properties that influence atomic mass, charge, and chemical behavior.

Diagram of proton, neutron, and electron with charge, mass, and location.

Subatomic particles and their key properties.

Q3: What is the difference between atomic number and mass number?

Answer:
The atomic number (Z) is the number of protons in an atom's nucleus and defines the element.
The mass number (A) is the total number of protons and neutrons in the nucleus.

Two atoms of the same element can have the same atomic number but different mass numbers — these are called isotopes.

Equation:

A = Z + N

A: Mass number
Z: Atomic number (protons)
N: Number of neutrons

Comparison of atomic number (Z) and mass number (A) with labeled equation A = Z + N.

Visualizing the difference between atomic number and mass number with the A = Z + N formula.

Q4: What is the Bohr Model and How Does It Explain Electron Shells?

The Bohr model of the atom, proposed by Niels Bohr in 1913, describes electrons as revolving around a small, dense nucleus in fixed circular orbits called energy levels or electron shells. These orbits represent quantized energy states in which electrons do not emit radiation.

Each shell is identified by a principal quantum number (n) and has a specific capacity governed by the formula:

Maximum Electrons per Shell = 2n²

K-shell (n = 1): 2 electrons
L-shell (n = 2): 8 electrons
M-shell (n = 3): 18 electrons

Electrons always fill the lowest available shell first, in accordance with the Aufbau principle. The Bohr model accurately explains hydrogen's spectral lines and provides a foundational framework for understanding atomic behavior, though it is limited for multi-electron systems.

Bohr model diagram showing atomic nucleus with labeled K, L, and M electron shells and maximum electrons formula

Visualization of electron shells in Bohr’s atomic model with labeled orbits (K, L, M).

Q5: What is the Quantum Mechanical Model of the Atom?

The quantum mechanical model is the modern and most accurate representation of the atom. Unlike the Bohr model, which depicts electrons in fixed orbits, this model describes electrons as existing in orbitals — regions of space where the probability of finding an electron is highest.

Electrons don’t follow defined paths but behave as both particles and waves, governed by the principles of quantum mechanics. This model arises from solving Schrödinger’s equation and introduces concepts like energy sublevels (s, p, d, f), electron clouds, and the uncertainty principle.

Key Equation (Schrödinger’s Time-Independent Equation):

Ĥ Ψ = E Ψ

Where:

Ĥ: Hamiltonian operator (total energy)
Ψ: Wavefunction describing the system
E: Energy of the electron

Quantum atom model with electron cloud and Schrödinger equation.

Schrödinger’s model shows electrons as a cloud around the nucleus, governed by a wave equation.

Q6: What are quantum numbers, and how do they define atomic orbitals?

Answer:
Quantum numbers are a set of four values that uniquely describe the energy, shape, orientation, and spin of each electron within an atom. They emerge from solving Schrödinger’s equation and are central to the quantum mechanical model. These numbers determine the structure of atomic orbitals and the probable electron location.

1. Principal Quantum Number (n):

Specifies the main energy level or shell (n = 1, 2, 3…). Higher n values indicate electrons are further from the nucleus and have greater energy.

2. Azimuthal Quantum Number (ℓ):

Defines the subshell or orbital shape (ℓ = 0 to n−1). Values correspond to:
ℓ = 0 → s-orbital (spherical)
ℓ = 1 → p-orbital (dumbbell)
ℓ = 2 → d-orbital (cloverleaf)
ℓ = 3 → f-orbital (complex)

3. Magnetic Quantum Number (m_ℓ):

Describes the orbital’s spatial orientation. It ranges from −ℓ to +ℓ.
For example, if ℓ = 1 → m_ℓ = −1, 0, +1 (representing p_x, p_y, p_z)

4. Spin Quantum Number (m_s):

Represents electron spin, either +½ or −½. According to the Pauli Exclusion Principle, no two electrons in an atom share all four quantum numbers.

Together, these four quantum numbers form a unique identifier for each electron, governing atomic behavior, periodic trends, and electron configurations.

Q7: What Are Orbits, Orbitals, and How Do the Bohr and Schrödinger Models Differ?

In the Bohr model, electrons revolve around the nucleus in fixed circular orbits. Each orbit corresponds to a distinct energy level, and electrons can transition between them by absorbing or emitting discrete energy quanta. This model effectively explains the hydrogen atom’s spectral lines but is limited in describing complex atoms.

In contrast, the Schrödinger (quantum mechanical) model replaces orbits with orbitals—three-dimensional regions of space where electrons are most likely to be found. These orbitals arise from mathematical solutions to the Schrödinger equation and come in various shapes (s, p, d, f). This model accurately accounts for the behavior of electrons in multi-electron atoms.

Bohr vs Schrödinger atomic model comparison – orbits vs orbitals with labeled visuals

Infographic showing the key differences between Bohr’s orbit concept and Schrödinger’s orbital model in atomic theory.

Summary and What's Next

Understanding the evolution from Bohr’s classical orbits to Schrödinger’s quantum orbitals marks a turning point in atomic theory. While Bohr’s model offered a structured view of electron energy levels, Schrödinger’s model introduced the concept of probability clouds that revolutionized our understanding of atomic behavior. These models form the backbone of modern quantum mechanics.

Wait for the next part in the Physics FAQ by ECAICO, where we’ll explore how these atomic models relate to chemical bonding and material properties.

ECAICO | Smarter. Greener. Engineering.

Physics FAQ by ECAICO – Part 1