Atom's structure

Timeline of atomic models

Timeline of atomic models

An atomic model is the representation of the internal structure of an atom.

Throughout history, the constitution of matter has had various models that have evolved until reaching the current atomic model.

Here is the timeline of the most important atomic models:

1. Model of Democritus of Abdera (year 450 BC)

The Democritus model of the atom was developed by the Greek philosopher Democritus of Abdera . Democritus believed that matter was made up of small indestructible or indivisible particles that he called atoms.

Atomic models, chronology and description of the models of the atomDemocritus, an ancient Greek philosopher, was one of the first to propose the idea that matter was made up of discontinuous, indivisible particles called "atoms." In his philosophy, atoms were the fundamental units of reality and were eternal and infinitely small.

Democritus believed that different types of matter were formed from combinations and arrangements of atoms in empty space.

Importance of the model

Although Democritus's model lacked experimental evidence and a solid scientific basis at the time, it laid the foundation for the idea that matter was composed of discrete, indivisible units. However, this model did not have an immediate impact on the scientific community of its time and was superseded by other ideas until it re-emerged with more force in the era of modern science.

Democritus's contribution is fundamental to the evolution of atomic theory throughout history, and his concept of the "atom" is the basis of how we understand the structure of matter today.

2. Model of John Dalton (1808)

Dalton's atomic theory was the first scientifically based model. Dalton defined the concept of atom and concluded that in chemical reactions, atoms are neither created nor destroyed, but rather changed their distribution.

The Dalton model of the atom, proposed by John Dalton in the early 19th century, was one of the first systematic attempts to explain the nature of matter and chemical reactions in terms of atoms.

Atoms as indivisible units

Dalton postulated that matter was composed of fundamental, indivisible particles called atoms. Each chemical element consisted of atoms identical to each other in terms of mass and chemical properties.

atoms of different elements

Atoms of different elements were different from each other in terms of mass and chemical properties. Dalton believed that atoms were the basic and indivisible units of matter and that chemical compounds were formed from the combination of atoms of different elements in fixed and defined proportions.

Conservation of mass

Dalton postulated that mass is conserved in chemical reactions. This means that the total mass of the atoms in the reactants is equal to the total mass of the atoms in the products after a chemical reaction.

Fixed proportions in compounds

Dalton proposed the Law of Definite Proportions, which states that elements combine in fixed, constant proportions to form specific chemical compounds. This means that the atoms of the elements combine in simple whole numbers to form compounds.

Law of multiple proportions

Dalton also formulated the Law of Multiple Proportions, which states that when two elements form more than one compound, the masses of one element that combine with a fixed mass of the other element are in the ratio of simple whole numbers.

3. Thomson 's model (1904)

Thompson discovered the electron as a subatomic particle with cathode ray tubes. These new negatively charged particles came to be considered part of the structure of atoms.

Thomson's model of the atom is also known as the raisin pudding model since JJ Thomson conceived of the atom as a raisin pudding. In this model, the atom would be formed by a positive mass to which the electrons would be attached like raisins in the cake.

Model features

Thomson conducted experiments with cathode ray tubes and discovered the existence of negatively charged subatomic particles that he called electrons. This discovery challenged the previous idea that atoms were indivisible and uniform.

According to Thomson's model, the negatively charged electrons were embedded in a positively charged matrix. This created a balance of electrical charges, resulting in a globally neutral atom. Visually, the atom was compared to a "raisin pudding", where the electrons represented the "raisins" and the positive charge the "pudding mass".

Thomson proposed that the positive charge was evenly distributed throughout the structure of the atom, thus keeping the electrons in equilibrium within it. This idea explained how atoms could be electrically neutral despite having separate electrical charges within them.

Model limitations

Although Thomson's model introduced the idea that atoms had subatomic components and charges, it did not explain why electrons did not collapse into the nucleus due to repulsive electrical forces between negative charges.

Thomson's model of the atom also failed to address how electrons were arranged in orbits around the nucleus.


Although Thomson's model was later superseded by more complete models, his discovery of electrons and his proposal for a subatomic structure dramatically changed scientific understanding at the time.

These findings paved the way for future developments in atomic theory and laid the foundation for more sophisticated models to explain the organization and behavior of electrons in atoms.

4. Rutherford model (1911)

In Rutherford's model, known as the "planetary" or "nucleus" model, Ernest Rutherford said that atoms were easily transposable by electrical charges. His model defined the atom as a heavy nucleus with a positive charge and that the electrons revolve around it so that most of the atom was made up of empty space.

Rutherford's model of the atom marked a crucial advance in the understanding of atomic structure in the early 20th century. The key aspects of this model are detailed below:


Rutherford and his team carried out an experiment in which they shot alpha particles (helium nuclei) at a thin sheet of gold. They expected the particles to pass through the foil with little deflection due to the common belief at the time that atoms were mostly empty space.

Discovery of the atomic nucleus

However, the result was surprising: some alpha particles deviated significantly and even bounced back. Rutherford concluded that this could only be explained if the atoms contained a small, dense, positively charged nucleus at their center, which concentrated most of the atom's mass.

Empty space in the atom

Based on his experiment, Rutherford also came to the conclusion that most of the atom consisted of empty space, since most of the alpha particles passed through the gold foil without deflecting at all.

Although Rutherford's model of the atom introduced the concept of a central nucleus, it did not offer a detailed explanation of how electrons were organized around the nucleus. Due to electrical forces between the opposite charges, the electrons should have collapsed into the nucleus according to classical electromagnetic laws.

Importance of the atomic model

Despite its limitations, Rutherford's model was an important milestone. He introduced the notion that atoms had a dense, positively charged nucleus at their center, which resolved some of the issues raised by earlier models.

Furthermore, this model paved the way for later developments in atomic theory, such as the Bohr Atomic Model.

5. Niels Bohr 's atomic model (1913)

Atomic models, chronology and description of the models of the atomBohr's model of the atom was a modification of Rutherford's model: the atom is like “a microscopic solar system” in which electrons orbit the nucleus. Bohr assumed that the electrons moved in circular orbits around the nucleus.

The atomic model proposed by Niels Bohr at the beginning of the 20th century was a significant advance in the understanding of atomic structure and the nature of electronic orbits.

Discrete energy levels

Bohr postulated that the electrons in an atom move in discrete, quantized orbits around the nucleus, rather than in continuous motions. Each orbit corresponded to a specific energy level for the electrons.

The electrons could not have intermediate energies, which gave rise to the stability of the atoms.

Absorption and emission of energy

According to Bohr's atomic model, electrons could absorb or emit energy in the form of quanta (discrete packets of energy) when moving between different energy levels.

When an electron absorbed energy, it jumped to a higher energy level; when it emitted energy, it returned to a lower energy level.

Atomic spectra

One of the main achievements of this model was the explanation of the emission and absorption spectra of atoms. The spectra were bright or dark lines in the light emitted or absorbed by the atoms. Bohr was able to relate the changes in the energy levels of the electrons to the specific lines in the spectra.

Model limited to simple atoms

Although the Bohr Model of the Atomic was successful in explaining certain aspects of atomic spectra and providing a quantitative description of electronic structure, it was applicable primarily to simple atoms with one electron, such as hydrogen. He had difficulty explaining more complex atoms and systems with multiple electrons.

Transitions and Planck's constant

Bohr introduced the idea that electrons could only exist in certain discrete states of energy, which were related to Planck's constant. This laid the foundation for the subsequent formulation of quantum theory.

Importance of the model

Although Bohr's atomic model was superseded by later developments in quantum theory, his concepts of quantized energy levels and electronic transitions provided a crucial approach to understanding the structure and behavior of atoms. His work paved the way for the evolution of quantum mechanics and represented an essential step in the modern understanding of atomic physics.

6. Schrödinger 's Atomic Model (1926)

Atomic models, chronology and description of the models of the atomThe Schrödinger model of the atom, also known as quantum mechanics, is a theoretical framework that describes the behavior of subatomic particles, such as electrons, in terms of probability and waves.

It was developed by the Austrian physicist Erwin Schrödinger in the 1920s and revolutionized our understanding of the atomic structure and properties of subatomic particles.

Probabilistic description

Unlike previous models that described the position and motion of particles in a deterministic way, the Schrödinger Atomic Model introduces the idea that subatomic particles, such as electrons, do not have definite trajectories, but are instead described by functions of wave that represent the probability of finding them in different positions.

Schrödinger equation

The basis of the model is the Schrödinger equation, a fundamental mathematical equation that describes how a particle's wave function changes over time in a given force field, such as the electric field created by the nucleus in an atom.

Solving this equation allows us to obtain information about the properties of the particle, such as its energy and its spatial distribution.

Orbitals and wave functions

In the context of atoms, wave functions are known as orbitals. Orbitals are regions of space where there is a high probability of finding an electron. The different types of orbitals (s, p, d, f) have characteristic shapes and probability distributions, which determine how electrons are organized around the nucleus.

Heisenberg's uncertainty principle

Schrödinger's model introduced the Heisenberg uncertainty principle, which states that there are fundamental limitations to simultaneously accurately measuring the position and momentum (or velocity) of a particle.

The more precisely the position of a particle is known, the less precisely its momentum can be known, and vice versa.

Evolution of quantum theory

Schrödinger's Atomic Model and quantum mechanics in general have led to a fundamental change in the way we understand the subatomic nature of the world. Quantum theory applies not only to atomic physics, but also to areas such as particle physics, quantum chemistry, and electronics.

7. Current atomic model (1926)

The current atomic model, based on quantum theory, is a complex and precise description of the structure and behavior of atoms and subatomic particles. It is derived from quantum mechanics, which is a fundamental theory in physics that describes the behavior of particles at subatomic scales.

The current atomic model is the result of contributions from various scientists throughout the 20th century. Its development cannot be attributed to a single person, since it was the result of the collaboration and cumulative advances of many scientists in the field of theoretical and experimental physics.

The works of scientists such as Max Planck, Albert Einstein, Niels Bohr, Werner Heisenberg, Erwin Schrödinger, Paul Dirac, Richard Feynman and many others, contributed significantly to the formulation of the principles and concepts that form the basis of the current atomic model and quantum theory in general.

Fundamental particles

The current atomic model recognizes a number of fundamental subatomic particles, such as electrons, protons, and neutrons, that make up matter. In addition, there are other, more exotic particles, such as quarks and leptons, which make up the most basic building blocks of matter.

Atomic nucleus

The atomic nucleus is made up of protons (positively charged) and neutrons (no electrical charge). The protons and neutrons are held together by the nuclear force, which is a strong interaction that keeps the nucleus cohesive despite the electrical repulsion forces between the protons.

Electrons and orbitals

Electrons, which are negatively charged, are distributed around the nucleus in high-probability regions called orbitals, or electron clouds. Each orbital has a specific set of quantum numbers that describe its characteristics, such as its shape, energy, and orientation.

Heisenberg's uncertainty principle

Quantum theory introduced the Heisenberg uncertainty principle, which states that there are fundamental limits to the precision with which we can simultaneously measure a particle's position and momentum (or velocity). This reflects the probabilistic and wave nature of subatomic particles.

Quantum field theory

Current quantum theory is based on quantum field theory, which unifies particles and interactions into a coherent framework. It describes how particles interact through quantum fields and how these interactions give rise to physical phenomena.

Publication Date: August 15, 2021
Last Revision: August 17, 2023