Scientists have been fascinated by the fundamental makeup of atoms, the basic building blocks of matter, for centuries. Atomic structure, which refers to the arrangement of the particles within an atom, is crucial to understanding the behavior of matter and the chemical and physical properties of substances. The study of atomic structure has evolved over time, as scientists have developed new models to explain the behavior of electrons and the nucleus within an atom. From the earliest models proposed by J.J. Thomson and Ernest Rutherford to the modern electron cloud model, our understanding of atomic structure has undergone significant changes. This topic is essential in the fields of chemistry and physics and has played a vital role in shaping our understanding of the world around us.
What is an element?
One type of atom makes up a pure substance called an element. Furthermore, elements are the basic building blocks of matter and chemical reactions cannot break them down into simpler substances.
To identify each element, scientists count the number of protons in the nucleus of its atoms, which they call the atomic number. For example, scientists determine carbon’s atomic number as 6 because it has six protons in its nucleus. They then organize the elements according to their atomic numbers in the periodic table.
Currently, scientists know 118 elements, of which 94 occur naturally on Earth. The remaining elements are artificially made in laboratories. Each element has its unique physical and chemical properties. As a result, they are useful for various applications.
What is an atom?
A nucleus of positively charged protons and neutral neutrons, surrounded by negatively charged electrons in shells or energy levels, forms the basic unit of matter called an atom. Notably, atoms measure around 0.1 to 0.5 nanometers in diameter and combine to make up everything, such as the air we breathe, the food we eat, and the water we drink.
To identify an atom’s element, scientists count the number of protons in its nucleus, which also determines its atomic number. Additionally, atoms of the same element can have different isotopes because of varying numbers of neutrons in the nucleus.
Atoms are held together by the electromagnetic force as the positively charged protons in the nucleus attract the negatively charged electrons in the shells. As a result, quantum mechanics explains the behavior of electrons. Furthermore, their arrangement in atoms affects the chemical and physical properties of elements.
| Particle | Location | Mass | Charge |
| Proton | Nucleus | 1 amu | +1 |
| Electron | Orbital | 0 amu | -1 |
| Proton | Nucleus | 1 amu | 0 |
What does an atom look like?
Thompson Model
J.J. Thomson proposed the first model of the atom, known as the “Thomson model” or the “plum pudding model,” in 1904. This model suggested that atoms consist of a positively charged substance with negatively charged electrons scattered throughout, similar to plums in a pudding or raisins in a cake. However, despite its initial popularity, scientists later found this model to be inaccurate since it did not account for the existence of a nucleus in the atom.
Rutherford/Planetary Model
In 1911, Ernest Rutherford proposed the “Rutherford model” or the “planetary model,” which became the second model of the atom. This model suggested that a tiny, central nucleus concentrated an atom’s positive charge and most of its mass, while negatively charged electrons orbited around it in a vast empty space.
Bohr Model
Danish physicist Niels Bohr proposed the Bohr Model of the atom in 1913. According to this model, electrons orbit the nucleus of an atom in distinct energy levels, or shells, with each energy level having a specific energy value and maximum number of electrons it can hold. Additionally, the model proposed that electrons can absorb or emit energy and jump between energy levels, explaining the characteristic spectra of different elements. The Bohr Model represented an advancement over the Rutherford Model and provided a framework for understanding the stability of atoms.
Quantum Model
The electron cloud model of an atom, also known as the quantum mechanical model, is a modern model of the atom that describes the behavior of electrons in atoms. It was developed in the 1920s and 1930s by several scientists, including Erwin Schrödinger and Werner Heisenberg, based on the principles of quantum mechanics.
In the electron cloud model, electrons are not thought of as particles orbiting the nucleus in specific paths, but instead are described by a probability distribution or “cloud” of possible locations where an electron may be found. The model includes the concept of energy levels and sub-levels, which determine the regions in space where the electrons are most likely to be found. The electron cloud model allows for a more accurate prediction of an atom’s properties, such as its electron configuration and the probability of electron interactions.