IRN-Modern Model of Atoms

The Modern Model of Atomic Structure

Since ancient times, men have been studying and questioning the world of matter around them. Over two thousand years ago, a Greek philosopher by the name of Democritus put forth a theory about matter. His theory proposed that all substances were composed of very small particles, or atomos, that were too small to be seen by the human eye. Today we define matter as anything that has mass and volume. Let’s explore the contributions of some important scientists and how their work led to the modern model of atomic structure.

Scientists have continued to study matter and the tiny particles called atoms using models to help explain the structure of the atom. In the 1800s, an English chemist and school teacher by the name of John Dalton published an atomic theory. His theory, based on experimental evidence, stated that all substances were composed, or made of small particles called atoms that could not be divided or destroyed. It also proposed that all atoms within an element were the same and the atoms of different elements were different. In addition, Dalton believed that atoms could join with each other to create new substances. Dalton’s model of an atom consisted of a simple, solid sphere.

Toward the end of the nineteenth century, Dalton’s atomic theory was revised in order to better understand and explain new scientific observations and discoveries. In the late 1800s, an English scientist by the name of Joseph John Thomson found an error in Dalton’s theory. His experiments showed that atoms were not solid, undividable spheres but actually contained smaller, negatively charged particles. His model became known as the plum pudding model, a popular English dessert, because it looked like a ball of pudding with small pieces of plum scattered evenly throughout. The pudding represented the positively charged part of the atom and the plum pieces represented the negatively charged corpuscles, or electrons as they are called today.

In 1909, another English scientist by the name of Earnest Rutherford revised Thomson’s atomic theory. The results of his experiments led him to believe that the plum pudding model of atomic structure was incorrect. His atomic model consisted of a positively charged, extremely dense region in the center of the atom called the nucleus. Surrounding the nucleus was an area of empty space that contained the electrons.

Four years later, the atomic model was revised once again. Niels Bohr, a Danish scientist, believed that an atom’s electrons travel in fixed orbits or paths around the nucleus. He believed that these paths were located at different distances from the nucleus, and later proposed that electrons could jump from one path to another. James Chadwick, a student of Earnest Rutherford, proposed that positive particles called protons and neutral particles called neutrons made up the nucleus. Eventually these discoveries were combined and resulted in Bohr’s model, which is also known as the “planetary model.” In this model, three subatomic particles are present. They include protons, neutrons, and electrons.

Protons are positively charged particles located in the nucleus. Protons have a mass of 1 atomic mass unit. Atomic mass unit or amu is a unit of measure developed by scientists to represent the relative mass of the tiny particles inside of atoms. Also located within the nucleus are neutrons. Neutrons have no charge. They are neutral. Although they are slightly larger than protons, neutrons are also given a mass of 1 amu. Protons and neutrons are made up of even smaller particles called quarks. Located outside of the nucleus are the negatively charged electrons. Electrons are given an amu of zero because of their tiny size. It would take 1,800 electrons to equal the mass of one proton! Size at the atomic level is measured on a nanoscale.

The modern model of atomic structure is called the “electron cloud” model. Edwin Schrodinger, an Austrian physicist, proposed that electrons do not travel in static, or fixed paths. Instead, electrons travel in regions called electron clouds. An electron cloud is an area around the nucleus where electrons are most likely to be found.

As scientists continue to study the atom, their understanding of its structure continues to evolve. The two atomic models most commonly used are the Bohr model and the electron cloud (Quantum Mechanics) model. Although the Bohr model does not represent the three-dimensional characteristics of the atom and has the electrons traveling in fixed orbits, it is used because it allows the atom to be easily shown in two dimensions. Scientists believe that the electron cloud model better supports our current understanding of the structure of the atom and its role as the basic building block of matter.