Atomic structure is a fundamental concept within physical chemistry that deals with the study of the structure of an atom. Basically, the atomic structure refers to the structure of an atom, which consists of a nucleus at its center and electrons orbiting around this nucleus.
The nucleus of an atom contains two types of subatomic particles, named protons and neutrons. Protons are positively charged particles, while neutrons have no charge (i.e. neutral). Electrons are negatively charged particles which orbit the nucleus at various energy levels or electron shells.
Who Discovered Atom First?
The word atom is derived from the Greek word “a-tomio” which means “uncuttable” or “indivisible”, i.e. an ultimate particle which cannot be further subdivided. An ancient Greek philosopher named Democritus first proposed an idea of atom in the 5th century BCE. He, along with his mentor Leucippus, said that atoms are the fundamental building blocks of matter.
According to them, everything in the universe like apple, wood, iron, you and me, all are composed of small particles. Everything can be cut into small pieces, but a limitation is it. And when we reach on that limitation, then we get the smallest piece, which is named atom by Democritus.
According to Democritus, an atom is the smallest unit of matter. The meaning of an atom is indivisible, meaning which would not be further divisible. He said that all the matters are composed of tiny indivisible particles called atoms, which are eternal, invisible, and vary in form, size, and position. He believed that the differences in the properties of materials are due to the differences in their constituent atoms, including the shape, arrangement, and type of atoms.
Although the ideas of Democritus were purely speculative and no scientific experimental evidence of it. Still, to be of the philosopher, he thought about the atom. And therefore, there is an importance to him because he was the first person that thought about the atom and said matter is made up of mixing of atoms.
Dalton’s Atomic Theory
After the time of Democritus, there is no significant progress to know the about the basic structure of an atom for almost 2300 years. In 1803, there was the next stage in the story of the atom when an English chemist and teacher, John Dalton, presented the first modern scientific theory about the basic structure of an atom, or simply atomic structure.
This theory is known as Dalton’s atomic theory, which is based on certain facts and experimental evidence that were known at the time. This theory was a landmark in the history of chemistry, which laid the foundations for our understanding of matter. The major postulates of this atomic theory are as follows:
- All matter in this universe is composed of tiny indivisible particles called atoms, and the atom is the smallest structural unit of an element.
- In terms of their mass and chemical and physical properties, atoms of the same element are identical in mass and properties and atoms of different elements are very different.
- Atoms chemically combine with each other in simple whole number ratios to form molecules or compounds. For example, water is always made of two hydrogen atoms and one oxygen atom (H2O), reflecting a fixed ratio of elements.
- In chemical reactions, atoms of elements retain their identity, but they simply rearrange and reorganize in the formation of other new substances.
- Atoms are neither created nor destroyed in chemical reactions but can be transformed from one form to another.
Dalton’s atomic theory successfully explained several fundamental laws of chemical reactions, such as the law of conservation of mass, law of constant composition (or definite proportions), law of multiple proportions, and law of reciprocal proportions.
Demerits of Dalton’s Atomic Theory
Dalton’s atomic theory was the first successful attempt which gave us some idea about the inner atomic structure of a matter. It successfully explained several laws of chemical combination by mass but it failed to explain Gay-Lussac’s law of combining volumes. The main failures of Dalton’s atomic theory are as follows:
- Dalton’s atomic theory failed to justify how atoms of distinct elements differ from each other, i.e. it did not tell anything about the structure of an atom.
- It could not explain how and why atoms of different chemical elements combine with each other to form molecules or compounds.
- It failed to explain the nature of forces that bind different atoms together in molecules.
- It failed to explain Gay-Lussac’s law of combining volumes, which states that when gases react together at constant temperature and pressure, the volumes of the reacting gases and the volumes of any gaseous products are in simple whole number ratios to each other. For example, two volumes of hydrogen gas react with one volume of oxygen gas to produce two volumes of water vapor (in gaseous form), all measured under the same temperature and pressure conditions.
Modern Atomic Theory for Atomic Structure
Dalton’s atomic theory laid the foundation for modern chemistry by providing a simple yet powerful explanation of the atomic structure of matter. It held a ground for about a century and no changes were made in this theory. However, towards the end of the 19th century and at the beginning of the 20th century, new discoveries were made by Sir J.J. Thomson, Ernest Rutherford, Niels Bohr, Chadwick, and other scientists.
These discoveries revealed the complex inner structure of atoms and laid the foundation for modern atomic theory and quantum mechanics. In the context of these discoveries, the atomic theory of John Dalton was suitably modified. The main modification made in this theory are as:
- An atom is no longer considered being indivisible particle. It is found that an atom has a complex structure that is composed of electrons, protons, neutrons, and many other sub-atomic particles. But still the atom is the smallest unit which takes part in the chemical reaction.
- Atoms of the similar element may not be alike in all respects. It has been discovered that atoms of the same chemical element can possess different relative atomic masses and are called isotopes. For example, hydrogen has three isotopes: protium (1H), deuterium (2H), and tritium (3H).
- Protium: This is the most common isotope of hydrogen. It has one proton and no neutrons in its nucleus. Therefore, its relative atomic mass is approximately 1.
- Deuterium: This isotope of hydrogen has one proton and one neutron in its nucleus, giving it a relative atomic mass of approximately 2. Deuterium is also known as heavy hydrogen.
- Tritium: Tritium contains one proton and two neutrons in its nucleus. This gives it a relative atomic mass of approximately 3. Tritium is radioactive, and it is the least common of the three hydrogen isotopes.
Such atoms of the same element which differ in their atomic masses are called isotopes.
- In certain cases, atoms of different elements may possess the same relative masses. For example, the atomic mass of two different elements, calcium and argon, is same (40 a.m.u). Such atoms of different chemical elements which possess the same atomic mass are called isobars.
- Though atom is made up of sub-atomic particles, as yet it is the smallest unit or particle which takes part in chemical reactions.
- It is not necessary to be a simple numerical ratio between the constituent elements of the compound. Some organic compounds, like sugar, proteins, carbohydrates, polymers, etc. do not possess a simple numerical ratio. For example, the ratio of C, H, and O in the sugar molecule (C12H22O11) is 12 : 22 : 11, which is not simple.
- With nuclear reactions, atoms of one element can be transformed into atoms of other elements. For example, atoms of nitrogen can be converted into oxygen atoms by interaction with alpha particles (helium nuclei). This is called transmutation.
- It is possible to transform the mass of an atom into energy. According to Einstein’s equation E = mc2 (where E represents energy, m represents mass and c represents velocity of light in a vacuum, which is approximately 3 × 108 meters per second (m/s)), mass and energy are interconvertible.
Thus, an atom is no longer indestructible. However, atoms remain unchanged during the chemical reactions.
Basic Structure of Atom
The basic structure of an atom refers to the arrangement and composition of its fundamental sub-atomic particles: protons, neutrons, and electrons. Atoms are the smallest units of matter and the defining structure of elements. Here is a brief overview of the atomic structure:
1. Nucleus: The nucleus was discovered by Ernest Rutherford in 1911. It is located at the center of the atom, which is a dense region containing protons and neutrons. Protons have a positive charge, while neutrons have no charge. The nucleus accounts for nearly all the mass of an atom, but only a tiny fraction of its volume.
2. Protons: The proton was first observed by Eugen Goldstein in 1886 through canal rays. The discovery of Goldstein was important in advancing the understanding of atomic structure, leading to further developments by scientists such as Rutherford, who identified the proton as a main constituent of the nucleus.
Protons are positively charged particles found within the atomic nucleus. The mass of one proton is 1.67 * 10-27 kilograms (kg), or 1.67 * 10-24 grams (g). Charge of a proton is 1.6 * 10-19 Coulombs (C). The number of protons in the nucleus defines the atomic number of the chemical element.
3. Neutrons: James Chadwick discovered the neutron in 1932. Neutrons are electrically neutral, meaning they carry no charge. Like protons, neutrons are also located in the nucleus of an atom. The mass of a neutron is very similar to that of a proton, approximately 1.67 × 10−27 kilograms (kg). The number of neutrons can vary within atoms of the same element, resulting in different isotopes.
4. Electrons: In 1897, J. J. Thomson discovered an electron when he was studying the properties of cathode rays. Electrons are negatively charged particles that orbit the nucleus at various energy levels or shells. The mass of an electron is 9 * 10-28 grams (g), or 9.11 * 10-31 kg. The charge on an electron is 1.6 * 10-19 C.
Despite their much smaller mass compared to protons and neutrons, electrons play an important role in chemical reactions and bonding between atoms. The number of electrons in a neutral atom equals the number of protons that balances the positive charge of the nucleus with an equal amount of negative charge. This balance results in an atom with no net electric charge.
FAQ on Atomic Structure
Here’s a set of frequently asked questions on atomic structure with concise answers that students often search in Google.
What is an atom?
An atom is the smallest fundamental particle of an element that has the properties of that element.
What is meant by atomic structure?
The atomic structure of an element refers to the composition of its nucleus and the arrangement of electrons revolving around it. Primarily, the atomic structure is concerned with three types of fundamental particles: protons, electrons, and neutrons. The protons and neutrons constituent the nucleus of the atom and electrons orbit around the nucleus.
Why is atomic structure important to understand in chemistry?
Atomic structure is important to understand to all aspects of chemistry because it provides a base for understanding properties of elements, chemical bonding, predicting chemical reactions, states of matter, periodic table organization, and many more concepts.
What are three historical models of the atom?
The three historical models of the atomic structures are: J J Thomson’s Atomic Model, Rutherford’s Atomic Model, and Bohr’s Atomic Theory.
What are subatomic particles?
Subatomic particles are those particles that make up an atom. This term generally refers to protons, electrons and neutrons. These three types of subatomic particles are fundamental to the structure of atoms and play significant roles in defining the properties and behaviors of elements.