Dalton’s Atomic Theory

Dhristi JEE 2022-24

Dalton’s Early Model of The Atom

The introduction to the early theory of the atom was done by a scientist named John Dalton (1766-1844). He was a British physicist, chemist, and meteorologist who is well known for many of his contributions to the pioneering research of atoms, the law of partial pressures, Daltonism, etc. Dalton's atomic model showed the way to many future works, researches regarding atomic theory, even though his conclusions were rather incorrect. He considered the atom as the smallest, indivisible unit of matter and wrote several postulates. Dalton’s model suggested the atom to be a ball-like structure that cannot be further divided. He also symbolized different atoms. Each atom was circular and bears different symbols.

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Dalton’s Atomic Model

The matter has been a subject of fascination since the beginning. Democritus was known to be the first to suggest that matter is made up of particles. Dalton’s model came almost two millennia later and brought further light to the topic. However, the ideas from researches of methane and ethylene might have helped define Dalton’s atomic theory at the time.  

 

The theory of Dalton was published in the paper “New Chemical Philosophy”. Dalton’s idea for the theory is believed to be inspired by the physical properties of gases. Although connections of his work have been made with several other chemists of the time. The definition of dalton's atomic theory brought the novel concept of calculating relative atomic weights. 

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Postulates of Dalton’s Atomic Theory

The law of conservation of mass and constant proportions are the basis that helps explain Dalton’s atomic theory. Based on these laws Dalton’s atomic theory states the following postulates:

  • Atoms are considered to be a matter which is made up of very small, indivisible particles.

  • All the atoms in an element are identical. So, they have the same size, shape, mass, and chemical properties. Different elements have different properties i.e. masses, sizes, shapes, and other chemical properties.

  • Atoms can neither be created nor be destroyed or subdivided into a chemical reaction.

  • Atoms of different elements combine in whole numbers in a simple but fixed ratio to form compounds. Different types of atoms are joined together to form compounds.

  • A chemical reaction is a rearrangement of atoms, where the formation of new products occurs due to the rearrangement of atoms in the reactant. The number of atoms before and after a chemical reaction remains the same. 

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Limitations of Dalton’s Atomic Theory

Although Dalton’s atomic theory marked a significant turning point in the research regarding the matter, the theory wasn’t entirely faultless. In further research, his theories were proven wrong.    

  • As it was found, later on, atoms are not indivisible. Subatomic particles like protons, electrons, and neutrons have been discovered since then.

  • Dalton’s atomic theory states atoms of an element are identical in mass. However, a single element having different atomic masses has been observed. These are called isotopes.

  • According to the theory of Dalton, compounds are formed when atoms combine in whole numbers. This fails to explain the formation of complex organic compounds.

  • Dalton’s model failed to explain the existence of isobars. Atoms of different elements when having the same mass, are called isobars.

  • The theory fails to explain the difference in properties of allotropes. It can not prove the difference in properties of charcoal, diamond, and graphite; allotropes of carbon. 

 

Influences on Modern Atomic Theory

Dalton’s atomic theory contributed a lot to modern atomic theory. Dalton’s model was revolutionary for the period and gave much to the new chemists to research upon. The Atomic theory got modified with the contribution of many after Dalton, namely, Chadwick, JJ Thompson, Ernest Rutherford, Niels Bohr etc. Later, JJ Thompson discovered electrons and Rutherford worked on the model to discover the nucleus. 

 

Finally, Niels Bohr’s model and the Quantum mechanical model provided the modern atomic model as it is known today. Modern atomic theory, though evolved in two centuries, holds much of Dalton’s atomic theory. 

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FAQs (Frequently Asked Questions)

1. What are the Laws Supporting Dalton’s Atomic Theory?

Dalton’s atomic model was mainly based on two laws. The two laws are: 


Law of Conservation of Mass: The law of conservation of mass states that mass cannot be created or destroyed. In a chemical reaction, the mass of the reactant side is equal to the mass of the product side i.e. the mass remains the same both before and after a chemical reaction takes place. Antoine Lavoisier in 1789 proposed the law.


Law of Constant Proportions: The law states that a compound will always have a constant proportion of its constituent element. For example, Table salt (NaCl) will have the exact proportion of Sodium (Na) metal and Chlorine (Cl) gas irrespective of where it came from.    

2. What are the Merits of Dalton’s Atomic Theory?

Despite its various shortcomings, Dalton’s atomic theory explained a lot and provided a framework for the modern atomic theory.

  • It explained that matter is made up of small particles called atoms. The atoms get combined, arranged, and rearranged during a chemical reaction.

  • Atoms of an element have various similar physical and chemical properties.

  • Compounds are formed upon a combination of two or more types of atoms. It explains the difference between elements and compounds.

  • The law of conservation of mass and constant proportions remain true for the atomic theory.

  • The theory explains the laws of constant proportions, multiple proportions, and reciprocal proportions.

3. What is Rutherford’s Atomic Model and What were its Limitations?

Ernest Rutherford was a British scientist who conducted an experiment and proposed the atomic structure of different elements. This was called the Rutherford Atomic Model. Through this experiment, Rutherford determined that the particles must have been scattered in all directions with very few exceptions and discovered that atoms are mostly empty spaces. In the year 1903, Ernest Rutherford conducted an experiment in which he bombarded a thin sheet of gold with α-particles and then studied the trajectory of these particles after their interaction with the gold foil. In the early 1900s, Thomson discovered that atoms had a nucleus held. Most of the mass of an atom was concentrated in an extremely small volume. He coined the name of this region of the atom as a nucleus of an atom. In the year 1911, Ernest Rutherford proposed that electrons revolve around the nucleus of an atom in circular paths and named these orbits electrons. Rutherford also claimed that a negatively charged electron is held together with a positively charged nucleus by an electrostatic force of attraction. named these circular paths as orbits. Electrons are negatively charged, and nuclei are densely concentrated masses of positively charged particles. These are held together by a very powerful electrostatic force of attraction. But there were a few limitations of this model and a perfect model was yet to be created.  


Maxwell thought that accelerated charged particles emitted electromagnetic radiation, and hence an electron revolving around the nucleus should emit electromagnetic radiation. If an electron is said to be a wave of energy and matter, then it would not collapse into the nucleus. The electrons in the atom are moving and therefore have kinetic energy. If an electron is given enough energy, it can escape from the nucleus’ orbit. The Rutherford model could not explain the stability of an atom. The Rutherford model had its flaws, but it was the first model of an atom that was accurate enough to be used in experiments. The next step in atomic theory came with Bohr's Rutherford Model of the Atom. In this model, electrons orbited a nucleus like planets orbits their suns. This model was more accurate than Rutherford's, but it still left something to explore in future and thus Rutherford's model was discarded.

4. What are Isotopes?

The total components of the nucleus of an atom are called nucleons. A nucleon can consist of either a proton or a neutron. All elements have a unique number of protons in them. This is described by their unique atomic numbers. An element can have many atomic configurations, each with a different total number of nucleons. Isotopes of the element are the versions of the element with a distinct nucleon number. These are also known as the mass number of that particular element. As a result, an element's isotopes have the same number of protons but different numbers of neutrons. The stability of an element's isotopes varies.  Isotopes have different half-lives. They do, however, have chemically comparable activity since they have the same electrical structures. The chemical symbol of the element, the atomic number of the element, and the mass number of the element are used to define the atomic structure of an isotope. For Example- The 3 known natural isotopes of the hydrogen atom are protium, deuterium, and tritium.

5. What was Bohr’s Atomic Model and What were its Limitations?

Neils Bohr was a scientist who proposed his atom model in 1915, which became popular. Nucleons are the constituents of an atom's nucleus. A proton or a neutron are both nucleons. Each element has a distinct number of protons, which is represented by its atomic number.  Based on Planck's theory of quantization, this is the most extensively used atomic model to describe an element's atomic structure. Atomic electrons are arranged in separate orbits known as stationary orbits. Quantum numbers can be used to represent the energy levels of these shells. Electrons can absorb energy to move to higher energy levels and lose or release energy to go to lower energy levels. There will be no energy emission or absorption as long as an electron remains stable or stationary. Only in these particular fixed orbits, do electrons spin surrounding the nucleus. The stationary orbits' energy is quantized. Only single-electron species like H, He+, Li2+, Be3+, and others operate with Bohr's atomic structure. Despite matching some conditions, Bohr's model had some flaws. Each line spectrum was revealed to be a composite of several smaller distinct lines when the emission spectra of hydrogen were studied using a more precise spectrometer.  Bohr's hypothesis failed to explain both the Stark and Zeeman effects.  Bohr's model was placed on hold because of these limitations.

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