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  • Last Updated : 20 Dec, 2021

The credit of origin of the history of atomic structure and quantum mechanics goes to Democritus. He was the first one to propose that matter is made up of atoms. The entire field of chemical reactions, bonds, physical properties is all related to the atomic structure. The first scientific theory for the atomic structure was given by John Dalton in 1800. Once the atomic structure was discovered it also laid the foundation for the discovery of subatomic particles, the fundamental particles that make up the structure of a matter.

An element’s atomic structure refers to its nucleus and it’s constitution, how and of what material it is made up of. It also involves the electrons that exists around the nucleus. The primary things that make an atomic structure of matter are protons, electrons and neutrons. The neutrons and protons are at the core of the atom and make the nucleus, and the nucleus is surrounded by the electrons that belong to the atom. The atomic number of an element is the total number of protons present in the nucleus of that element’s atom.

Atoms which are neutral in terms of the charge, have equal number of protons and electrons. This makes the atom neutral because protons are positively charged and electrons are negatively charged. But atoms have the ability to lose or gain electrons in order to increase their own stability and the resulting entity is charged in nature and is called an ion.

No two elements have equal number of protons and electrons, this makes the atomic structure of every element different. This is the main reason behind every element’s unique characteristics. Many scientists around the world, in 18th and 19th century, attempted to explain the atomic structure with help of their own atomic models. Each of such models had their own plus points as well as negative points. But they all played a huge role in the development of modern atomic model. The most noted contributors in this field were J.J. Thompson, John Dalton, Emest Rutherford and Neils Bohr.  

Helium atom as per Bohr’s model.

Important Terminology

  • Atom – The defining structure and basic units of matter of an element are called atoms. The term “atom” came from a Greek word that meant indivisible because earlier atom was thought to be the smallest things in the universe that could not be divided
  • Atomic Structure – The structure of an atom comprising of a nucleus, in which the protons and neutrons are present. The negatively charged particles called electrons revolve around the centre of the nucleus.
  • Nucleus – A collection of particles called protons and neutrons is called Nucleus. Protons are positively charged and neutrons, are electrically neutral. Protons and neutrons are made up of particles called quarks. The chemical element of an atom is determined by the number of protons, or the atomic number, Z, of the nucleus.
  • Proton – Positively charged particles found within atomic nuclei is given the name of Proton. Rutherford discovered proton in his famous cathode ray experiment that was conducted between 1911 and 1919. Protons are about 99.86% as massive as neutrons. The number of protons in an atom is unique for each element
  • Electron – Electrons are very tiny compared to protons and neutrons, about1800 times smaller than either a proton or a neutron. Electrons are just 0.054% as massive as neutrons. Electrons were discovered in 1897 by Joseph John (J.J.) Thomson, a British physicist. Electrons have a negative charge and are electrically attracted to the positively charged protons
  • Neutron – Rutherford theorized the neutron’s existence in 1920 and later discovered by Chadwick in 1932. Neutrons were found during experiments where atoms were shot at a thin sheet of beryllium. Subatomic particles with no charge were released – and were named as neutron. Neutrons are uncharged particles found within all atomic nuclei
  • Isotopes – Members of same family of an element that all have the same number of protons but different numbers of neutrons are named as isotopes. The number of protons in a nucleus determines the element’s atomic number on the Periodic Table. All the isotopes have unique properties, just like all family members have their own qualities. 

Dalton’s Atomic Theory

John Dalton was a British Chemist and he suggested that every matter is made up of atoms. Also that atoms are indivisible and indestructible. He also suggested that all atoms of a particular matter are all same, but atoms of different elements had different size and mass.

According to Dalton’s atomic theory, Chemical reactions involve the rearrangements of atoms in order to form the products. According to the postulates proposed in his theory, atomic structure is made up of atoms and they are the smallest particles responsible for chemical reactions to occur.

Postulates of Dalton’s theory:

  • Every matter that exists is made of atoms.
  • Atoms are indivisible.
  • A particular element has only one type of atom in it.
  • An atom has a constant mass that varies for every element.
  • During a chemical reaction, atoms undergo rearrangement
  • Atoms can neither be created nor be destroyed but can only be transformed from one form to another.

Dalton’s atomic theory was able to explain the Laws of chemical reactions successfully, named as the Law of conservation of mass, Law of constant properties, Law of multiple proportions and Law of reciprocal proportions.

Demerits of Dalton’s Atomic Theory

  1. This theory was not able to explain the existence of isotopes.
  2. No appropriate explanation was provided regarding the structure of atoms.
  3. Later the atoms were found to be divisible, and Dalton’s claim of atoms being indivisible was proved to be wrong.

The discovery of constituting particles of atoms led to a better understanding of chemicals, these constituting particles are called subatomic particles.

Thomson’s Atomic Model

Sir Joseph John Thomson who was also an English chemist famous for his discovery of electrons for which he also got the Nobel Prize. He conducted a cathode ray experiment to invent electrons. In this experiment, a tube of glass that has two openings is taken. One opening is for the vacuum pump and the other is for intake through which the gas to be filled in the tube is pushed in. Using the vacuum pump a partial vacuum pump is maintained inside the glass chamber. With the help of electrodes, a high voltage power supply is linked. In simple words, a cathode and anode are placed inside the glass tube.

The following observations were made when the current was allowed to flow between the cathode and anode.

  1. When the high voltage power is connected and switched ON, rays were transmitted from cathode towards the anode. Fluorescent spots were observed on the ZnS screen and it confirmed the fact the rays being transmitted. These rays were given the name of ‘Cathode Rays’.
  2. When an external electric field was projected on the tube, the rays got deviated towards the positive electrode. But in the absence of the electric field the rays got back in the straight line.
  3. But when rotor blades were fixed in the path of the cathode rays, the rays seemed to rotate. This proved that cathode rays were made of some particle that had some mass in them.
  4. Using all the evidences, Thomson reached to the conclusion that cathode rays are composed of negatively charged particle called electrons.
  5. By applying electric and magnetic fields on the cathode ray, the charge to mass ratio (e/m) was found. The e/m for electron came out to be 17588 × 1011 e/bg

Mullikin did an oil-drop experiment to find the charge of the electron using the e/m ratio. He found the charge of the electron = 1.6 x 10-16 C and Mass of electron = 9.1093 × 10-31 kg. Thomson described the structure of the atom as a positively charged sphere in which negatively charged electrons were entrenched. The popular name given to this model is the “plum pudding model” because it can be observed as a plum pudding dish where the positively charged atom signifies the pudding and the plum pieces stand for the electrons.

The reason for the failure of Thomson’s model is that this model is not clear about the stability of an atom. This model could not adjust to other subatomic particles discovered in the future.

Alpha Ray Scattering Experiment & Rutherford Atomic Theory

Rutherford who was a student of J. J. Thomson discovered another subatomic particle called Nucleus. This discovery made huge changes to the atomic structure. The observations made by Thompson in his experiment were used by Rutherford to propose his theory for atomic structure.

Rutherford used the radioactivity phenomenon in conducting his experiment. He used radioactive material radium bromide (RaBr). RaBr emits α particles which is a form of radiation. A thin gold metal sheet was put up in the setup. Then the α particles were bombarded on this sheet. To observe the deflection of the particles a screen of Zinc Sulfide (ZnS) was used and it was placed behind the Gold foil. Rutherford further developed a detector in order to count the number of radioactive particles. Initially, he recorded the count rate of RaBr as he kept a count of α particles emitted per minute.

Observation of Rutherford’s model

Following observations were made by Rutherford and conclusions were drawn:

  • Most of the α particles passed through thin sheets. This means most of the atom’s space is empty.
  • Another observation made was that some of the α  particles deflected a bit in every direction. This leads to the conclusion that positive charge is not distributed uniformly throughout the atom.
  • Very few α particles get deflected back along the path that they were travelling on. This happened because of charges repelling each other. Seeing this Rutherford concluded that the positive charge in an atom exists in a very small volume.
  • Not only the positively charged particles but a lot of mass is also concentrated in a very small volume. Rutherford named this region as Nucleus.
  • Rutherford also came up with the argument that electrons are present in orbits around the orbits, much like the planets in the solar system. Electrons are negatively charged and they revolve around the nucleus.
  • The electrons and nucleus are held by the electrostatic force of attraction because they are negatively and positively charged respectively.

Conclusion of Rutherford’s model

Drawing conclusions from all the above observations, Rutherford proposed his Atomic structure which had the following properties –

  1. The nucleus lies at the center of the atom, and maximum of the charge and mass is concentrated there only.
  2. Atoms are spherical in nature.
  3. Electrons revolve around the nucleus in a circular orbit.

Limitations of Rutherford’s model

Just like other atomic models, Rutherford’s model also had many shortcomings.

  1. In order to revolve around the nucleus they will have to spend energy and that also against the strong force of attraction from the nucleus. Electrons will spend a lot of energy and eventually they will lose the entire energy and fall into the nucleus. This raises serious questions about the stability of the atom.
  2. If the electrons are revolving continuously around the nucleus, then the spectrum that they emit should be a continuous spectrum, but what we observe is a line spectrum.

Subatomic Particles

Protons

  • Protons have a positive charge. This charge is 1e, which is approximately 1.602 × 10-19
  • The mass of a proton is approximately 1.672 × 10-24
  • Protons are over 1800 times heavier than electrons.
  • The total number of protons in the atoms of an element and the atomic number of the element is always equal.

Neutrons

  • The mass of a neutron is almost similar to that of a proton i.e. 1.674 × 10-24
  • Neutrons are always electrically neutral particles and do not carry any charge.
  • Isotopes of an element have the same number of protons but a different number of protons in their respective nuclei.

Electrons

  • The charge of an electron is -1e, which is approximately -1.602 × 10-19
  • The mass of an electron is approximately 9.1 × 10-31.
  • The mass of an electron is almost negligible as compared to the mass of an atom, so an electron’s mass is ignored while calculating the mass of an atom.

Atomic Structure of Isotopes

All the components are put together under the name of the Nucleons of an atom. It can either be a neutron or a proton. The number of protons is unique for every element, which is denoted by its unique atomic number. There can be multiple atomic structures for an element, but have different numbers of nucleons.  These variants of an element have different numbers of nucleons. These variants are called isotopes of the elements. This leads to the fact that isotopes have the same number of protons but different number of neutrons.

To describe the structure of an isotope, the symbol of the element is used along with the atomic number and the mass number of the isotope. To give an example, Hydrogen has 3 isotopes named protium, deuterium and tritium. The stability of isotopes is different. The half-lives are also different. But they generally have similar chemical behaviour because they have the same electronic structures.

Bohr’s Atomic Theory

Bohr proposed his model in 1915 and it is the most widely used atomic model and is based on Planck’s theory of quantization. Bohr’s theory is applicable to the modified atomic structure. It explains that electrons always move in fixed orbitals only, and they are not present everywhere in the atom. Bohr also explained that each orbit is a fixed energy level. An orbit is also called a shell. Rutherford only explained the nucleus of the atom. Bohr made changes to that model and added electrons and energy levels.

Bohr’s model is a small nucleus that is positively charged which is surrounded by negative electrons and they move around in orbits. Bohr found out that larger the distance of an electron from the nucleus, the larger is its energy.

Postulates:

  1. Inside atoms electrons are present in discrete orbits called “stationary orbits”.
  2. Quantum numbers are used to represent the energy levels of these shells.
  3. Electrons can go to higher levels by absorbing energy and move to lower energy levels by losing or emitting some energy.
  4. Only when an electron stays in its own orbit, no absorption or emission of energy take place.
  5. Electrons revolve in these stationary orbits only.
  6. The energy of the stationary orbits is quantized.

Limitations of Bohr’s Atomic Theory:

  • It works only for single-electron species such as H, He+, Li2+, Be3+, ….
  • When a more accurate spectrometer was used to observe the emission spectrum of hydrogen, each line spectrum was seen to be a combination of multiple smaller discrete lines.
  • Bohr’s theory was unable to explain Stark and Zeeman effects.

Sample Questions

Question 1: What are the advantages of the Periodic Table?

Answer:

In the periodic table, elements are arranged in a tabular form. This makes it is easy to remember the properties of elements if one knows the position of the element. Also the compounds formed by the elements are predictable if the position of the element is known. Periodic table made it easy and systematic to study chemistry

Question 2: If the number of electrons and number of protons is equal in an atom, then why is it wrong to say that an atomic number of the element is the same as the number of electrons?

Answer:

It is wrong to say that the atomic number of the element is the same as the number of electrons because an atom can lose or gain electrons, so the number of electrons keeps changing and they are never constant. Whereas the number of protons never changes for an element. This is why atomic number is taken from number of protons.

Question 3: Electrons contribute negative charge, protons contribute positive charge. An atom has both but why there is no charge?

Answer:

As per Thomson’s model of an atom, the number of electrons and the number of protons are equal in an atom. Electrons are negatively charged and protons are positively charged, hence the + and – charges are neutralized by each other that making atoms neutral as a whole

Question 4: Why Rutherford’s model could not explain the stability of an atom?

Answer:

According to Maxwell’s electromagnetic theory, when charged particles are accelerated, they should emit electromagnetic radiation. Similarly, an electron in its orbit moves at a very high velocity and it will emit radiation; the orbit will then continue to shrink and the electron ultimately falls into the nucleus which does not happen in an atom.

Question 5: What are the Flaws in Dalton’s Atomic Theory?

Answer:

There were multiple flaws found in dalton’s atomic theory. The biggest flaw was that atoms are not the elementary particles of matter, but they are further divided into three main categories known as electrons, protons, and neutrons. But they are the smallest entities that take part in a chemical reaction. The discovery of isotopes further led to a crucial flaw in this theory. Initially, it was said that the same elements have similar atoms, but isotopes proved that atoms in some elements can have variable densities and masses. Similarly, isobar’s discovery denied the fact that atoms are different for different elements. The whole number ratio theory was also found incorrect as it was not observed in complex compounds.


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