We know that an atom is the smallest part of the elements which makes up everything on the Earth. Particles of energy make up an atom, and only nuclear reactions can further split up an atom. A variety of different models have been used over the past decades to speculate on how an atom works and what particles it contains.
1. Billard Ball Model
We know that an atom is the smallest part of the elements which makes up everything on the Earth. Particles of energy make up an atom, and only nuclear reactions can further split up an atom. A variety of different models have been used over the past decades to speculate on how an atom works and what particles it contains.
In the very beginning of the 1800s, John Dalton proposed that atoms were like tiny, hard billiard balls. His view of completely solid atoms seems like a very basic idea now, but in 1803 it was groundbreaking. The experts at Colorado State University say this theory was a major contribution to chemistry. He also proposed that all atoms of one element are identical, and that each element had a different type of atom.
In the very beginning of the 1800s, John Dalton proposed that atoms were like tiny, hard billiard balls. His view of completely solid atoms seems like a very basic idea now, but in 1803 it was groundbreaking. The experts at Colorado State University say this theory was a major contribution to chemistry. He also proposed that all atoms of one element are identical, and that each element had a different type of atom.
2. Plum Pudding Model
J.J. Thompson's Plum Pudding Model introduced the idea of positive and negatives charges existing in atoms. According to Visionlearning, he used cathode ray tubes and positively charged plates to demonstrate the existence of negative particles named electrons. He hypothesized that an atom resembled a plum pudding, or a sphere filled with positively charge liquid and dotted with negative electrons.
J.J. Thompson's Plum Pudding Model introduced the idea of positive and negatives charges existing in atoms. According to Visionlearning, he used cathode ray tubes and positively charged plates to demonstrate the existence of negative particles named electrons. He hypothesized that an atom resembled a plum pudding, or a sphere filled with positively charge liquid and dotted with negative electrons.
3. Two Solar System Model
In 1910 to 1911, Ernest Rutherford proposed the planetary, or nuclear, model of the atom. He believed that atoms were composed mostly of empty space, with a dense nucleus. His experiments involved shooting alpha particles at gold foil. He concluded that the positive nucleus contains most of the atom's mass. With his orbit model, Niels Bohr refined the idea of the atom as a tiny solar system in 1913. Bohr's model had electrons orbiting the nucleus in shell-like layers.
■ Rutherford's Atomic Model
His model was baee upon the gold foil experiment (which was also called Geiger–Marsden experiment), carried out in 1911, focuses on the Discovery of the Nucleus.
In this experiment, he had a large circular detector, which was coated with phosphorescent material. He used a very thin sheet of gold-foil in the middle and had a radioactive source of Alpha Particle.
Here, the Alpha Particles, are emitted from the source and pass through a lead plate (not shown in the figure), and bombard with the Gold Foil.
On the basis of this experiment, he found out that the size of the core (later named Nucleus, which was the positively charged) is around 10^(-15)m which was smaller than the size of atom which was known to have a size of 10^(-10)m.
After Rutherford’s model was introduced, the atom, now had a new orientation with positive charge in the middle and electrons revolving around it.
Although there are some limitations of this atomic model.
- According to electromagnetic theory, an acceleration rated charged particle must radiate electromagnetic energy. An electron revolving around the nucleus is under continuous acceleration towards the center. It should continuously lose energy and move in orbits of gradually decreasing radii. The electron should follow a spiral path and finally it should collapse into the nucleus. Thus the Rutherford's model can't explain the stability of an atom.
- In Rutherford's model, an electron can revolve in orbits of all possible radii. So it should emit a continuous spectrum. But an atom like hydrogen always emits a discrete line spectrum.
■ Bohr's Atomic Model
Neils Bohr made some modifications in Rutherford’s model, which described above. In his model, structure of atom was compared to planetary motion in which, motion of electron around the nucleus was compared to motion of planets revolving around sun.
- Electrons revolve around the nucleus in circular path. The circular path is called orbits or energy shells or energy level.
- An atom may have infinite number of orbits. Only few orbits contains electrons in them. These orbits are known as discrete orbits. Electrons present in discrete orbits have stable energies. Hence, electrons do not lose energy during the revolution due to which electron do not fall in nucleus. Hence, Bohr’s model explains stability of atom.
- In Bohr’s model, energy of electron is stable. But electron may undergo change in energy. If electron jumps from higher orbital to lower orbital or from lower orbital to higher orbital, in such case energy of electron changes.
- Orbits or shells are represented by alphabets K,L,M,N,….. Or by numbers n = 1,2,3,4,…….
After the confirmation of structure of atom through Bohr’s model, scientists put forth the distribution of electrons in the shell of atom.
- Number of electrons in single orbit is 2 n^2, where, n shows energy level. Therefore, K shell is first shell i.e. n=1, therefore, K shell will have 2(1)^2 = 2 electrons. Similarly,
- Term duplet is allowed if shell has two electrons. Term octet is used of shell has 8 electron.
- Electrons are filled in shells from inner shell towards the outer shell.
But it also leads to some concepts of failures
- Bohr treated electrons as particles where according to de Broglie’s hypothesis, having a very low mass, electron also exhibits wave nature.
- Bohr’s model was adequate only for nucleus having only one electron e.g. Hydrogen, He+1, Li+2 etc. Bohr’s model could not explain the spectra of multi-electronic atoms.
- Bohr’s model was two-dimensional where an atom is three-dimensional. In other words is not a flat as suggested by Bohr, but has a three dimensional existence.
- Using a better spectrometer, the spectra showed very fine lines. Bohr’s model could not explain the origin of those fine lines. (Solved by Arthur Somerfield who imagined electrons orbiting in different planes and having elliptical orbits.)
- Bohr’s model could not explain the effect electric field and magnetic field on spectra. (Stark effect {spectral lines are split up into components when the source emitting lines is placed in a strong magnetic field} and Zeeman effect {spectral lines are split up into components when the source emitting lines is placed in a strong magnetic field.}
- In Bohr’s equation, the momentum and position of electron, revolving around the nucleus were well defined. But, according, Heisenberg’s Uncertainty principle, it is impossible to measure the position and momentum of electrons precisely. If the position is measure with maximum precision, there will be uncertainty in the value of momentum and vice versa.
In 1910 to 1911, Ernest Rutherford proposed the planetary, or nuclear, model of the atom. He believed that atoms were composed mostly of empty space, with a dense nucleus. His experiments involved shooting alpha particles at gold foil. He concluded that the positive nucleus contains most of the atom's mass. With his orbit model, Niels Bohr refined the idea of the atom as a tiny solar system in 1913. Bohr's model had electrons orbiting the nucleus in shell-like layers.
■ Rutherford's Atomic Model
His model was baee upon the gold foil experiment (which was also called Geiger–Marsden experiment), carried out in 1911, focuses on the Discovery of the Nucleus.
In this experiment, he had a large circular detector, which was coated with phosphorescent material. He used a very thin sheet of gold-foil in the middle and had a radioactive source of Alpha Particle.
Here, the Alpha Particles, are emitted from the source and pass through a lead plate (not shown in the figure), and bombard with the Gold Foil.
On the basis of this experiment, he found out that the size of the core (later named Nucleus, which was the positively charged) is around 10^(-15)m which was smaller than the size of atom which was known to have a size of 10^(-10)m.
After Rutherford’s model was introduced, the atom, now had a new orientation with positive charge in the middle and electrons revolving around it.
Although there are some limitations of this atomic model.
- According to electromagnetic theory, an acceleration rated charged particle must radiate electromagnetic energy. An electron revolving around the nucleus is under continuous acceleration towards the center. It should continuously lose energy and move in orbits of gradually decreasing radii. The electron should follow a spiral path and finally it should collapse into the nucleus. Thus the Rutherford's model can't explain the stability of an atom.
- In Rutherford's model, an electron can revolve in orbits of all possible radii. So it should emit a continuous spectrum. But an atom like hydrogen always emits a discrete line spectrum.
■ Bohr's Atomic Model
Neils Bohr made some modifications in Rutherford’s model, which described above. In his model, structure of atom was compared to planetary motion in which, motion of electron around the nucleus was compared to motion of planets revolving around sun.
- Electrons revolve around the nucleus in circular path. The circular path is called orbits or energy shells or energy level.
- An atom may have infinite number of orbits. Only few orbits contains electrons in them. These orbits are known as discrete orbits. Electrons present in discrete orbits have stable energies. Hence, electrons do not lose energy during the revolution due to which electron do not fall in nucleus. Hence, Bohr’s model explains stability of atom.
- In Bohr’s model, energy of electron is stable. But electron may undergo change in energy. If electron jumps from higher orbital to lower orbital or from lower orbital to higher orbital, in such case energy of electron changes.
- Orbits or shells are represented by alphabets K,L,M,N,….. Or by numbers n = 1,2,3,4,…….
After the confirmation of structure of atom through Bohr’s model, scientists put forth the distribution of electrons in the shell of atom.
- Number of electrons in single orbit is 2 n^2, where, n shows energy level. Therefore, K shell is first shell i.e. n=1, therefore, K shell will have 2(1)^2 = 2 electrons. Similarly,
- Term duplet is allowed if shell has two electrons. Term octet is used of shell has 8 electron.
- Electrons are filled in shells from inner shell towards the outer shell.
But it also leads to some concepts of failures
- Bohr treated electrons as particles where according to de Broglie’s hypothesis, having a very low mass, electron also exhibits wave nature.
- Bohr’s model was adequate only for nucleus having only one electron e.g. Hydrogen, He+1, Li+2 etc. Bohr’s model could not explain the spectra of multi-electronic atoms.
- Bohr’s model was two-dimensional where an atom is three-dimensional. In other words is not a flat as suggested by Bohr, but has a three dimensional existence.
- Using a better spectrometer, the spectra showed very fine lines. Bohr’s model could not explain the origin of those fine lines. (Solved by Arthur Somerfield who imagined electrons orbiting in different planes and having elliptical orbits.)
- Bohr’s model could not explain the effect electric field and magnetic field on spectra. (Stark effect {spectral lines are split up into components when the source emitting lines is placed in a strong magnetic field} and Zeeman effect {spectral lines are split up into components when the source emitting lines is placed in a strong magnetic field.}
- In Bohr’s equation, the momentum and position of electron, revolving around the nucleus were well defined. But, according, Heisenberg’s Uncertainty principle, it is impossible to measure the position and momentum of electrons precisely. If the position is measure with maximum precision, there will be uncertainty in the value of momentum and vice versa.
4. Electron Cloud Model
The electron cloud model is the most updated atomic model available, and it was developed in the 1920s. The Colorado State University website states that Erwin Schrodinger and Werner Heisenburg changed the specific rings of the Bohr model into clouds that surround the nucleus. Each cloud contains a certain number of electrons, but this model best reflects how difficult it is to pinpoint where each electron might be in relation to the nucleus.
The electron cloud model is the most updated atomic model available, and it was developed in the 1920s. The Colorado State University website states that Erwin Schrodinger and Werner Heisenburg changed the specific rings of the Bohr model into clouds that surround the nucleus. Each cloud contains a certain number of electrons, but this model best reflects how difficult it is to pinpoint where each electron might be in relation to the nucleus.
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