About Rutherford
Ernest Rutherford was a New Zealand born physicist who in 1911 described the structure of an atom, which was an improvement on the plum in pudding model of atom Rutherford model is also known as the Rutherford atomic model, planetary model of the atom, or the nuclear model of the atom. The Rutherford atomic theory has defined the atom as a tiny, dense, positively charged core called a nucleus, which is surrounded by negative charges called electrons. It describes the atomic model as to where all the atom’s mass is concentrated in the centre called the nucleus, around which the negative charges called the electrons revolve.
Rutherford’s Atomic Model
According to Rutherford’s atomic model, the positively charged particles and most of the atom’s mass was concentrated in a minimal volume. He called this region of the atoms a nucleus. Rutherford’s nuclear model also proposed that the negatively charged electrons encircle the nucleus of an atom. Rutherford also proposed that the electrons move at the speed of light around the nucleus. He named these circular paths orbits.
Rutherford Nuclear Model
By improving on Thomson’s model of the atom Rutherford in 1911 depicted that the atom has a dense nucleus with the help of the gold-foil experiment, and thereby improved the understanding of the atomic model. Five years earlier, Rutherford observed that alpha particles transmitted through a hole onto a graphic plate would make a sharp-edged picture. In contrast, alpha particles transmitted through a sheet of mica only 20 micrometres thick would create an effect with blurry edges. For some particles, the blurring resembled a two-degree deflection. Remembering those results, Rutherford had his postdoctoral fellow, Hans Geiger, and an undergraduate student, Ernest Marsden, refined the experiment by beaming alpha particles through gold foil and recognised them as beams of light or scintillations on a screen.
The gold foil was only 0.00004 cm thick. Most alpha particles went directly through the foil, but some were diverted by the foil and hit the spot on a screen placed off to one side. However, the Rutherford atomic model was not readily accepted by all physicists, as it did not conform to the then chemical understanding of the atom. The model suggested that the charge on the nucleus was the essential characteristic of the atom, determining its structure. On the other hand, Mendeleyev’s periodic table of the elements was organised according to the atomic masses, implying that the mass was responsible for atoms’ structure and chemical behaviour.
Rutherford Atomic Structure
The present-day understanding of the atom is based on the nuclear model of the atom proposed and explained by Rutherford, which says that the atom has a large, dense central mass called the nucleus, which is encircled by the negatively charged electrons. The protons that make up the nucleus are positively charged, and it is represented by Z, the atomic number of an element. For an atom to be electrically neutral, it must cover the same number of extranuclear electrons as there are protons in the nucleus. Hence, the number of electrons in a neutral atom of atomic number Z is also Z. For example, the hydrogen atom has one proton and one electron, whereas the carbon atom comprises six electrons.
We know that an atom's structure is made up of electrons, protons, and neutrons. This was accurately presented after several scientists developed various models. Ernest Rutherford developed the Rutherford atomic model, also known as the Rutherford model of the atom. However, it is no longer considered an accurate representation of an atom. Please tell us more about this model.
The Rutherford Atomic Model
Rutherford proposed that an atom is mostly made up of space, with electrons orbiting in predetermined paths around a fixed, positively charged nucleus.
History
Democritus, a Greek philosopher, first proposed the concept of the atom in 400 BCE. However, it wasn't until 1803 that John Dalton proposed the idea of the atom again. Atoms were thought to be indivisible at the time. This concept of an atom as indivisible particles persisted until 1897, when British physicist J.J. Thomson discovered negatively charged particles known as electrons.
Based on that, he proposed a model in which electrons were embedded uniformly in a positively charged matrix. The plum pudding model was given that name. J.J. Thomson's plum pudding model, on the other hand, had some limitations. It was unable to explain certain experimental results relating to element atomic structure.
"Ernest Rutherford," a British physicist, proposed a model of atomic structure known as Rutherford's Model of Atoms. He conducted an experiment in which he bombarded -particles in a thin gold sheet. He investigated the trajectory of the -particles after interaction with the thin sheet of gold in this experiment.
Experiment with the Rutherford Atomic Model
In Rutherford's experiment, he bombarded a thin gold foil with high-energy streams -particles. A radioactive source was used to direct the streams of -particles. He conducted experiments to investigate the defect created in the trajectory of -particles after interaction with a thin sheet of gold. To investigate the deflection, he surrounded the gold foil with a zinc sulphide screen. J.J. Thomson's plum pudding model was contradicted by Rutherford's observations.
Rutherford Model Experiment Results
Rutherford concluded that major space in an atom is empty based on his observations during the experiment – a large fraction of -particles passed through the gold sheet without being deflected. As a result, the majority of an atom must be empty.
The positive charge in an atom is not distributed uniformly and is concentrated in a very small volume – When bombarded, the gold sheet deflected a few -particles. They were deflected at minute and small angles. As a result, he came to the above conclusion.
Very few -particles had been deflected at large angles or returned. Furthermore, only a few particles had deflected at 180o. As a result, he concluded that positively charged particles covered a small proportion of an atom's total volume.
Rutherford atomic model postulates based on observations and conclusions
Positively charged particles make up an atom. The majority of an atom's mass is concentrated in a very small region. The nucleus of an atom was named after this region of the atom. Later, it was discovered that the very small and dense nucleus of an atom is made up of neutrons and protons.
The nucleus of an atom is surrounded by negatively charged particles known as electrons. At a very high speed, electrons revolve around the nucleus in a fixed circular path. Orbits were named after these fixed circular paths.
Because electrons are negatively charged and the densely concentrated nucleus is positively charged, an atom has no net charge or is electrically neutral. The nucleus and electrons are held together by a strong electrostatic force of attraction.
The size of an atom's nucleus is very small in comparison to the total size of an atom.
Rutherford Atomic Model Limitations
Certain things could not be explained by Rutherford's experiment. They are as follows:
Rutherford's model was unable to explain atomic stability. According to Rutherford's postulate, electrons revolve at a very high speed in a fixed orbit around an atom's nucleus. Maxwell, on the other hand, explained that accelerated charged particles emit electromagnetic radiations. As a result, electrons revolving around the nucleus emit electromagnetic radiation.
The electromagnetic radiation will contain energy from the electronic motion, causing the orbits to gradually shrink. Finally, the orbits in an atom's nucleus will contract and collapse. If Maxwell's explanation is followed, Rutherford's model will collapse in 10-8 seconds, according to the calculations. As a result, Rutherford's atomic model did not conform to Maxwell's theory and was unable to explain the stability of an atom.
Rutherford's theory was lacking in detail because it did not address the arrangement of electrons in the orbit. One of the major flaws of the Rutherford atomic model was this.
Conclusion
Despite the fact that early atomic models were inaccurate and could not adequately explain the structure of atoms and experimental results. However, it served as the foundation for quantum mechanics and aided in its future development.
FAQs on Rutherford Atomic Theory
1. What is Rutherford's nuclear model of the atom?
Rutherford's nuclear model, proposed in 1911, describes the atom as having a tiny, dense, positively charged core called the nucleus, where nearly all the mass is concentrated. This central nucleus is surrounded by negatively charged electrons that revolve around it in circular paths called orbits. The model suggests that most of the atom is empty space, and the atom as a whole is electrically neutral because the positive charge of the nucleus is balanced by the negative charge of the electrons.
2. What were the three key observations from Rutherford's alpha-particle scattering experiment?
Rutherford's conclusions were based on the following key observations from his gold foil experiment:
- Most alpha particles passed straight through: A vast majority of the alpha particles went through the gold foil undeflected, indicating that most of the space inside an atom is empty.
- A few alpha particles were deflected by small angles: A small fraction of particles were deflected from their original path, suggesting the presence of a concentrated positive charge within the atom that repelled the positively charged alpha particles.
- Very few alpha particles bounced back: Approximately 1 in 20,000 particles were deflected by large angles (more than 90°) or even rebounded completely, implying that the entire positive charge and mass of the atom are concentrated in a very small, dense region, which he named the nucleus.
3. What are the main postulates of Rutherford's atomic theory?
Based on the gold foil experiment, Rutherford proposed the following postulates for his atomic theory:
- An atom consists of a positively charged, dense, and very small central core called the nucleus.
- The nucleus contains almost the entire mass of the atom.
- Negatively charged electrons revolve around the nucleus at high speeds in fixed circular paths known as orbits.
- The atom is electrically neutral, as the total positive charge on the nucleus equals the total negative charge of the electrons.
- The electrostatic force of attraction between the positive nucleus and negative electrons holds the atom together.
4. What were the major drawbacks of Rutherford's atomic model?
Despite being a major step forward, Rutherford's model had two significant drawbacks:
- It could not explain the stability of the atom: According to Maxwell's theory of electromagnetism, any accelerating charged particle must emit energy. An electron revolving in an orbit is accelerating and should continuously lose energy, causing it to spiral into the nucleus and making the atom unstable. Rutherford's model offered no explanation for why this collapse doesn't happen.
- It did not explain the arrangement of electrons: The model did not provide any details about the specific arrangement of electrons in the orbits or the distribution of energy among them.
5. Why is Rutherford's model also known as the planetary model, and what is the main flaw in this analogy?
Rutherford's model is often called the planetary model because it draws an analogy to the solar system. The massive, positively charged nucleus is compared to the Sun, and the light, negatively charged electrons are compared to the planets orbiting it. However, this analogy is flawed. Planets are electrically neutral and are governed by gravitational forces, whereas electrons are charged particles governed by electrostatic forces. According to classical physics, an orbiting electron, being a charged particle, would continuously radiate energy and spiral into the nucleus, which planets do not do.
6. How did Rutherford’s discovery of the nucleus challenge the prevailing Thomson’s plum pudding model?
Rutherford's discovery fundamentally contradicted J.J. Thomson's plum pudding model. Thomson proposed that an atom was a sphere of uniform positive charge with electrons embedded within it, like plums in a pudding. If this were true, the alpha particles in Rutherford's experiment should have passed through the gold foil with minimal or no deflection. The observation that some particles were deflected at large angles proved Thomson's model incorrect. It showed that the positive charge was not spread out but was concentrated in a tiny, dense nucleus, leading to a complete rethinking of atomic structure.
7. Why was the stability of the atom a major unanswered question in Rutherford's theory?
The stability of the atom became a critical problem because Rutherford's model relied on classical mechanics and electromagnetism, which could not explain phenomena at the atomic scale. According to classical electromagnetic theory, an electron orbiting a nucleus is constantly accelerating and should therefore continuously emit electromagnetic radiation. This loss of energy would cause its orbit to shrink rapidly, leading to the electron spiralling into the nucleus in a fraction of a second. Since atoms are known to be stable, this contradiction represented a fundamental failure of the model, which was later addressed by Niels Bohr's quantum-based model.
