Let’s start the introduction of the Rutherford Model of the Atom
Rutherford’s model of the atom
The Rutherford model of the atom is also called a nuclear atom or planetary model of the atom, a description of the structure of atoms proposed by Ernest Rutherford.
- An atom consists of a positively charged, dense, and very small nucleus containing all the protons and neutrons. Almost the entire mass of an atom is concentrated in the nucleus.
- Rutherford’s model proposed that the negatively charged electrons surround the nucleus of the atom. The electrons are revolving around the nucleus in circular paths at very high speeds. The circular paths of electrons are called orbits.
- Electron being negatively charged and nucleus being a densely concentrated mass of positively charged particles are held together by the strong electrostatic force of attraction.
- An atom is electrically neutral because the number of protons and electrons in an atom is equal.
- Most of the atom is empty space.
- According to Rutherford’s theory, a hydrogen atom consists of a small nucleus containing one proton and one electron revolving around it. The nucleus is almost at the center of the atom. Since the hydrogen atom contains an equal number of protons and electrons, it is electrically neutral. The nucleus of the ordinary hydrogen atom does not contain any neutrons.
Limitations of Rutherford Model of the atom
The main limitation of the Rutherford Model of the atom:
- When we apply electromagnetic theory to Rutherford’s atomic model, it will mean that the negatively charged (-ve) particles’ electrons revolving around the nucleus with accelerated motion will lose their energy continuously by radiation. Thus, the energy revolving electrons will decrease gradually and their speed will also go on decreasing. The electrons will be attracted more strongly by the oppositely charged nucleus due to which they will come closer and closer to the nucleus and ultimately the electrons should fall into the nucleus by taking a spiral path. This should make the atom very unstable then the atom should collapse. But we know that atoms do not collapse on their own. Rutherford’s model does not explain the stability of an atom.
- One of the limitations of the Rutherford model was also that he did not say anything about the arrangement of electrons in the shell of the atom which made his theory incomplete.
- Rutherford’s model does not explain the stability of an atom. According to Rutherford’s model, electrons revolve at very high speed around a nucleus of the atom in a fixed orbit. However, Maxwell explained accelerated charged particles release electromagnetic radiation. Therefore, electrons revolving around the nucleus will release electromagnetic radiation.
Rutherford’s Experiment – Discovery of Nucleus
When fast-moving alpha particles are allowed to strike a very thin gold foil in a vacuum, it is found that:
- Most of the α particles pass straight through the gold foil without any deflection from their original path; it shows that there is a lot of empty space in the atom.
- A few alpha particles are deflected through small angles and a few are deflected through large angles to show that there is a center of positive charge in the atom which repels the positively charged alpha particles and deflects them their original path. This center of positive charge in the atom is known as the nucleus.
- The observation that a very few alpha particles completely rebound on hitting the gold foil and turn back on their path shows that that the nucleus is very dense and hard which does not allow the alpha particles to pass through it.
Characteristics of the Nucleus
- The nucleus of an atom is positively charged.
- The nucleus of an atom is very dense and hard.
- It is very small as compared to the size of the atom.
- The nucleus was discovered by Ernest Rutherford.
- The nucleus is a small positively charged part at the center of an atom. It contains all the protons and neutrons.
- Protons and neutrons take together are known as nucleons because they present in the nucleus.
The number of protons in one atom of the element is known as the atomic number of the element.
- Atomic number is denoted by Z.
- Two elements cannot have the same element number.
- The atomic number of an element is equal to the number of electrons in a neutral atom of that element.
- All the atoms of the same element have the same number of protons in their nuclei, and hence they have the same atomic number.
The total number of protons and neutrons present in one atom of the element is known as its mass number. An atom consists of electrons, protons, and neutrons. Since the mass of electrons is negligible, the real mass of an atom is determined by the protons and neutrons only.
- The mass number is also known as the atomic number.
- The mass number is denoted by A.
Mass number = No. of protons + No. of neutrons
Mass number = Atomic number + No. of neutrons
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