The atom is that the smallest unit of an element and has the same number of protons and neutrons. The model of an atom may be a scientific model that describes the structure of atoms. The three most well-known models are Thomson's atomic model, Rutherford's atomic model, and Bohr's atomic model. These models differ in their assumptions about how electrons behave around a nucleus (the location where protons reside).
J.J. Thomson's Model of Atom
Sir J.J. Thomson came forward with the primary model of an atom in 1897. This model is understood as the J.J. Thomson model or plum-pudding model of an atom. According to Thomson, an atom is Sphere of positive charges during which electrons are embedded. The important features of this model are as follows:
- Atom consists of a consistent sphere with a radius of about 10-8 cm
- The charge ( due to protons ) is spread over the entire surface of the sphere.
- Electrons are embedded during this spheres in such a way that the net charge of a sphere is zero, therefore atom is electrically neutral.
Rutherford's experiment and nuclear model of the atom
In 1911, E. Rutherford, H. Geiger, and E. Marsden performed a historical alpha particle scattering experiment which provided remarkable information regarding the harassment of constituent particles in atoms. The experiment involved the study of scattering of the alpha particle by a thin gold foil the mode of scatting of Alpha particle provided a wealth of information regarding the composition of an atom.
The apparatus employed by Rutherford is shown in the figure. The apparatus consists of a source of alpha particle enclosed in a lead block. The highly energetic alpha particle coming out from a source was collimated into a fine beam with the help of a lead slip system and permitted to attack on a thin gold foil. The gold coin was surrounded by a circular fluorescent zinc sulphide screen. Whenever an alpha particle struck the screen, a small flash of light was produced at that point a movable microscope was used to observe this flashes. Rutherford found that the majority of the Alpha particles crossed the gold foil and struck the fluorescent screen placed around the foil and produces flashes in front of the foil. He debated this experiment several times and made the subsequent observations:
- Most of the Alpha particles (about 99% ) proceed through the gold foil and undeflected therefore, they travel in the same direction in which they were incident on the foil.
- Some Alpha particles got deflected through small angles.
- Few Alpha particles got deflected through very large angles. about one in 20,000 suffered or diffraction of even 180° and are available straight back.
Nuclear model of the atom
On the idea of conclusions derived from the Alpha particle scattering experiment ( discussed above ), rutherford proposed a model of an atom. This model is understood as the nuclear model of an atom or rutherford's model of an atom. The important features of this model are as follows:
- Most the mass and entire positive charge of an atom is concentrated in its center called the nucleus. Due to the presence of the entire mass nucleus is very heavy. The density of a nucleus is of the order of 10¹⁴ g cm-1(approximately 100 million tons per ml).
- The size of the nucleus is extremely small as compared to the size of an atom. The radius of the nucleus is of the order of 10-13 cm while that the atom is 10-8 cm the remaining space within the atom is hollow.
- The nuclei of the atoms of various elements possess different nuclear charges. The magnitude of the charge present within the nucleus of an atom depends upon its nature and varies from one element to the other. The nuclear of the atom of the identical elements possess similar charges.
- Electrons move at a really high speed around the nucleus in orbit or closed circular path. The space round the nucleus in an atom is empty the electron move in this space in orbits. The charge on the electrons is the same as the total positive charge present in the nucleus. Thus, the full atom is electrically neutral.
Bohr's model (for H and H like atoms)
We have already seen that rutherford's model of the atom can neither explain the stability of an atom nor account for the atomic spectrum of hydrogen. In 1913, Niles Bohr ( a Denish physicist ) proposed a replacement model for H and H like Atoms known as Bohr's model of an atom. This model successfully explains the spectrum of hydrogen and the stability of an atom. the idea describing this model of an atom is popularly known as Bohr's theory and involves certain assumptions.
Theorize of Bohr's model of the atom
The bores model of an atom is predicated upon the following hypothesis/theorizes
- The electrons move round the nucleus in certain specifically permitted circular orbits known as energy levels or energy States. An electron during a particular energy level is associated with a definite amount of energy. The energy levels are numbered as 1, 2, 3, 4, ..., etc., or designed as K, L, M, N, ..., etc., Shells. The energy state closest to the nucleus is numbered 1 are designated to as K shell in the figure. Successive energy levels are numbered 2, 3, 4, ... or designated by successive alphabetic letters.
- While occupation a particular energy level or energy state, an electron neither loses nor gains energy. The energy of an electron during a particular energy state always remains fixed or stationary. This suggests that if undisturbed, an electron will move during a particular orbit forever. Since the energy of an electron in an energy level is fixed or stationary, the energy level is also referred to as the stationary energy State.
- Only those energy levels or energy states are permitted during which angular momentum of an electron is an integral ( whole number ) multiply of h/2π , where h is Planck's constant .
If an electron of mass m moves with a velocity v during a circular orbit of radius r, its momentum is given by:
Momentum of Electron = mvr
Hence, consistent with the above theorizes => mvr = n h/2π
Thus, an electron can have the values h/2π, 2h/2π, 3h/2π, etc., For momentum . An electron at no time have a fractional value of h/2π. This suggests that the angular momentum of an electron in an atom is quantised. On the absorption of energy from one external source and electron jump from a lower energy level to a higher energy level. Incoming back from a higher level to a lower energy level, the electron emits energy. The energy gained or lost in these transitions is usually quantised and is equal to the difference of energy of the two-level involved in the transition. If E¹ and E² respectively are the energy of lower and higher levels involved in the transition of an electron, the ever theories implies that
∆ = E² – E¹= hv
Where hv means the quantised energy absorbed or emitted by an electron.
In summary, the atom is composed of a positively charged nucleus with electrons surrounding it. The negatively charged electrons are in constant motion around the nucleus. In order to do their job they move in waves called orbits which describe specific paths around the nucleus.