Question

write a report on the history of unveiling the structure of atom from john Dalton to Neils Bohr

Answer 1

John Dalton, an English chemist, meteorologist and physicist, began a series of experiments which made him propose the theory of atomic compositions, which are known as Dalton’s Atomic Theory. This theory is one of the cornerstones of modern physics and chemistry.



According to Dalton's Atomic Theory



1) All matter is made of atoms. Atoms are indivisible and indestructible.

2) All atoms of a given element are identical in mass and properties

3) Compounds are formed by a combination of two or more different kinds of atoms.

4) A chemical reaction is a rearrangement of atoms.



However, the modern atomic theory is a little more involved than Dalton's theory but the essence of Dalton's theory remains valid.



The 4 postulates of Bohr's atomic model are:-

1) Electrons assume only certain orbits around the nucleus. These orbits are stable and called "stationary" orbits.

2)Each orbit has an energy associated with it. For example, the orbit closest to the nucleus has an energy E1, the next closest E2 and so on.

3)Light is emitted when an electron jumps from a higher orbit to a lower orbit and absorbed when it jumps from a lower to the higher orbit.

4) The energy and frequency of light emitted or absorbed is given by the difference between the two orbit energies, e.g, E (light) = Ef - Ein = E(light)/hh = Planck's constant = 6.627 x 10⁻³⁴ Jswhere "f" and "i" represent final and initial.

Answer 2

The word atom comes from the Ancient Greekadjective atomos, meaning "indivisible"

The idea that matter is made up of discrete units is a very old one, appearing in many ancient cultures such as Greece and India. The word "atom" was coined by the ancient Greek philosophers Leucippus and his pupil Democritus. However, these ideas were founded in philosophical and theological reasoning rather than evidence and experimentation.


John Dalton

John Dalton studied and expanded upon this previous work of The first was the law of conservation of mass, formulated by Antoine Lavoisier in 1789. The second was the law of definite proportions. First proven by the French chemist Joseph Louis Proust in 1799, this law states that if a compound is broken down into its constituent elements, then the masses of the constituents will always have the same proportions, regardless of the quantity or source of the original substance. And he developed the law of multiple proportions: if two elements can be combined to form a number of possible compounds, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers.

Dalton proposed that each chemical element is composed of atoms of a single, unique type, and though they cannot be altered or destroyed by chemical means, they can combine to form more complex structures (chemical compounds). This marked the first truly scientific theory of the atom

Dalton estimated the atomic weights according to the mass ratios in which they combined

However, Dalton did not conceive that with some elements atoms exist in molecules—e.g. pure oxygen exists as O2. He also mistakenly believed that the simplest compound between any two elements is always one atom of each (so he thought water was HO, not H2O)

Avogadro

The flaw in Dalton's theory was corrected in principle in 1811 by Amedeo Avogadro. Avogadro had proposed that equal volumes of any two gases, at equal temperature and pressure, contain equal numbers of molecules (in other words, the mass of a gas's particles does not affect the volume that it occupies).

or PV=nRT


Discovery of subatomic particles

Atoms were thought to be the smallest possible division of matter until 1897 when J.J. Thomson discovered the electron through his work on cathode rays

Thomson suggested that atoms were divisible, and that the corpuscles were their building blocks. To explain the overall neutral charge of the atom, he proposed that the corpuscles were distributed in a uniform sea of positive charge; this was the plum pudding model as the electrons were embedded in the positive charge like plums in a plum pudding (although in Thomson's model they were not stationary).


Discovery of the nucleus

Thomson's plum pudding model was disproved in 1909 by one of his former students, Ernest Rutherford, who discovered that most of the mass and positive charge of an atom is concentrated in a very small fraction of its volume, which he assumed to be at the very center.


First steps toward a quantum physical model of the atom

Main article: Bohr model

Quantum theory revolutionized physics at the beginning of the 20th century, when Max Planck and Albert Einstein postulated that light energy is emitted or absorbed in discrete amounts known as quanta (singular, quantum). In 1913, Niels Bohr incorporated this idea into his Bohr model of the atom, in which an electron could only orbit the nucleus in particular circular orbits with fixed angular momentum and energy, its distance from the nucleus (i.e., their radii) being proportional to its energy. Under this model an electron could not spiral into the nucleus because it could not lose energy in a continuous manner; instead, it could only make instantaneous "quantum leaps" between the fixed energy levels. When this occurred, light was emitted or absorbed at a frequency proportional to the change in energy (hence the absorption and emission of light in discrete spectra).

Bohr's model was not perfect. It could only predict the spectral lines of hydrogen; it couldn't predict those of multielectron atoms. Worse still, as spectrographic technologyimproved, additional spectral lines in hydrogen were observed which Bohr's model couldn't explain. In 1916, Arnold Sommerfeldadded elliptical orbits to the Bohr model to explain the extra emission lines, but this made the model very difficult to use, and it still couldn't explain more complex atoms.





the theory is very large dear
we cannot complete it here
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