DISCOVERY OF THE PRESENT MODEL OF THE ATOM (Ernest Rutherford- 1910 (His…
DISCOVERY OF THE PRESENT MODEL OF THE ATOM
Democritus - 440BC
A greek philosopher who proposed the idea that there was a limit in how many times matter could be divided.
He called the smallest piece of matter “atomos” meaning not to be cut.
He was dismissed due to lack of evidence
He believed matter was not made up of atoms but elements such as water, air, earth and fire. He also thought it could be divided forever.
These ideas were incorrect but they weren’t questioned for a very long time.
John Dalton- 1808AD
2000 years later Dalton created the foundation for for modern models of the atom
Dalton proposed his theory in which he described the atom as a invisible andtiny hard sphere. Each have their own properties due to their element.
His theory included that
atoms are building blocks of matter
Can not be created or destroyed or even subdivided
Atoms of the same element are the same size, mass and other properties.
Compounds are formed by a combination of two or more different kins of atoms.
Joseph John Thompson- 1897
In the 19th Century, J.J. Thompson’s research in cathode rays led to the discovery of electrons
Thomson concluded that atoms had an overall neutral charge. Therefore there must be a source of positive charge within the atom to counterbalance the negative charge on the electrons.
He proposed that atoms are uniform spheres of positively charged matter with negatively charged electrons embedded within.
Ernest Rutherford- 1910
Rutherford's gold foil experiment was determined to discover the structure of an atom. Already he knew that there were electrons in atoms that are negatively charged. he just wanted to test the plum pudding model
His students Geiger and Marsden fired dense alpha particles (positively charged) at a thin gold foil.
They expcted all the particles to pass straight through the gold atoms
Air was removed from the apparatus so ionisation would not occur and collision between air particles.
Most particles went through. They were detected when they hit zinc sulfidee screen. Some alpha particles came straight back
Due to some particles bouncing back this meant that the nucleus of the atom is small and positively charged therefore repelled the particles.
Their results were used to create a new model for the atom called the Nuclear model.
The nuclear model also shows that most of the mass must be concentrated at the centre and most of an atom is empty space.
Lastly, his model proposed that electrons orbit in clouds around the nucleus which contains dense positively charged protons like alpha particles.
Niels Bohr- 1914
Bohr thought this model was incorrect as the atom would just collapse if the electrons where a 'cloud' surrounding the nucleus.
He wanted to answer the question why electron were not attracted to the nucleus.
He suggested that the electrons were orbiting the nucleus at set distances in certain fixed energy levels
The energy must be given out when excited electrons fall from a high to a low energy level.
Futher research- 1932
They had no evidence of protons but a second sub-atomic particles in the nucleus was also proposed to explain the missing mass. There neutrons must have no charge and the same mass as protons.
Due to having no charge it was difficult to detect them in experiments. Until James Chadwick conducted an experiment to prove the existence of neutrons.