Atomic Structure & timelineAtom is a basic unit of matter that is not indivisible & is composed of many smaller subatomic particles. Atoms have three main important subatomic particles that are proton, neutron & electron. Proton & neutron have a much greater mass than the electron as one proton equals to 1/1840 electrons. Neutrons & protons are tightly bound together to form the nucleus of the atom hence nucleus contains all the mass. Neutron is electrically neutral where protons & electrons carry +ve & -ve charge respectively. Electrons occupy shells around the nucleus whereas protons are there in the nucleus as we knowEvolution of atoms: -1. Democritus was the first one to come up with the postulates stating that everything is made of atoms, atoms are indivisible & are solid & small particles & called them atomos, this all happened between 460BC-370BC2. The Dalton came up with his theory & research stating that atoms cannot be created or destroyed in chemical reactions, all atoms of the same element are perfectly identical & each element has a different atom and a different atomic weight. year 1766-1844 3. Then Thomson came with his Plum Pudding Model & Discovered that atoms ARE divisible & Inferred that atoms have a positive charge; Atoms have a much larger mass than electrons & discovered electrons through his Cathode-Ray experiment. year 1856-1940 4. Rutherford a well scientist of his time as we know who conducted the gold foil experiment & stated that atoms are mostly empty space. Electrons move rapidly in the empty space around the nucleus & predicted the existence of neutrons & discovered that most of an atoms mass is in its nucleus which is small, dense positively charged. year 1871-1937 5. Bohr Stated that electrons can only travel at specific distances from the nucleus, & stated that electrons can jump between levels/paths around the nucleus & Stated that the chemical properties of an element are determined by how many electrons are in the outer shell. year 1885-19626. Chadwick- Discovered the previously unknown part of the nucleus of an atom- the Neutron, Named the neutron because of its lack of an electrical charge, His discovery made it possible to create new elements in the laboratory (heavier than Uranium) which was not possible before. Year 1891-19747. Electron cloud model - This is our most modern and accepted model of the atom, It differs from Bohrs model because it states that the electrons can be found in a general region around the nucleus, not specific paths

1808: Daltons model of the atom was the billiard ball model. Hethought the atom was a solid, indivisible sphere. Atoms of eachelement were identical in mass and their properties. Atoms of oneelement differed from that of another atom.

1897: JJ Thomsons model of the atom was the plum pudding model.He discovered the electron and believed that a massive positivelycharged substance filled the atom. The electrons were arrangedwithin this substance.

1898: Ernest Rutherfords model of the atom was the nuclear model.He directed particles at a thin sheet of gold foil. Most of the particles passed through the foil but some were deflected at largeangles. These results could only be explained if all the positive chargeof the atom was concentrated in a tiny, massive central core calledthe nucleus. an atom is 10000 times larger than the nucleus. It is mostlyempty space.

1911: Neils Bohrs began work on the atom. There was aninconsistency with the nuclear atom and classical theory. Rutherfordsmodel did not account for the lack of emission of radiation aselectrons move about the nucleus (at the rate an electron would loseenergy, it would spiral into the nucleus) and the emission of light atonly certain wavelengths (an accelerated electron should radiateenergy at all wavelengths).

Bohrs Postulates- Electrons exist in circular orbits. It is the electrostatic forcethat holds them in orbit rather than a gravitational force.- Electrons only exist in allowed orbits. In each orbit there is atotal amount of energy so these orbits are described as energylevels. This means that the energy of electrons in atoms isquantized.- If an electron remains in orbit, it does not radiate energy.- When electrons jump between orbits, they absorb or emit anamount of energy that is equal to the difference in the energylevels.