Recognize discoveries from Dalton (atomic theory), Thomson (the electron), Rutherford (the nucleus), and Bohr (planetary model of atom) and understand how these discoveries lead to the modern theory.
Describe Rutherford’s “gold foil” experiment that led to the discovery of the nuclear atom. Identify the major components (protons, neutrons, and electrons) of the nuclear atom and explain how they interact.
Write the electron configurations for the first twenty elements of the periodic table.
All matter is composed of atoms Atoms cannot be made or destroyed All atoms of the same element are identical Different elements have different types of atoms Chemical reactions occur when atoms are rearranged Compounds are formed from atoms of the constituent elements Atomic theory proposed by John Dalton
In 1912, Rutherford discovered the nucleus by doing scattering experiments . He concluded the atom was mostly empty space, with a large dense body at the center, and electrons which orbited the nucleus like planets orbit the Sun.
The model created by Rutherford had still some serious discordance. According to the classic science, electron moving around the nucleus should emit an electromagnetic wave. Electron should than move not by the circle but helical and finally collide with the nucleus. But atom is stable.
Following Rutherford’s planetary model of the atom, it was realized that the attraction between the electrons and the protons should make the atom unstable Bohr proposed a model in which the electrons would stably occupy fixed orbits, as long as these orbits had special quantized locations
Electrons move around the nucleus at stable orbits without emitting radiation.
Electron in one of these stable orbit has a definite energy.
Energy is radiated only when electrons make transitions from high energy orbit to a low energy orbit.
In the Bohr model, the electron can change orbits, accompanied by the absorption or emission of a photon of a specific color of light.
Bohr’s Model 1913 – Atomic Model with Fixed Orbits proposed – The Atom has No Charge Number of Protons = Number of Electrons
An easy way to calculate the total number of electrons that can be held by a given energy level is to use the formula 2*n 2 , 98 7 72 6 50 5 32 4 18 3 8 2 2 1 Maximum number of electrons 2*n 2 Principle energy level ( n )
The most electrons possible in the first shell are 2. After the first shell is filled, the second shell starts filling up, according to the number of positive charges in the nucleus. The most allowed in the second shell is 8 electrons. Then the third shell starts to fill.