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In the second term you studied Rutherford´s atomic model, which stated: Atoms are divided into a tiny core, called the nucleus, and an electron shell. Between the nucleus and the electron shell there is empty space. Most of the atom is empty space. The nucleus of an atom is positively charged and contains almost all the mass of the atom. We know now that it is composed of protons and neutrons. Electrons orbit around the nucleus in circular orbits. The nucleus holds the electrons by means of an electric force between positive and negative charges. But other models were made after Rutherford´s model to explain better the properties of atoms. Bohr´s atomic model (1913) Electrons orbit around the nucleus (which, as we know now, is formed by protons and neutrons) in different layers. In each layer there is a maximum number of electrons: In the first layer there are 2 electrons at most, 8 in the second layer, 18 in the third layer... In the layer n there are 2n2 electrons at most. Each layer has different energy. The energy and the radius of each orbit is quantized, that means it has an specific value (it can´t be any number). Bohr´s atomic model (1913) Bohr´s atomic model (1913) Inner layers have lower energy and outer layers have higher energy. An electron can absorb energy and go to a higher layer. Or it can go to a lower layer and release energy. Bohr´s atomic model (1913) As atoms absorb or release energy in the form of light, absorption and emission spectra of the elements are explained. The spectra of each element is specific and unique for that element.