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st 1 unit of Chemistry Atomic structure Dalton´s atomic theory (1803) Matter is made of indivisible particles called atoms (we now know that atoms are not indivisible). Elements consist of the same kind of atoms, atoms that are exactly the same (we now know that there are atoms of the same element that are different, those are the isotopes). Atoms of different elements are different (they have different mass and properties). Chemical compounds are made up of molecules, in which atoms are combined according to a fix whole-number ratio. Chemical reaction It was soon discovered that atoms are formed by smaller particles. The first particle that was discovered was the electron, a particle with a negative charge. The atom was not indivisible, so new models of the structure of an atom were needed. Thomson´s atomic model (1904) Atoms can be considered as uniform positively charged spheres in which electrons are embedded. The amount of positive and negative charge is always the same so that the atom is electrically balanced. Rutherford´s experiment (1911) It consisted on shooting alpha particles (which are positively charged) at a thin gold foil. Some of the particles went through the gold foil without diverting, some of them were slightly diverted and some were strongly diverted. This experiment showed that the atom was not homogeneous. Rutherford´s experiment Rutherford´s atomic model Rutherford explained the results of his experiment proposing a new atom model: Atoms are divided into a tiny core, called 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. Rutherford´s atomic model How the model of Rutherford explained his experiment Soon, new particles were discovered: protons and neutrons Particles of the atom: Protons: they have a positive charge. Neutrons: they don´t have charge. Their mass is the same as protons. Electrons: they have a negative charge. Their mass is negligible compared with the mass of protons. Atomic number, Z (número atómico) The number of protons in an atom is called the atomic number. Z = number of protons All the atoms of the same element have the same atomic number. Therefore, it is the atomic number that defines an element. Mass number, A (número másico) It is the number of protons and neutrons in an atom. A = Z + number of neutrons The mass number is not necessarily the same in all the atoms of an element, because the number of neutrons can change in an element. Atoms with the same Z (therefore of the same element) and with different A (therefore with different number of neutrons) are called isotopes. An example of isotopes: isotopes of hydrogen Periodic table Elements are ordered in the periodic table in increasing atomic number. Cations and anions The number of electrons and protons in a neutral atom is the same. An atom can lose electrons and become positively charged, a cation: K → K+ + 1eAn atom can gain electrons and become negatively charged, an anion: O + 2e- → O2- 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. The energy and the radius of each orbit is quantized, that means it has an specific value (it can´t be any number). Each layer has different energy. 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) Bohr´s atomic model (1913) 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 spectrum of each element is specific and unique for that element.