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The Atom The model of the atom has evolved from Dalton's concept of it as a "solid billiard ball" to a highly complicated model. At first it was felt that it could not be broken down. It was soon discovered the atom could be broken down into 3 sub-atomic particles known as electrons, protons and neutrons. ELECTRON: a particle which has a relative charge of -1 much smaller in mass than neutrons and protons PROTON a particle which has a relative charge of +1 NEUTRON: uncharged particle very similar in size and mass to protons Protons and neutrons (nucleons) are located in the middle of the atom in an area known as the nucleus (Rutherford Gold Foil Experiment). ATOMIC NUMBER refers to the number of protons in the nucleus it determines the identity of the element (every atom of each element has its own unique number of protons) ATOMIC MASS the mass of the atom expressed in atomic mass units It was impossible for scientists to determine the mass of individual atoms (they are too small!), therefore they attempted to assign relative atomic masses that agreed with the known compositions of compounds. a new unit was developed to mass atoms . C-12 was chosen as the reference standard. An atom of C-12 was arbitrarily assigned a mass of 12 atomic mass units. The masses of all other atoms are compared with the mass of this type of carbon atom. According to this definition, an atomic mass unit is defined as 1/12 the mass of a carbon-12 atom. the most important sub-atomic particles you have studied thus far have the following atomic mass units: electron: 0.000 549 u proton: 1.0073 u neutron: 1.0087 u MASS NUMBER the total number of protons and neutrons in an atom (symbolized by the letter A) ISOTOPE All atoms of the same element must have the same number of protons in the nucleus, but they do not have to have the same number of neutrons atoms whose nuclei contain the same number of protons but a different number of neutrons NEUTRAL ATOM an atom in which the number of protons equals the number of electrons All the elements can be represented as symbols that are organized in the periodic table. Carbon is represented by 6 protons 6 neutrons 12 refers to the number of protons 6 C In most atoms the number of protons does NOT equal the number of neutrons. There is usually more neutrons than protons. MASS NUMBER - number of protons and neutrons A ATOMIC NUMBER - number or protons Z X CHEMICAL element SYMBOL of the Summary: SUBATOMIC PARTICLE proton MASS LOCATION CHARGE 1 amu nucleus 1+ nucleus shell No charge 1- (atomic mass unit) neutron electron 1 amu Almost = 0 Each element has a unique type of atom based on the numbers of protons in the nucleus. There fore: Hydrogen atoms have (1) one proton 1 protons 0 neutrons 1 refers to the number of protons 1 Helium atoms have (2) two protons 2 protons 2 neutrons 4 refers to the number of protons 2 H He Lithium atoms have (3) three protons 3 protons 4 neutrons refers to the number of protons 7 3 Li Atomic Mass Calculations Isotopes are atoms of the same element that have different numbers of neutrons. For example, carbon has two naturally occurring isotopes: Carbon-12 and Carbon-13. In a sample of carbon 98.89% of the carbon is 1.11% of the carbon is 13 12 C and C. To calculate the Average Atomic Mass of carbon both isotopes must be taken into consideration. Average = (Atomic mass of A)(% of A) + (Atomic mass of B) (% of B) Atomic Mass = (12 u)(0.9889) + (13 u)(0.0111) = 11.87 u + 0.144 u = 12.01 u