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Atomic Structure Chapter 4 Subatomic particles Electrons-negatively charged, located outside the nucleus in the electron cloud Protons-positively charged, located in the nucleus Neutrons-neutral charge, located in the nucleus Atomic Number Summary The number of protons in the nucleus Also the number of electrons in a neutral atom Located just above the element symbol on the PT (whole number) Elements are listed on the PT according to their atomic number increasing from left to right Mass Number Mass number = protons + neutrons Atomic Mass units Atomic Mass Unit The atomic weight scale has traditionally been a relative scale... the first atomic weight basis was suggested by John Dalton in 1803: 1 amu = mass of 1H (a hydrogen atom) Number of Neutrons of neutrons = mass number – atomic number (protons) # Nuclear Symbols A X Z A = mass number = sum of protons and neutrons in nucleus Z = atomic number = # of protons A – Z = number of neutrons Examples: 80 Br A – Z = (80 – 35) = 45 neutrons 35 Z = 35 protons and 35 electrons Atomic Structure worksheet Atom: electrically neutral, number of protons is equal to the number of electrons Ion: charged particle If an ion has a + charge, it has lost electrons If an ion has a – charge, it has extra electrons (has gained electrons) What is the charge on the Fluorine Ion? Remember: Atoms ONLY GAIN OR LOSE ELECTRONS, they do not gain or lose protons so the charge of an ion comes from either losing electrons (positively charged ion) or gaining electrons (negatively charged ion) **When atoms gain or lose electrons they become ions! Example: 40 K+1 19 A – Z = 40 - 19 = 21 neutrons 19 protons (# of protons can not change) 19 electrons – 1 = 18 electrons Net charge of +1 Example: 32 P-3 15 A – Z = 32 – 15 = 17 neutrons 15 protons 15 electrons + 3 = 18 electrons Net charge of -3 Isotopes o Isotopes are atoms with the same number of protons, but different numbers of neutrons. o Since number of protons and electrons stays the same, isotopes of an element are chemically almost identical. Representing Mass #: Cobalt-58 has a mass number (A) of 58 and cobalt-60 has a mass number (A) of 60 We write: 58 60 Co 27 Co 27 Atomic Mass Number located just below the chemical symbol on the PT Not usually a whole number Isotopes of the same element have different atomic masses (and therefore have different mass numbers) To calculate: Multiply the mass of each isotope by its percent abundance Add the products and divide by 100 to give total atomic mass of the element Calculating Average Atomic Mass: Chlorine has 2 naturally occurring isotopes, 75.0% Chlorine – 35 and 25.0% Chlorine – 37. What is the average atomic mass of Chlorine? Chlorine 35 35 x 75 = 2625 Chlorine 37 37 x 25 = 925 Added together = 3550/100 Average atomic mass = 35.5 amu An Unknown element consists of 2 isotopes. The first has a percent abundance of 20.0% and a mass of 266 and the second an abundance of 80.0% and a mass of 272. What is the average atomic mass for this element? (20.0 *266 + 80.0 *272) 100 = (5320 + 21760 )/100= 27080/100 To 3 sig figs 271 amu Isotopes WS Periodic Table Preview Periodic Table Preview Arranged in order of atomic number Grouped in columns that have similar properties A row is known as a period A column is known a s a group or family Groups All elements in a group have similar properties. Atom - History The smallest particle of an element that retains the properties of that element Idea dates back to ancient Greece https://www.youtube.com/watch?v= AiSC6nxHzCI The structure of the nuclear atom JJ Thomson credited with discovering the electron Cathode ray tube experiment Cathode Ray Experiment Glowing beam when electricity flowed through a gas. The “ray” was attracted toward the (+) plate and was deflected away from the (-) plate (magnet) http://www.youtube.com/watch?v=7YHwM WcxeX8&feature=related What exactly did Thomson learn? Since a beam of electrons (cathode rays) is deflected by an electric field toward the positively charged plate, electrons MUST have a negative charge. When the metal used in the electrodes and the gas used in the tube were changed, the same results were evident. Thus, all atoms must contain electrons. What exactly did Thomson learn? Since matter carries a neutral charge, he deduced that the structure of the atom is a positively charged sphere with negative charges “floating” around in it = “Plum Pudding” model Plum pudding (raisin bread) model Ernest Rutherford Credited with discovering the nucleus Gold Foil Experiment Gold Foil Experiment Alpha particles (positively charged) were expected (according to the then current theory of Dalton) to go through the atoms of gold foil Instead some were deflected Evidence of a concentrated mass of positive charge located in the center of the atom 1 in 8000 particles were deflected Concluded atoms are mostly empty space. The nucleus is 1/10,000 of the size of the atom. Rest of the size of the atom is the electron cloud http://www.youtube.com/watch?v=XBqHkr af8iE&feature=related Rutherford (Planetary) model Protons and neutrons, most of the mass, is in the center, with electrons “orbiting” the nucleus. Which of the following are results of Rutherford’s work? A. Atoms have mass B. Electrons have a negative charge C. The center of the atom has a concentrated positive charge D. The atom is mostly empty space Perspective If the atom was the size of football stadium, the nucleus would be a small marble or pea in the center of that stadium. Remember that most of the mass of the atom is concentrated in the nucleus.