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The Structure of an Atom Ch.4 Notes Section 1: Early Theories of Matter •Atom- the smallest particle of an element that retains the properties of the element. •Size? The world’s population is 6,840,000,000 •The number of atoms in a penny is 29,000,000,000,000,000,000,000 Early Theories of Matter • Ancient Greek philosopher Democritus speculated matter was made up of atomos • The theory was rejected for 2000 years due to being challenged by Aristotle Discovery of the Electron • John Dalton- proposed his atomic theory in 1803 based on his research that was not completely accurate • Atoms can be subdivided • Atoms of the same element can have slightly different masses Discovering the Electron •Discovering the electron •Sir William Crookes - used a cathode ray tube to do experiments on the atom based on the relationship between mass and charge. Electrons are negatively charged Discovering the Atom •JJ Thomson (1856-1940)- in the 1890’s found a cathode ray could be deflected by magnetic and electric fields. He concluded the ray was a stream of negatively charged particles dislodged from an atom with a mass much less than a hydrogen atom. http://www.yout ube.com/watch? v=GzMh4q2HjM Discovering the Atom •Robert Millikan (1868-1953) – in 1909 he accurately determined the mass of an electron through the Oil Drop Experiment, it had a charge of -1 •The actual mass of an electron is 9.1 x 10-28 g Nucleus • Plum Pudding Theory-( aka the chocolate chip cookie model) • Matter is neutral, so Thomson proposed: spherical atoms have uniform (+) charge with (-) electrons embedded throughout. Nucleus http://www.youtube.com/wa tch?v=wzALbzTdnc8 • Ernest Rutherford, in 1911 • To test the theory, he aimed a thin beam of (+) charged Alpha particles at a thin sheet of gold. • He expected the alpha particles to pass through with little deflection • Instead, most passed through, but few bounced back and deflected at big angles. • He concluded atom is mostly empty space with large (+) charged nucleus in center Atomic Summary *Nucleus -protons + neutrons -contains all the mass -positive charge *Electrons -located in the cloud -fast moving -very small Properties of Subatomic Particles Particle Symbol Location Relative Charge Mass Electron e Electron Cloud -1 Proton + p Nucleus +1 1 Neutron 0 n Nucleus none 1 1/1840 Actual Mass 9.11 x 10-28 1.673 x 10-24 1.675 x 10-24 How atoms differ: •Atomic number is how an atom is identified, it is equal to the number of protons in an atom, & determines the location of the atom in the periodic table. •The periodic table is arranged left to right & top to bottom by increasing atomic number. atomic # Chemical Symbol Average atomic mass 1 H 1.008 Atomic Number Number of protons in nucleus of an element This number identifies the element Periodic table is arranged in sequence of increasing atomic numbers Atoms are neutral: (+) charges must equal (-) charges We know: atomic # = #protons = # electrons •How many protons and electrons are there in: •Be: p= 4 e= 4 •F: p= 9 e= 9 Ions • Charged particles: Number of electrons has changed- proton # stays the same • Be2+ has lost 2 electrons: p= 4 e= 2 • F1- has gained one electron: p=9 e=10 Isotopes •To identify an isotope a number is added to the end of the name, it is called a mass number. •Mass Number = # of protons + # of neutrons •Ex: Carbon-14, Neon-22, Potassium-41 • Isotopes • All atoms of same element have some number of protons • Most have different numbers of neutrons Mass Number • An isotope is identified by its mass number • Mass number= number of protons + number of neutrons • Number of neutrons = mass number – atomic number Atomic Mass • Atomic mass units (amu) = ½ the mass of a Carbon-12 atom • Atomic mass given in Periodic Table is not given in whole numbers • Atomic mass is the weighted average of the atomic masses of all naturally occurring isotopes. This weighted average takes into account the mass and abundance of each isotope Atomic Mass • Average atomic mass= mass * abundance for each isotope: then add Copper-63 Copper-65 Number of protons 29 29 Number of neutrons 34 36 Atomic mass 62.930 amu 64.928 amu Abundance 69.17% 30.83% • Mass Contribution = mass x abundance • Cu-63= (62.930)(.6917)= 43.53 • Cu-65= ( 64.928)(.3083) =20.02 • 43.53+20.02= 63.55