Atomic Theory

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Atomic Theory

Section 1: Early Theories of Matter!• Atom- the smallest particle of an element that retains theproperties of the element.• Size? The worlds population is 6,840,000,000• The number of atoms in a penny is29,000,000,000,000,000,000,000


Early Theories of Matter!• Ancient Greek philosopher Democritus speculated matterwas made up of atomos!• The theory was rejected for 2000 years due to beingchallenged by Aristotle!


Discovering the Atom!• JJ Thomson (1856-1940)- in the 1890’s found acathode ray could be deflected by magnetic andelectric fields. He concluded the ray was a stream ofnegatively charged particles dislodged from an atomwith a mass much less than a hydrogen atom.""


Discovering the Atom!• Robert Millikan (1868-1953) – in 1909 he accuratelydetermined the mass of an electron, it had a charge of-1"• The actual mass of an electron is 9.1 x 10 -28 g "


Nucleus!• Ernest Rutherford, in 1911!• To test the theory, he aimed a thin beam of (+) chargedAlpha particles at a thin sheet of gold.!• He expected the alpha particles to pass through with littledeflection!• Instead, most passed through, but few bounced back anddeflected 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"


How atoms differ:!• Atomic number is how an atom is identified, it is equal to thenumber of protons in an atom, & determines the location of theatom in the periodic table.• The periodic table is arranged left to right & top to bottom byincreasing atomic number.


atomic #!1!Chemical Symbol!H!Average atomic mass!1.008!


Atomic Number!" Number of protons in nucleus of an element!" This number identifies the element!" Periodic table is arranged in sequence ofincreasing 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!• Be^+2 has lost 2 electrons: p= 4 !e= 2!• F^-1 has gained one electron: p=9!e=10!


Isotopes!• To identify an isotope a number is added to the end ofthe 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 havesome number of protons!• Most have different numbers ofneutrons!


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 wholenumbers!• Atomic mass is the weighted average of the atomic masses ofall naturally occurring isotopes. This weighted average takesinto 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!

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