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Chemistry – Atomic structure & Nuclear Chemistry Chapter 4 & 25 History of the Atom • Democritus (400 BC) – Named the atom as indestructible and indivisible. – His ideas of the atom did not explain chemical behavior. • John Dalton (1766-1844) – Studied the ratios in which elements combine. – Discovered the real nature of atoms and the connection between observable changes. Dalton’s Atomic Theory Dalton’s Atomic Theory • 5 parts (in 1906) 1. All matter is made of atoms 2. **All atoms of the same element are identical in mass, volume, and properties. 3. Atoms can’t be created, destroyed, or divided. 4. Atoms combine in small, whole number ratios 5. In chemical reactions, atoms are combined, separated or rearranged, NOT turned into another element. Sizing up the atom • A coin the size of a penny has 2.4 x 1022 atoms of pure copper. (all atoms act like copper) • Earth’s population is 6 x 109. • Despite their small size, atoms are observable with scanning tunneling microscopes. More History • In 1897, Thompson discovers the electron using a Cathode Ray Tube. – A Cathode Ray Tube charge is negative (-). A magnetic field pushed the stream away. – The electron is discovered!!!! • Atoms redefined: Smallest particle of an element that retains its properties. More History • Discovery of the Nucleus – Rutherford (1909) discovered the nucleus in his Gold foil Experiment. – He shot alpha particles (Helium) through gold and determined: • The nucleus is a very densely packed bundle of matter with a positive charge. • That atoms are mostly made of empty space. Atomic Structure • Electrons: Negatively charged, outside the nucleus of the atom. – Mass is 1/1840 the mass of a hydrogen atom. • Nucleus- Tiny central core of the atom consisting of Protons & Neutrons. • Protons- Positively charged particles. – Each proton is about the size of a hydrogen atom. • Neutrons – Particles with no charge, but equal to the size of a proton. Distinguishing Among Atoms • Atomic Number – Equal to the number of protons in the nucleus in an atom of that element. • Almost all elements are electrically neutral, therefore the number of electrons equals the number of protons. • Mass Number – The total number of protons and Neutrons. – Always a whole number. Never a decimal. Distinguishing Among Atoms • Determining # of Neutrons # of neutrons = mass number – atomic number • Example: Gold has a mass number of 197 and an atomic number of 79. How many neutrons does Gold have? Whole Numbers of e-, n◦, p+ • There can only be a whole number of electrons, neutrons, and protons in a substance. • The elements are written in either of these 2 ways: 108 𝐴𝑔 : 108 = Mass #; 47 = Atomic # 47 – Neutrons = (mass # - Atomic #) = (108 – 47) = 61 – Ag – 108. Means the same as above. – • Silver always has 47 protons, so it is unnecessary to write it in the above format. Isotopes • Isotopes: – Atoms that have the same number to protons, but different numbers of neutrons. This will also create different mass numbers. • Examples: 108 109 111 – Ag , Ag , Ag or Ag-108, Ag-109, Ag-111 47 47 47 – These are still all Silver and have all of the same properties and characteristics as Silver. AMU’s An Atomic Mass Unit (amu) is defined as 1:12 the mass of a carbon-12 atom. Any proton or neutron is equal to 1 amu’s. The numbers on the periodic table are written as amu’s in decimal form. Atomic Mass • Atomic mass of an element is a weighted average mass of the atoms in a naturally occurring sample of the element. • Example: Silver has an atomic mass of 107.87. – This means that a sample of Silver will have some Ag-107, Ag-108, Ag-109, etc., and that the average of those atoms is 107.87 Calculating Atomic Mass • In order to calculate the atomic mass of an element you must know: a) The number of stable isotopes b) The mass of each isotope c) The natural abundance (percentage) of each isotope. Calculating Atomic Mass • Carbon has two stable isotopes: Carbon-12, which has a natural abundance of 98.89%, and carbon-13, which has a natural abundance of 1.11%. • Atomic Mass of carbon = =(12.000 amu x 0.9889) + (13.003 amu x 0.0111) = 12.011 amu • This is why there are decimals on the periodic table. Periodic Table • Elements are listed in order by Atomic #, which is the # of protons. • The # of protons defines the element. • The mass of elements can change: Isotopes.