T HE S TRUCTURE O F T HE A TOM Models and Theories Subatomic Particles Atomic Number and Mass Unstable Nuclei

M ODELS AND T HEORIES chemistry/chapter_8/pages/history_of_atom.html eature=related ature=related

A H ISTORICAL L OOK 400 BC, Democritus, Greek philosopher, “atomos” meaning indivisible 400 BC, Aristotle – Air, Earth, Fire, Water, Aether 1805 (AD), Dalton’s Atomic Theory: Elements are composed of indivisible particles, called atoms. Atoms from the same element are identical (except isotopes). Atoms combine in simple, whole number ratios. In chemical reactions, atoms can be separated, combined, or rearranged. Atoms cannot change into atoms of a different element (by chemical means).

1897, Thomson: used the cathode ray tube, discovered negatively charged particles, electrons, developed the plum pudding model 1909, Millikan: used the oil drop experiment, discovered the charge & mass of an electron 1911, Rutherford: used the gold foil experiment, discovered the nucleus, a positively charged, dense center to the atom 1932, Chadwick: discovered the neutron The nucleus contains protons and neutrons. The atom is electrically neutral.

Start of the Modern Atomic Theory 1900, Planck: matter absorbs and releases energy in specific amounts called quanta 1905 – Einstein: proposed that light can act as both waves & particles called photons 1913, Bohr: described electrons in energy levels called orbits around the nucleus, developed the planetary model – only worked for Hydrogen 1920s, Heisenberg – The Heisenberg Uncertainty Principle – can’t precisely know the location and velocity of an electron 1926, Schrodinger: mathematical equation to describe the location of electrons in areas called orbitals Modern Era – Quantum Mechanical Model: based on the probability of electrons in specific regions of space

S UBATOMIC P ARTICLES : Electron Proton Neutron

A TOMIC N UMBER AND M ASS

In the periodic table Equal to the number of protons Defines an element’s physical and chemical properties Also equal to the number of electrons (in a neutral atom) Mass Number: Not on the periodic table Equal to the number of protons plus the number of neutrons (totals the number of particles in the nucleus) Atomic Mass: On the periodic table Is a weighted average of the mass numbers of various isotopes Isotopes = atoms of the same element, having different numbers of neutrons Atomic NumberMass

I SOTOPE N OTATION : Isotope name vs. symbol? Number of protons, electrons, neutrons? Carbon – 12 Carbon – 14

C ALCULATING A VERAGE A TOMIC M ASS : Boron has two naturally occurring isotopes. Boron – 10 has an abundance of 19.8 %. Boron – 11 has an abundance of 80.2 %. Average atomic mass of Boron? Atomic mass unit (amu)

H ISTORY CONTINUED … 1790s Lavoisier composed a list of 23 known elements Mid-1800s About 70 elements are known Mendeleev organizes the Periodic Table by atomic mass 1913 Moseley re-organizes the Periodic Table by atomic number Periods  Groups (families) 

U NSTABLE N UCLEI AND N UCLEAR R EACTIONS

N UCLEAR C HEMISTRY Radioactivity = (radioactive decay) Process where nuclei of unstable isotopes (radioisotopes) give off energy and particles (radiation) to gain stability Discovered by Marie Curie Uranium Radiation Alpha (): 4 2 He, mass = 4 amu, charge = +2 Beta (an electron): 0 -1 β, mass = 1/1840 amu, charge = -1 Gamma (energy): 0 0 γ, mass = 0, charge = 0

R EACTION C HARACTERISTICS Occur when bonds are broken and formed. Atoms remain unchanged, though they may be rearranged. Involve only outer electrons. Associated with small energy changes. Reaction rate is influenced by environment. Occur when nuclei emit particles and/or rays. Atoms are often converted into atoms of another element. May involve protons, neutrons, and electrons. Associated with large energy changes. Reaction rate is not normally affected by environment. Chemical ReactionsNuclear Reactions

N UCLEAR R EACTIONS ( SEE TEXT PAGE 812) Alpha radiation: Beta radiation:

Gamma radiation: Electron-capture: Positron emission:

Half-life: Characteristic rate of decay for each radioisotope t 1/2 = time required for one-half of the radioactive atoms to decay; one-half remaining radioactive Can be seconds or billions of years (p. 818) Decay series Keeps on going p. 814 # 11, 12 and p. 820 # 21 Fission: Splitting of nucleus into smaller fragments by impact with moving neutron Atomic bombs, nuclear power plants, nuclear reactors (submarines) Fusion: Nuclei combine to produce a nucleus of greater mass In the sun (at very high temperatures, matter exists as plasma, gas-like)