Chapter 4: atoms

Atomic models Democritus – philosophy Atom = indivisible, not-cut; that which is too small to be divided any further

Atomic models Dalton – scientific theory - studied ratios of elements in reactions 1. All elements are composed of tiny indivisible particles (atoms) 2. Atoms of the same element are identical. Atoms of any one element are different from atoms of any different element. 3. Atoms of different elements can physically mix together or can chemically combine in compounds of simple ratios (whole numbers). 4. Chemical reactions= separating, joining, or rearrangement of atoms. Atoms of one element are never changed into atoms of another element as the result of a chemical reaction.

Subatomic particles J.J. Thomson – electrons Cathode ray tube: electrons flow from cathode (-) to anode (+) through a low pressure gas, forming a glowing beam Opposite charges attract, like charges repel Negatively charged plates in the tube deflected the straight ray Charge to mass ratio of electrons is constant despite changes in element

Atomic models Thomson’s atomic model – a.k.a. “plum-pudding model” Electrons are stuck in a lump of positively charged material

Subatomic particles Robert A Millikan – charge of an electron Oil-drop experiment 1.60 ×10-19 coulomb Charge to mass ratio calculated electron’s mass

Subatomic particles Eugene Goldstein – Protons cathode ray tube Rays in opposite direction of cathode ray : positively charge James Chadwick – Neutrons alpha particles, math Neutral charge, mass equal* to protons

Atomic models Ernest Rutherford – Gold foil experiment Alpha particles : helium minus electrons (2 protons) Expected slight deflection from positive alpha particles and positive material of atom Results led to model including a nucleus (tiny core of an atom with all the positive charge and nearly all the mass) Rutherford’s atomic model is known as the nuclear atom.

p+ n0 e- +1 1 -1 1/1836 or 5.45×10-4 Relative charge Relative mass Symbol +1 1 p+ n0 -1 1/1836 or 5.45×10-4 e-

Identifying atoms Atomic number Gives you the number of protons Atomic mass / mass number Gives you the number of protons and neutrons combined # protons = # electrons (in an electrically neutral atom) # of neutrons = mass number – atomic number

Chemical symbols

Chemical Symbols

Atomic mass AMU (atomic mass units): defined as 1/12 of the mass of a carbon-12 atom Isotopes (atoms of an element with a different # of neutrons) The given mass of an element is the average of the mass of its isotopes based on percentages of each isotope found in nature.

Atomic mass of isotopes Hydrogen, Deuterium, Tritium. All have 1 e- and 1 p. Hydrogen-1 has no neutron (mass=1) Hydrogen-2 has one neutron (mass=2) Hydrogen-3 has 2 neutrons (mass=3) Atomic mass of hydrogen = 1.008 because deuterium and tritium are almost non-existent in nature.