atomic theory and nuclear chemistry

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element
a substance that cant be broken down by ordinary chemical means
chemical reaction
transformation of a substance into one or more new substances
basic chemical law
law of conservation of mass
law definite proportions
law of multiple proportions
law of conservation of mass
mass is neither created nor destroyed during chemical reactions
ex: 32g O2 + 2g H2-> 34g H2O
law of definite proportions
all samples of a compound contain the same proportion (by mass) of the elements, regardless of sample size or source of compound
EX: 10gH2O 6% H 94% O
law of multiple proportions
applies to different compounds made from same element, ratios
EX: N +O -> NO
N +O+O -> ONO
Ratio: 2:1
Dalton’s atomic theory
1. all matter is composed of extremely small particles called atoms (all)
2. atoms of the same elements are identical in size, mass and other properties; atoms of different elements differ in size, mass and other properties (all)
3. atoms can not be subdivided, created or destroyed (cm)
4. atoms of different elements combine in simple whole number ratios to form compounds (DP)
5. In chemical reactions atoms are combined separated or rearranged (cm)
Dalton’s model of the atom
indivisible, uniformly dense, participated in but not changed by chemical reactions
modern atomic theory (differences between Dalton’s and modern)
1. atoms are not indivisible- made up of smaller particles
EX: protons, neutrons, electrons, electrons are temporarily altered when chemical change takes place
2. atoms can be changed by nuclear reactions
3. atoms of the same element are not exactly alike
EX: isotopes have different number of neutrons, Ions have different number of electrons
discovery of electron
discovered through the use of the cathode-Ray tube
cathode-ray tube
electric current (glowing beam) flows from cathode to anode when electrons are connected to voltage source, magnet placed in closed tube deflected beam away from negatively charged object, able to push paddle down tube. beam consisted of charged particles, NOT LIGHT SINCE LIGHT HAS NO CHARGE. cathode rays are negatively charged
JJ Thomson
measured the ratio of the charge of cathode-Ray particles to their mass. ratio was always the same regardless of the mental used or the gas inside the tube. concluded that all cathode rays are composed of identical negatively charged particles called electrons.
Thomson’s model of the atom
“plum pudding” model, negatively charged electrons embedded in a ball of positive charge
Robert A. Millikan
used thomsons charge to mass ratio and calculate the mass of the electron. (1/1836 of H atom, 9.109 x 10 to the -31 KG) also created the oil drop experiment
Millikans oil drop experiment
adjusted the voltage until droplets were held motionless between charge plates, found the gravity mass of electron and electrical force between plates, then calculated the charge of the electron
electron
negatively charged, present in all atoms, very small particles very light compared to mass of the atom, moves rapidly within atom
discovery of atomic nucleus
knew atoms were neutral weighed more then electrons, must contain other particles to balance out negative charge and make up mass (Ernest Rutherford)
Ernest Rutherford
created the gold foil experiment, discovered atomic nucleus
gold foil experiment
when a beam of particles is directed at a thin gold foil most particles pass through the foil undefeated, small numbers are defeated at large angles, few bounce back towards the particle source, he discovered that most of an atom is empty space, nucleus of an atom is densely packed bundle of matter with a electrical charge, volume of the nucleus is small compared to total volume of atom, suggested that electrons surround nucleus
protons
positively charged, equal in magnitude to electron, all atoms are electrically neutral (#p=#e), found in nucleus of atom, mass is 1836 times greater than that of electron (1.672x 10 -27kg), number of protons determines atoms identity
Discovery of Neutron
Rutherford model did not explain atomic masses, 1920 Rutherford proposed that neutral particles with roughly the same mass as proton existed in nucleus
James chadwick
knew beryllium emitted some kind of neutral radiation, proposed that neutral radiation came from neutrons
neutron
found in nucleus, same mass as proton, no electrical charge, energy corresponds to alpha particle energy
the nucleus holds
protons and neutrons
nuclear forces hold nucleus together by
short range attractions between protons-protons protons-neutrons neutrons-neutrons
radius
distance from center of nucleus to edge of electron cloud, measured in picometers (10 to the -12m)
atomic radii
range from 40-270pm
nuclear radii
.001pm
high nuclear density =
2×10 to the 8 metric ton/cm cubed
hydrogen and carbon isotopes
same # protons = atomic number, behavior of chemical property, different masses, mass # = p + n
radioactive decay
spontaneous disintegration of a nucleus, becomes a lighter nucleus
radioactive nuclei
unstable
nuclear radiation
particles or electromagnetic are emitted when nucleus is disintegration (alpha particles – helium nuclei) all atoms of atomic number < than 84 are unstable
types of radiation
alpha (stopped by paper), beta( stopped by clothing), gamma rays ( stopped by concrete)
half life
time required for half the atoms of radioactive substances to decay, half life’s are constant, the more stable a radioactive substance is the longer the half life ( more time it takes to decay)
nucleons
p + n
nuclide
atom
Categories: Nuclear Chemistry