Ch. 4 Notes – Atomic Stucture
What is an atom? What do you know?
-Nucleus, particles, basic structure of matter
-Table for subatomic particles, location charge and mass
-Basic picture of the atom = does it really make sense? Protons should repel, electrons attract, empty?
- All matter is made up of atoms
- Atoms of the same element are chemically alike
- Atoms of same element can differ in charge or mass
- Atoms of different elements can weigh the same
- Atoms cannot be divided or changed in a chemical rxn
Timeline
Democritus – tiny indestructible indivisible particles (like bb’s)
- began this idea that everything is made up of something (is there truly a fundamental particle?)
Dalton – began experimentation to test for difference between elements, compounds and mixtures
-critique the theory
- matter is made up of atoms, but they can be divided by a nuclear rxn or radioactivity
- atoms of the same element have identical protons, but not always same electrons or neutrons
- still true
- still true except a nuclear rxn can change an atom
At this point atoms are still bb’s = neutrally charged solid spheres of unique properties
Series of experiments that established subatomic particles, their charges and masses
Thomson = cathode ray tube established negative charged particles
Goldstein = protons
Chadwick = neutrons
Plum Pudding Model = matter is made of solid spheres that have + and – charges inside of them
- an attempt to corroborate the solidity and neutral charge of matter with the idea of charged particles
- chocolate chip cookie ball (negative chips in a positive batter)
- contrast this concept with what we know an atom looks like today = how could it change so much
Rutherford Gold Foil Experiment = truly revolutionary, paradigm shifting experiment (never the same)
Actually performed by Marsden and Geiger (student assistants)
Radioactive material (radium) in a lead box (ray gun) shot at gold foil with a fluorescent (ZnS)
screen around it
Expected the radiation to pass mostly straight through the foil due to the intense nature of radiation
Observations
the radiation went largely through but was also deflected more than they thought it would
accidentally noticed some of the radiation was not just deflected, but bounced
Consequences/Conclusion
Radiation shouldn’t defect much, and it definitely shouldn’t bounce
There must be something amazingly dense in the center
but it still weighs the same, so it must be mostly empty space with a dense center
But it can’t just be dense, it must also be positively charged
=a dense positively charged nucleus surrounded by 99.9% empty space and e- outside
= yet that doesn’t make any sense, yet it’s the only explanation
= what holds the protons in the center = deduce the strong force
- what are the other 3 forces? Weak, gravity, electromagnetic
= electrons stay outside? = incredible speeds
Bohr/Rutherford Planet Model
Similar to the solar system with the sun/nucleus in the center and the planets/electrons orbiting outside
Limitations = 2 dimensional, orbits vs. orbitals
Must be even more complex than this = Ch. 5 and the Modern Atomic Theory
Basic Atomic Properties = given this new picture, lets study the atoms
Protons are the key, they are the identity of the element, only the protons determine the type of element
Atomic Number = number of protons
Neutrons add mass, but do not change the element
Mass Number = the protons plus the neutrons, the mass of one specific atom in amu
Isotopes = a different version of a given element that has a different # of n, and so weighs more
Electrons change the charge of the atom, but do not change the element
Charge = the amount of difference between the protons and the electrons considering + or –
Ions = an atom that has a charge due to an unbalanced number of p and e
PBS Atom Builder
Complete Chemical Symbol
Hydrogen-2H-2238U+2
Atomic Weight = the average mass of a given element’s atom
Found using each Isotope (version) of the element and its relative abundance
Some isotopes are stable, others are not = a just right balance in the spacing of the nucleus
Large atoms have more neutrons for strong force, but eventually are too big = radioactive
Stability Band by graphing neutrons vs. Protons
Radioactivity (Ch. 25)
Radioactivity = Radioactive Decay = the spontaneous process by which an unstable atom emits subatomic
Particles (all atoms above #83 are unstable)
Radiation = the actual charged particles or electromag radiation (energy) emitted during radioactive decay
3 Common forms of Radiation
Alpha = two protons and two neutrons = clothes can protect you but don’t breathe
Beta = an electron from the nucleus = typically a foot of concrete
Gamma = massive amount of energy = several feet of concrete and or lead
Radioactive Decay Formula = a mathematical expression that must follow the Laws of Conservation
238U 4A + 234Th
238U 0B + 238Np
Nuclear Half Life = a math eqn to calculate the time it takes for ½ of stable material left after a certain time No way to predict which atom will decay, and its gradual not instantaneous
The answer is always ½ of the previous amount
Half life of 10 years, 40 years later = 1/16
1/32 sample left = 5 half lives
10,000 years have gone by, ¼ left = half life is 5000 years
Polonium 214 = 163.7 microseconds
Carbon 14 = 5730 years
Uranium 238 = 4,460,000,000 years
Nuclear Fission = process where a large unstable atom (plutonium) is bombarded with a smaller atom
Large atom splits into two more stable atoms, yet their combined weight is slightly less
E = mc2 the small amount of “missing” mass becomes a HUGE amount of E
The two split pieces bombard neighboring atoms, and so on = Nuclear Chain Reaction
Nuclear Fusion = Lighter unstable atoms combine to form a heavier more stable atom and a LOT of E