Unit 2: Atomic Theories Flashcards
Philosopher Democritus’ theories and hypothesis
2500 years ago, Democritus reasoned that if a solid was repeatedly cut into smaller pieces, eventually, it will get to the point where it can’t be cut –> he called this “atomos” (Greek for uncuttable)
His theories about atoms
- There is empty space between atoms
- Atoms are completely solid WRONG: atoms are mostly empty space
- Atoms have no internal structure (he believed it was a solid marble: WRONG: we have subatomic particles: protons, neutrons, electrons
- Each atom (of a different substance) is different is shape, size, & weight CORRECT
- Particles of different sizes and shapes might be components of all the different types of matter CORRECT?
Aristole’s belief
came after Democritus and proposed that all types of matter were actually made up of different proportions of 4 basic elements: earth, air, fire, water
- his idea, and other variations were accepted for almost 2000 years in the practice of alchemy
John Dalton
(1766-1844)–> came up with bowling ball method
He adapted Democritus’ theory to the first modern atomic model and created the law of multiple proportions
John Dalton’s modern atomic model
Matter is made up of tiny solid spheres called atoms
- atoms are indestructible & indivisible
- Each element is it’s own kind of atom, characterized by the weight of their atoms
- The atoms of a given element are all identical to each other & different from atoms of other elements –> partially right, rmr isotopes
Law of Multiple Proportions
Personal note: I am 80% sure that it was created by John Dalton
law:
- atoms of different elements combine in SPECIFIC RATIOS to form compounds e.g water has to have 2 hydrogen & 1 oxygen atom
- In a chemical reaction, atoms are separated, rearranged and recombined to form new compounds
J.J Thomson & his disoveries
1856-1940–> discovered the electrons
- He was British & won nobel prize
Thomson Proposed
- atoms contain negatively charged sub-particles called electrons. Thompson was able to predict the mass of this particle
Plum Pudding / Raison Bun Model
model created by J.J Thomson to explain structure of atom
- stated that each atom is a sphere filled with a positively charged fluid
- the electrons are stuck or suspended in this fluid
- the negative fluid balances out the positively charged fluid, making the atom neutral
Cathode ray experiment
Conducted by J.J Thomson
SET UP (see page 4)
- You have a nearly empty tube ( air is sucked out)
- Electricity passes through the tube from the cathode (negative end) to anode (positive end)
- It will travel through to a florescent screen (phosphor coating) at the end of tube. Where the beam hits, a dot will illuminate
CONDUCTION & RESULTS OF EXPERIMENT
- When Thomson placed oppositely charged plates around the tube, the beam bent towards the positive plates.
- therefore he concluded that particles must be negatively charged, and that atoms have internal structures
Explanation of Rutherford’s observations
1)Most particles travel straight through: Atoms are not solid (as Thomson predicted) , but mostly empty space
2) Some particle were deflected by small angles: because they come close to electrons
3) some were deflected by large angles: there must be positive “something” in the atom that repels the positive particles
4) a few particles reflected from the foil as if turned away by something: the “something” in the atom must be really positive, and very small. –> Rutherford called this “something” the Nucleus
Rutherford’s Atomic model
- electrons move in space around the nucleus like a beehive
- the electrons account for the volume of the atom, and the nucleus accounts for the mass (neutrons+protons) –> Rutherford didn’t know about neutrons –> his student Chadwick discovered it
2 problems with Rutherford’s model
1) what was stopping electrons from being attracted to the nucleus
2) What stops protons in nucleus from repelling –> he didnt know about neutrons
Neutrons act as
peacekeepers to keep the protons together
Chadwick discovered…
the neutrons –> he won a nobel peace prize
Niels Bohr
1885-1962 - came up with planetary model of atom
He proposed that
1) electrons can exist only in a series of “allowed” energy levels or shells of “fixed” energies
–> electons cant exist in between shells (energy levels)
–> lower shells have lower energies.
–> higher shells have higher energies
2) If the electron is found in the lowest possible energy shell, it is in its ground state
–> to go from lower level to higher, an electron must gain energy –> the electron is said to be in an excited state
–> To go from a higher level to a lower, an electron must release the same amount of energy it observed. –> this energy is observed as light
Atomic Number (z)
The number of protons in the nucleus
–> it is the identity of an element
Atomic mass (A)
The mass of an atom of a chemical elements
–> Atomic Mass= protons + neutrons
Cations
positive ions
–> atom loses electrons
Anions
negative ions
–> atom gains electrons
Valence shells
the outermost shell/energy level
- electrons in this shell are called valence electrons
Isotopes
- atoms of the same element but have different atomic masses
–> protons stay same, # of neutrons vary - Isotopes have identical chemical properties but slightly different physical properties
- some isotope are unstable & radioactive
Natural Abundance
Isotopes are not equally stable and therefore aren’t equally abundant
–> The relative isotope abundance is the percentage of a particular isotope that occurs in nature–> all isotope abundances for a particular element add up to 100%
Average Atomic Mass
- This is the atomic mass you see on the periodic table
- It is the weighted average of the masses of all the isotopes of that element –> it reflects both the mass & the natural abundance of the isotopes as they occur in nature
EQUATION
Average atomic mass=(%of isotope 1 (be sure to convert to decimals) x mass isotope 1) + (%of isotope 2 (be sure to convert to decimals) x mass isotope 2)…
Mass Spectrometry
-AKA mass spectrometer
- used to identify isotopes and their respective abundances
Mass spectrometers separate atoms and molecules according to their mass-to-charge ratio (m/z ratio)
How do mass spectrometers work?
SEE PAGE 19
1) The substance is first converted into a gas
2) The ion source- the substances is heated in a vacuum & then ionized into positive ion
3) Analyzer- The ions produced are accelerated through a magnetic field that separates ions of different masses. The greater the m/z ratio (the lighter the atom), the more deflections occur, and the faster they move. The # of hits at each deflection point determines the relative abundances of isotopes in the sample
4) Detectors- records the separation, the percent abundance, & the mass of each isotope
Mass spectrum
Bar graph that presents the findings of the mass spectrometer
APPLICATION OF MASS SPECTROMETRY
- identify isotope percentages in elemental samples
- confirm identity of unknowns, such as illegal drugs in a system
- identifies if samples are pure or mixtures
Nucleons are
protons and neutrons –> they are called nucleons cuz they are found in the nucleus
Nuclear Force
A short-distance attractive force that acts between all nuclear particles that are extremely close together –> neutrons and protons
Electron-magnetic Repulsions
the repulsion force amongst the protons.
–> its a long distance force
What is the purpose of neutrons
neutrons add strong nuclear force to stop the protons from repelling
- thats why the more protons you add, the more neutrons you need
–> neutrons act like the glue
What makes elements radioactive
When there are too many protons compared neutrons–> the repulsion wins
Radioactive elements
the like-charged protons would repel one another and fly apart –> the nucleus will undergo radioactive decay to release energy
when do radioactive elements begin
at the 83 protons (element bismuth)
what element onwards are so unstable that they cant be found in nature
from Neptunium onward (93 protons)
Ideal ratio of stability
the stability of an element depends on the neutron-to-proton ratios
- For elements less that 20, the ideal ratio 1:1
- At an atomic mass of 20, this starts to increase till it gets to 1.5:1
Nuclear Chemistry
- the study of the nucleus and its reactions
- this involves the the nucleons rather than the electrons in typical chemical reactions
Differences between nuclear reactions & chemical reactions
CHEMICAL
- electrons are moved around (bonding)
- same identity of elements (no new elements being made)
NUCLEAR
- neutrons & protons are moved around
- change in the element identity (because protons change–> atomic # changes)
Band of Stability
is the range in the number of neutrons for a given number of protons for isotopes that are found in nature
Island of stability
represents stability range of man-made elements
Radioisotope
is an isotope of an element that is unstable & undergoes radioactive decay to release energy
- a radioactive isotope will attempt to reach stability by ejecting nucleons as well as other particles, or releasing energy(radiation)
Decay Series
elements may continue to repeatedly decay until a stable daughter isotope is formed
Half life
the time required for one half (50%) of the atoms of the original radioactive isotope to decay to another isotope
- note that a radioactive substance never fully decays to zero (you cant break 1 atom)
Half-life of Carbon-14
5,730
-scientists use the known half-life of carbon-14 to date plants, animals, and artifacts less than 50,000 years old –> archaeology dating
Alpha Radioactive Decay
- nuclear decay process where an unstable nucleus changes to another element by shooting out a helium nucleus / alpha particle
- alpha particle has a mass of 4 amu & a charge of 2+
Half life equation
present value(A)= A0(initial amount)/2^n(# of half lives)
Gamma Radioactive Decay
Gamma radiation is a wave, not a particle, meaning there is no mass or charge change
- gamma radiation=high energy –> makes particle stable
- Gamma radiation can be done alongside alpha or beta emission, or on its own
Beta Radioactive Decay
occurs when there are too many neutrons relative to protons
A neutron is converted into a proton & electron –> atomic # increases by 1, atomic mass stays the same, & and a electron particle (beta particle) is created
Can alpha decay or beta decay happen alongside other decays?
No
Ionizing Ability
particles have ionizing ability if they can pull electrons off from atoms –> requires energy as electrons are attracted to their atomic nuclei –> hence particles that have strong ionizing ability, have low penetrating power
Penetrating power
the ability to go through different materials
Alpha Particles
- slow and heavy
- mass: 4 amu (2p+2n)
- high ionizing ability
- low penetrating power –> u can stop then with a sheet of paper/tissue
Beta Particles
- Fast and light
- mass: one electron
- medium ionizing ability
- medium penetrating power –> u can stop then with a sheet of aluminum or plastics
Gamma Rays
- Very fast
- mass: 0
- low ionizing ability
- high penetrating power –> u can stop then with a thick sheet of metals such as lead, or concrete –>most dangerous
