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
