Unit 03 - The Atom

Question 1

Democritus

Answer 1

(442 BC) proposed that everything is ultimately made of tiny, indivisible, particles called atoms

Question 2

Joseph Priestly

Answer 2

(1774) 1. discovered oxygen (dephlogisticated air); first to recognize that air is a mixture of gases.
2. invented carbonated water and rubber eraser
3. revealed plants were made of oxygen

Question 3

Antoine Lavoisier

Answer 3

(1789) 1. performed controlled experiments where he measured the mass of the reactants and products in a reaction and found the masses to be the same.
2. proposed Law of Conversation of Mass. helped to isolate oxygen from the air and gave oxygen its name.
3. proposed a nomenclature/naming system for chemistry (its basic principles are still in use)

Question 4

Joseph Proust

Answer 4

(1794) 1. found that in any compound, the mass ratio of one element to another is always the same. ex. H2O - for every 1 g. of hydrogen, there is 8 g. of oxygen; became known as Law of Constant Composition/Definite Proportions
2. there is also the Law of Multiple Proportions - when two elements form multiple compounds with one another, the ratio is a whole number multiple of the mass of that element in the second compound

Question 5

John Dalton

Answer 5

(1803) explained the work of Lavoisier and Proust by developing the first model of the atom. views atoms as tiny, little, solid spheres similar to billiard balls (Billiard Ball Model of the Atom) it states:
1. elements are made of small, indivisible particles called atoms (not indivisible)
2. all atoms of a given are identical (incorrect, isotopes)
3. atoms of one element are different from atoms of other elements
4. compounds are composed of atoms of different elements combined in whole number ratios
5. chemical reactions involve the rearrangement of atoms as they go from reactants to products, but the total number of atoms always stay the same

Question 6

William Crookes

Answer 6

(1870) developed Crooke’s Tube to study the behavior of electricity under the pressure of low-pressure gases; noticed that a green light was formed when electricity passed from one electrode (the Cathode) to the other electrode (the Anode) in the tube and became known as cathode rays
the green light was the result of the cathode rays striking the glass tube and causing electrons to become “excited” in the atoms of the glass

Question 7

JJ Thompson

Answer 7

(1898) 1. used the cathode ray tube to determine that all atoms contain electrons
2. yielded the charge-to-mass ratio (e/m) for electrons, which is
−1.76 x 108 Coulombs per gram (C/g of e⁻).
3. Plum Pudding Model - views an atom as having a sphere of positive charge with negatively charged electrons spread throughout the positive sphere

Question 8

Robert Millikan

Answer 8

(1908) using the Oil-Drop experiment, he determined the exact charge on an electron (−1.60 x 10−19 Coulombs), and calculated the mass of an electron (9.11 x 10−28 grams) by utilizing the exact charge, and Thomson’s charge-to-mass ratio.

Question 9

Ernest Rutherford

Answer 9

(1909) 1. studied radioactivity and found that there were three different types, one with a positive charge, one with a negative charge, and one with no charge.
2. Gold Foil Experiment - this experiment bombarded a thin piece of gold foil with alpha particles from a radioactive source, and observations were made. results in the development of the Planetary Model of the Atom, replacing the Plum Pudding model.
- majority of alpha particles passed through the foil, therefore atoms are mostly empty space
- some bounced off of the front of the foil, therefore the particles were hitting something in the gold atom that was very small, but carried a great deal of mass. He termed this the nucleus of the atom.
- Some particles passed through the foil, but were deflected at an angle. alpha particles have a positive charge, so they must have come near the nucleus, which also must be positively charged, and this repelled the alpha particles at an angle. Thus, the nucleus has a positive charge.

Question 10

Henry Mosely

Answer 10

(1913) started working under Rutherford
1. Initial research focused on radioactivity and beta radiation in radium.
2. began to study the X-ray spectra of the elements where he found a relationship between the frequencies of corresponding lines in the X-ray spectra of each element.
3. He concluded that the frequencies were essentially proportional to the integer value of positive charge of the element. (the creation of the atomic number)

Question 11

James Chadwick

Answer 11

(1932) bombarded beryllium atoms with alpha particles and a type of particle with no charge was emitted from the nucleus (discovered the neutron)