Atomic Structure and Atomic Theory

Flashcards based on lecture notes covering Dalton's atomic theory, subatomic particle discoveries (electrons, protons, neutrons), and early atomic models proposed by J.J. Thomson and Ernest Rutherford.

What are the five postulates of John Dalton's atomic theory?

1) Matter is made up of very tiny particles called atoms. 2) Atoms are indivisible and indestructible. 3) Atoms of the same element are identical in mass and properties. 4) Atoms can combine chemically with each other in simple whole number ratios to form compounds. 5) Atoms of different elements have different masses.

Which two postulates of John Dalton's atomic theory were later found to be incorrect, and why?

Postulate 3 was incorrect because atoms of the same element can differ by their isotopes. Postulate 2 was incorrect because atoms are not indivisible as they contain smaller subatomic particles, though they are not destroyed during ordinary chemical reactions.

Who discovered electrons, and what setup was used?

John Joseph Thomson discovered electrons using a cathode ray tube where most of the air was removed to create a low-pressured gas inside.

In John Joseph Thomson's experiment, how were electrons produced and directed in the cathode ray tube?

High voltage was applied to the tube, causing the cathode to emit electrons that traveled toward the anode, creating a green glow.

What three factors affect the amount of deflection of cathode rays/electrons in electric or magnetic fields?

1) Charge, 2) Mass, 3) Strength of the electric/magnetic field.

Who discovered positive rays (protons), and how were they produced?

Eugen Goldstein observed positive rays (positively charged ions) generated when gaseous atoms in a cathode ray tube lost electrons ($X \rightarrow X^+ + e^-$).

What did Henry conclude about atomic number and the relationship between electrons and protons?

Henry concluded that atomic number corresponds to the positive charge of the nucleus. Because atoms are neutral, the number of electrons must be the same as the number of protons.

Who discovered the neutron, in what year, and by what experiment?

James Chadwick discovered neutrons in $1932$ by showing that bombarding beryllium with high-energy $\alpha$ particles produces an uncharged particle with a mass slightly greater than a proton.

What nuclear reaction equation represents James Chadwick's discovery of the neutron?

$$^9_4\text{Be} + ^4_2\text{He} \rightarrow ^{12}_6\text{C} + ^1_0\text{n}$$

How do neutrons behave when exposed to electric or magnetic fields?

Neutrons carry no charge and are not deflected by electric or magnetic fields.

How did J.J. Thomson describe atomic structure in his plum pudding model?

J.J. Thomson proposed that protons were evenly distributed throughout the atom and electrons were like dried fruit in a pudding.

What atomic model did Ernest Rutherford propose, and what experimental setup did he use?

Ernest Rutherford proposed the planetary model after directing positively charged $\alpha$ particles at a thin sheet of gold foil and using a fluorescent zinc sulfide screen to detect where the $\alpha$ particles went.