Honors Chemistry Chapter 4 - Atomic Structure

atom

the smallest particle of an element that retains it's identity in a chemical reaction

Democritus

suggested the existence of atoms, he called them “Atomos” which means “cannot cut”; said they were indivisible and indestructible

John Dalton

began modern atomic theory

John Dalton’s Atomic Theory

a. all matter is made of atoms

b. all atoms of a given element are identical in mass and properties (false - isotopes have different numbers of neutrons

c. compounds are combinations of two or more different types of atoms

d. a chemical reaction is a rearrangement of atoms

symbols for protons, neutrons, electrons

p+, n0, e-

Ernest Rutherford

discovered the atomic nucleus; suggested the atom is mostly empty space except for all the mass being in the nucleus

Rutherford’s experiment

directed a narrow beam of alpha particles at a thin sheet of gold foil

result of Rutherford’s experiment

most alpha particles went through the foil or were deflected; a small fraction of them bounced off at large angles

JJ Thompson

discovered electrons and that they are in the atoms of all elements; made the plum-pudding model

Thompson’s experiment

created a cathode ray by passing an electric current through gas in a glass tube, the end with the anode became positively charged and the end with the cathode became negatively charged

cathode ray

a glowing beam that traveled from cathode to anode; positively charged metal plate attracts cathode ray while negatively charged plate repels it; Thompson concluded that the cathode ray is a stream of electrons moving at high speed

cathode ray (simple)

stream of electrons produced at the cathode of a tube containing a gas at low pressure

Thompson’s 2nd experiment

measured the ratio of an electron’s charge to its mass; the ratio was constant; concluded that electrons are in all atoms of all elements

plum-pudding model

made by Thompson; the atom has material with electrons suspended in the material

Robert A. Millikan

discovered the electron’s mass

Millikan’s experiment

suspended negatively charged oil droplets between two charged plates, then changed the voltage on the plates to see how this affected the droplets’ rate of fall

result of Millikan’s experiment

the charge on each oil droplet was a multiple of 1.60 x 10-19 coulomb, meaning this must be the charge of an electron

an electron’s mass

1/1840 the mass of an hydrogen atom

Eugen Goldstein

discovered protons

Goldstein’s experiment

observed a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays; concluded that these were protons

James Chadwick

discovered neutrons

atomic theory timeline

1. Democritus: Atomos
2. Dalton: started atomic theory, not divisible
3. Thompson: plum-pudding model, discovered electron
4. Rutherford: nuclear model, discovered nucleus

subatomic particles

protons, electrons, neutrons

atoms are electrically ----- because they have no net ---------

neutral; electric charge

electric charges always exist in ----, no -------

whole numbers; fractions

an electron’s symbol, relative charge, actual mass

e-, 1-, 9.11 x 10-28

a proton’s symbol, relative charge, actual mass

p+, 1+, 1.67 x 10-24

a neutron’s symbol

n0, 0, 1.67 x 10-24

nucleus

the tiny and dense part of an atom; composed of protons and neutrons

atomic number

number of protons; identifies the element

in an atom, the number of electrons and the number of protons is ----

the same

mass number

total number of protons and neutrons; the rounded atomic mass

atomic mass

weighted average mass of an atom, a decimal due to isotopes, unit amu

isotope

an element with the same number of protons but a different number of neutrons; chemically alike

write Au in it's two-number form and hyphen form when the atomic mass is 197 and the atomic number is 79

197, 79 Au or Au-197

atomic mass unit (amu)

1/12 the mass of a carbon-12 atom

how to calculate the atomic mass of an element with two isotopes

(mass x natural abundance) + (mass x natural abundance)