Chemistry Ch 3 & 21

Democritus’ beliefs

believed matter was made up of small particles called “atomos” = “indivisible”

Aristotle’s beliefs

• disagreed with Democritus

• believed matter was continuous

• 4 categories of matter: Earth, Air, Fire, Water

Law of Conservation of Matter

Mass of the reactants = Mass of the products

Law of Definite Proportions

whenever a compound is made, it has the same ratios of each element

Law of Multiple Properties

2 elements can combine in different ratios to form different compounds

Dalton’s atomic theory

1. All matter is composed of tiny particles called atoms

2. Atoms of a given element are identical of size, mass, and other properties

3. Atoms cannot be subdivided, created, or destroyed

4. Atoms of different elements combine in simple whole number ratios to form chemical compounds

   1. In chemical reactions, atoms are combined, separated, or rearranged

What has changed from Dalton’s atomic theory to modern atomic theory? What hasn’t changed?

Changes
- atoms are divisible into subatomic particles
- a given element can have atoms with different masses

Unchanged
- atoms are combined, separated, or rearranged in chemical reactions
- atoms cannot be created or destroyed

What was Democritus the first to propose?

Matter is not infinitely divisible

T/F: Philosophers thousands of years ago tested their ideas using the scientific method.

False

How does a cathode ray work?

electric charge is applied —> current passes through cathode ray tubes —> surface of tube opposite of cathode glows

a ray of radiation (cathode ray) travels from the cathode to the anode

Observations of Thomson’s cathode ray experiment

cathode rays (electrons) were deflected by a magnetic field in the same manner as a wire carrying electric current, which was known to have a negative charge
tldr the rays were deflected away from a negatively charged object

Conclusions of Thomson’s cathode ray experiment

• charge to mass ratio of cathode rays = 1.76 × 10^11 C/kg

• this ratio is always the same regardless of the metal or gas used

• discovery of the electron

• electrons are present in all elements

• atom is divisible

• e^- is a fundamental particle

• charge:mass ratio is very large

Milikan’s oil drop experiment

• measured the charge of an electron

• scientists later found electron mass

Plum pudding model: who and what

JJ Thomson

• atom has a spherical shape

• plum = protons, pudding = electrons

In what experiment was the nucleus discovered?

Rutherford’s gold foil experiment

Rutherford’s gold foil experiment

• studied how positively charged alpha particles interacted with solid matter

• bombarded a thin piece of gold foil with alpha particles

Discoveries and conclusions of Rutherford’s gold foil experiment

• some particles deflected back towards the source, but most went through

• therefore the plum pudding model was incorrect

• led to the conclusion that the atom is composed of a dense, positively charged nucleus surrounded by negative electrons

Nuclear strong force

holds nuclear particles together

Who discovered the neutron?

Chadwick

Isotopes: Hyphen notation

X-A (X = chemical symbol, A = mass number)

Isotopes: Nuclear symbol notation

A X (Z = atomic number)
Z

Nuclide

general term for any nucleus of an isotope

Hydrogen isotopes

Protium (H-1), Deuterium (H-2), Tritium (H-3)

What is the mass in amu of C-12?

12 amu

How to calculate average atomic mass?

weighted average, abundances in decimals

(mass1)(abundance1) + (mass2)(abundance2)

Nucleons

neutrons and protons

Mass defect

difference in mass between the nucleus and its component nucleons

Nuclear binding energy

the energy released when a nucleus is formed from nucleons
the amt of energy needed to break 1 mol of nuclei into individual nucleons
measure of the stability of a nucleus (strong force)

T/F: The atom is spherical

True

T/F: In the Rutherford nuclear-atom model, heavy subatomic particles reside in the nucleus

True

Band of stability

the stable nuclei cluster over a range of neutron-proton ratios
*outside this band, all nuclei are radioactive and all atoms undergo reactions (decay) to become stable; band ends at Pb-208 (all elements atomic numbers >82 radioactive)

Stable nuclei n-p ratio

1:1, in higher atomic numbers it gets closer to 1.5:1

Magic numbers

numbers of electrons that complete shells

Transmutation

change in the identity of a nucleus as a result of a change in the number of its protons

What ratio does the type of radioactive decay depend on?

n:p

Alpha decay

• an alpha particle is emitted from the nucleus

• restricted to only very heavy particles w more than 82 protons

• 4/2 He

Alpha particle

2 protons and 2 neutrons; Helium-4

Mole

SI unit for the amount of substance
- amt of substance that contains as many particles as there are atoms in exactly 12g of C-12

Avogadro’s Number

6.022 × 10^(23)
number of particles in 1 mole of a pure substance

Molar mass

• numerically equal to the atomic mass

• mass of one mole of a pure substance

  • g/mol or kg/mol

Mole to gram conversion

multiply by molar mass

Gram to mole conversion

divide by molar mass

Mole to number of atoms conversion

multiply by Avogadro’s number

Number of atoms to moles conversion

divide by Avogadro’s number

Grams to number of atoms conversion

divide by molar mass and multiply by Avogadro’s number

Number of atoms to grams conversion

multiply by molar mass and divide by Avogadro’s number

Einstein’s equation relates which quantities?

Energy, mass, speed of light

The mass of the nucleus is always _____ the sum of the masses of the individual protons and neutrons that make up the nucleus

less than

Beta Decay

• radioisotope as too many neutrons relative to its number of protons

• decreases the n:p ratio by converting a neutron into a proton and electron

• atomic number +1, mass unchanged

Beta particle

an electron emitted from the nucleus
0/-1 B

Positron

a particle that has the same mass as an electron but as a positive charge, and emitted from the nucleus
0/+1 B

Positron emission

• a proton is converted into a neutron by emitting a positron

• this decreases the number of protons

• atomic number -1, mass unchanged

Electron capture

• decreases the number of protons in unstable nuclei below the band of stability

• an inner orbital electron is captured by the nucleus of its own atom

• to increase the number of neutrons, an inner orbital electron combines with a proton to form an neutron

• atomic number -1, mass unchanged

Gamma ray decay

• high-energy electromagnetic waves emitted from a nucleus as it changes from an excited state to a ground energy state

Alpha particle stopped by:

a few sheets of paper

Beta particle stopped by:

a few cm of lead

Gamma ray stopped by:

several cm of lead

Parent nuclide

unstable nucleus

Daughter nuclide

stable nucleus

When does decay stop?

When the atom becomes stable

Half-LIfe

the time required for half the atoms of a radioactive nuclide to decay

More stable nuclides decay ____ and have _____ half-lives

slowly, longer

Half-Life equation

Artificial Transmutation

made by artificial radioactive nuclides (not found on Earth)

How are artificial radioactive nuclides made?

bombardment of stable nucleus with a neutron or alpha, beta, or gamma radation

Transuranium elements

elements atomic numbers 93 and above
- all were produced in a lab by induced transmutation

Stability of transuranium elements

UNSTABLE!!

What determines whether an atom spontaneously decays and the type of radiation it emits?

its n:p ratio

What type of radiation has the greatest penetrating power?

Gamma radiation

Alpha particles are ____ charged

positively

Beta particles are attracted to a ______ charged plate, meaning they are _____ charged

postitively, negatively

Which kind of radiation is most difficult to shield a person from?

Gamma ray

Radiation exposure

nuclear radiation can transfer the energy from nuclear decay to the electrons of atoms or molecules and cause ionization

Rem

unit that measures exposure to human tissue

R/Roentgen

unit that measures nuclear exposure

Film badges

use exposure of film to measure the approximate radiation exposure of people working with radiation

Geiger-Müller counters

instruments that detect radiation by counting electric pulses carried by gas ionized by radiation

Scintillation counters

instruments that convert scintillating light to an electric signal for detecting radiation

Radioactive dating

• the approximate age of an object is determined based on the amount of certain radioactive nuclides present

  • age is estimated by measuring either the accumulation of a daughter nuclide or the disappearance of the parent nuclide

• ex. C-14 dating

Radioactive nuclides in medicine

• used to destroy certain types of cancer cells

• radioactive tracers = radioactive atoms that are incorporated into substances so that movement of the substances can be followed by radiation detectors

Radioactive nuclides in agriculture

• used to determine the effectiveness of the fertilizer

• radiolabeled pesticides as an alternative

• also used to prolong shelf life

Which disease is radiation therapy used to treat?

Cancer

What unit accounts for the type of living tissue that absorbs a dose of radiation?

Rem

Nuclear fission

a very heavy nucleus splits into more stable nuclei of intermediate mass
- releases a lot of energy, less energy per nucleon however
- can be spontaneous or induced

Chain reaction

a reaction in which the material that starts the reaction is also one of the products and can start another reaction

Critical mass

minimum amount of a nuclide that provides the number of neutrons needed to sustain a chain reaction

Subcritical mass

not massive enough to sustain a chain reaction

Supercritical mass

chain reaction rapidly escalates

Nuclear reactors

use controlled fission chain reactions to produce energy and radioactive nuclides

Shielding

outer covering; absorbs excess radiation + gamma rays; usually thick concrete

Fuel

uranium fuel rods, usually U-235

Coolant

liquid water under high pressure; absorbs excess energy

Control rods

absorb excess neutrons to contain chain reaction

Moderator

controls the speed of the neutrons (slows them down)

Problems in nuclear reactors

• produce highly radioactive waste

• not a lot of U-235 to use as fuel

Which type of nuclear change releases more energy per amu?

fusion

Nuclear Fusion

low mass nuclei combine to form a heavier more stable nucleus

What could nuclear fusion be used for?

generating energy

Advantages of fusion

• releases more energy per gram of fuel than fission

• lightweight isotopes used as fuel are abundant

• products generally not radioactive

Limitations of fusion

• requires extremely high energies to initiate and sustain a reaction

• confinement of extreme temperature reaction (hard to confine)