Download Unit 2 * Chapter 11 - Dr. Wall`s Science

Unit 2 – Chapter 11
The Atom
Atomic Theory
• Democritus
– 440 BCE
– Idea that cutting something until it cannot be cut
anymore
• Aristotle
– Disagreed with Democritus
• Atom – Smallest particle an element can be
divided and still be the same substance
Atomic Theory
• John Dalton
– 1803
– British chemist and teacher
– Elements combine in certain proportions because
they are made of single atoms
John Dalton’s Atomic Theory
• All substances are made of atoms
• Atoms of the same element are exactly alike,
and atoms of different elements are different
• Atoms join with other atoms to make new
substances
JJ Thomson
• 1897
• JJ Thomson discovered a mistake in Dalton’s
theory
• There are actually smaller particles INSIDE the
atom
• Cathode ray tube
JJ Thomson
• Passed an electrically charged beam through a
metal plate
• The positively charged plate attracted the
beam’s particles
• This meant that there are negatively charged
particles as well
• Electrons – negatively charged particles in an
atom
JJ Thomson
• Proposed a model of an atom
• Plum pudding model
– Electrons mixed throughout the atom
Ernest Rutherford
• Student of JJ Thomson
• 1909
• Sent positively charged particles through thin
sheets of gold foil
• Could see where the particles went after they
went through the foil
Gold Foil Experiment
• Most particles passed right through
• Some were deflected slightly
• Some were deflected right back
Rutherford’s Atomic Theory
• Nucleus: Tiny, dense, positively charged center
of atom
• Positive repels positive
• Empty space between particles
Niels Bohr
• 1913
• Danish scientist who worked with Rutherford
• Proposed that electrons move around the
nucleus in paths
• Electrons can jump from path to path
Modern Atomic Theory
• Erwin Schrödinger (Schrödinger’s cat)
• Werner Heisenberg
• Discovered electrons do not travel in definite
paths or orbits
• Most likely within regions
• Electron Clouds: Regions within an atom
where electrons are found
The Atom
• How small is an atom?
– One penny contains about 2x1022 atoms, or
20,000,000,000,000,000,000,000 atoms
– One Al (aluminum) atom has a diameter (how
large across the entire atom) of 0.00000003 cm
– Three one hundred millionths of a centimeter
Subatomic Particles
• Electrons
– Negatively charged
– VERY tiny!!!!!
– Found in electron clouds
– Mass of 9.1x10-31kg
– Because the mass is so small, we ignore it and call it
zero
Subatomic Particles
• Protons
– Positively charged
– Found in nucleus
– Mass of 1.67x10-27kg
– Mass of 1 AMU (atomic mass unit)
Subatomic Particles
• Neutron
– No charge (neutral)
– Found in nucleus
– Mass of 1.67x10-27kg
– Mass of 1 AMU
Nucleus
•
•
•
•
Proton and neutron together make up nucleus
Has a positive charge
EXTREMELY DENSE!!!
If the nucleus were 1 cubic meter in size, its
mass would be over 100,000,000,000,000 tons
• Makes up 99.99% of mass of atom
Outside the nucleus
• Electrons in electron clouds
• Negative charge – cancels out positive charge
of nucleus, so overall charge of atom is neutral
• Atoms can gain or lose electrons
– These are called ions
– Can be positive or negative
– Positive – lose electrons (less negative)
– Negative – gain electrons (more negative)
How do elements differ?
• About 119 elements overall
• Protons and electrons found in all atoms of every
element
• Number of protons = number of electrons
• Number of neutrons can differ, C-12 and C-14
• Atomic number: Number of protons in the
nucleus of an atom
– This is what makes that atom that specific element
– Carbon has atomic number of 6, Oxygen is 8, etc
– Every atom of that element has same atomic number
Isotopes
• Atoms that have same number of protons, but
different number of neutrons
– Carbon-12 has 6 protons and 6 neutrons
– Carbon-14 has 6 protons and 8 neutrons
• Isotopes can be unstable
– Nucleus will DECAY over time
– This is called radioactivity
– Spontaneous
– Gives off energy and particles
Isotopes
• Mass number: Total of protons + neutrons
• To calculate, add protons plus neutrons
• To name an isotope:
– Write name of element, followed by mass number
– C-12
– O-18
• The isotope must be given to you to find the
number of neutrons, or vice versa
Mass of element
• Most elements contain a mixture of two or
more isotopes
• Copper contains Cu-63 and Cu-65 atoms
• Atomic mass:
– Average mass of all naturally occurring isotopes
– Shown as a decimal on periodic table
Forces
• There are four main forces at work in the
universe
– Gravitational
– Electromagnetic
– Strong Nuclear
– Weak Nuclear
Gravitational
• Gravitational force depends on size and mass
of objects
• Pulls other objects towards the gravitational
center of object
• You toward center of Earth
• Earth toward Sun
• Sun toward middle of Milky Way galaxy
• Considered weakest force (short range)
Weak nuclear force
• Responsible for decay of subatomic particles
• Can turn neutrons into protons and/or
electrons
• Plays a part in radioactivity
Electromagnetic
• Same charges repel (positive repels positive,
negative repels negative)
• Opposite charges attract (like magnets)
• Electrons are held in place around the nucleus
due to electromagnetic force
• Magnetic field around Earth
• Larger range (planetary)
Strong nuclear force
• Holds protons and neutrons together in
nucleus
• Must be larger than electromagnetic force or
protons would fly away from each other in
nucleus (positive repels positive)
• Considered strongest force in universe at short
distances (inside nucleus)
Strongest?
• What happens if electromagnetic force is
larger than strong nuclear force?
• Can gravitational force be the weakest AND
the strongest force in the universe??