Transcript Atoms and Elements

Scientists of the Atom &
Atomic Structure
Scientists of Atoms
• Greeks
– Democritus:
• Matter is composed of small units called atoms
• Nothing is smaller than an atom
• Atoms of a solid were round and spikey, Atoms of a
liquid were round and smooth
– Aristotle:
• Atoms are forever divided into smaller parts
Dalton
• Dalton
– All elements are composed of atoms.
– All atoms of the same element have the same
mass. Atoms of different elements have
different masses.
– Compounds contain atoms of more than 1
element.
– In a particular compound, atoms of different
elements always combine in the same way.
Thomson
• Experiment
– Like charges repel, opposite charges attract
– Negative charges attract positive charges and vice versa.
– Beam of charged particles sent through a tube with a positive disk
and a negative disk. The beam bent toward the positively charged
disk.
• Discovery
– Atoms are made up of subatomic particles that have charges.
– Plum Pudding model: negative charges are spread out in a
positively charged mass
Rutherford
– Experiment
• Alpha particles = positively charged
• Some went through, a few bounced back
Rutherford
– Discovery
• An Atom’s positive charges are concentrated in
center of atom (called the nucleus).
Bohr
• Bohr Model
– Focused on electrons
– Move in fixed orbits around nucleus
Electron Cloud Model
– Nucleus has Protons and Neutrons
– Electrons are moving at high speeds around nucleus
– Electrons do not orbit in fixed distances around
nucleus, however exists in a cloud (where electrons
probably are).
– Cloud is denser at those locations where the probability
of finding an electron is high.
– Airplane propeller analogy.
Parts of an Atom
• Nucleus - inside
– Made of
• Protons (positive charge)
• Neutrons (neutral)
Parts of an Atom
• Electron Cloud - outside
– Made of
• Electrons (negative charge)
• Same number of protons and
electrons! (Same number of
positives as negatives!!)
If the protons & electrons are the same
in every atom the same element….
• …then what is the charge of a typical atom?
• 2 protons & 2 electrons
• +2 + (-2) = 0 NEUTRAL!!
• Atoms are typically neutral or they have no charge!
Atomic Number and Mass Number
• Atomic Number = number of protons in an
atom (PAN)
• Mass Number = number of protons and
neutrons (massive)
How many Protons? Electrons?
Neutrons?
Atomic Mass
• Notice how it has decimals?
• It is an AVERAGE!!!!
• There are different atoms of the same
element…we call these ISOTOPES.
Isotopes
• Atoms of the same element can have
different numbers of neutrons; the different
possible versions of each element are called
isotopes.
The Nucleus is the Heaviest Part…
• The Neutrons and Protons
are VERY HEAVY!!
– They are made of even
smaller part called
QUARKS.
• The Electrons are so small
they are almost
WEIGHTLESS!!
• The “Rings” don’t actually
exist, they are just drawn in
the picture to illustrate the
path of the orbits.
Orbits in the Electron Cloud
• Electrons spin and
circulate around the
nucleus. Electrons orbit
the nucleus in “rings” or
energy levels.
• Notice how the electrons
orbit the nucleus in many
different dimensions.
Remember, the atom is
3D!
Energy Levels or “Rings”
• For our purposes (very
simplified), there are four rings
around the nucleus.
• The closest ring holds
2 electrons.
• The second ring holds
8 electrons.
• The third ring holds
8 electrons.
• The fourth ring holds
2 electrons.
• Different areas of the
electron cloud have different
levels, each representing a
different amount of Energy.
(Think of levels in a parking
garage).
• There are up to SEVEN
levels where electrons can
exist, depending on how
much energy they have.
• How many rows are in the
Periodic Table?
Periodic Table
• Energy levels closest to the nucleus are filled with
lower energy electrons. The farther away the
levels get from the nucleus, the higher the energy
of the electrons in that level.
• Electrons always fill up the lower energy levels
first, just like cars in a parking garage fill in the
spaces closest to the ground floor, then spread
farther up as the spots get filled.
• The energy levels are
sometimes also called
orbitals.
• The chemical properties of
atoms depend on how many
electrons are in the outermost
or “valence” energy level.
• Think of it this way: an atom
“likes” to have its outermost
level filled with electrons.
• The Octet Rule states that
atoms are most stable when
their valence shell contains
eight electrons.
• How does the location of an element in the
periodic table tell us where its electrons of
its atoms are located?
• Which atoms will react similarly, those in
the same row, or those in the same column?
Something to Think About…
• What is the difference
between an atom and a
cell?
• Cells are the smallest
LIVING things. Atoms are
the smallest part of
EVERYTHING and they
are not alive!