Transcript Atomic Structure

Atomic Structure
Chapter 4, Section 1
Atomic History
• In the fourth century BC, the
Greek philosopher Democritus
suggested that the universe was
made of invisible units called
atoms.
• The word atom is derived from
the Greek work meaning unable
to be divided.
• The problem: Democritus was
unable to provide the evidence
needed to convince people that
atoms really existed.
Atomic History, Cont.
• In 1808, an English school teacher
named John Dalton proposed that
atoms could not be divided and that
all atoms of a given element were
exactly alike.
• Dalton’s theory is considered the
foundation for the modern atomic
theory.
• Dalton’s theory was developed with
scientific basis and was accepted
by others.
Atomic History, Cont.
• In 1872-1937, Rutherford ran
experiments to determine the
structure of an atom. When
positively charged particles are fired
into gold foil, most pass straight
through while a few are violently
deflected.
• This implies a dense, positively
charged central region containing
most of the atomic mass and that
the atom is mostly space.
The Gold Foil Experiment
• Rutherford expected
the alpha particles to
go straight through the
gold foil.
• Instead, some of the
alpha particles were
deflected, implying a
central positively
charged region
(nucleus).
Atomic History, Cont.
•
•
In 1913, the Danish scientist
Niels Bohr suggested that
electrons in an atom move in
set paths (energy levels)
around the nucleus much like
the planets orbit the sun.
Electrons can only be in
certain energy levels and
must gain energy to move to
a higher energy level or lose
energy to move to a lower
energy level.
Atomic History, cont.
• In the 1920’s deBroglie &
Shrodinger showed that
the “solar system” model
of the atom was
incorrect. Instead,
electrons orbit the
nucleus in orbitals. This
is called quantum
mechanics.
The Atomic Model
• PROTONS
– Charge is positive
– Found in the nucleus
• NEUTRONS
– Charge is neutral
– Found in the nucleus
• ELECTRONS
– Charge is negative
– Found in the energy levels (electron cloud).
Anything Smaller?
• Protons & Neutrons are each made of
3 quarks.
• What is a quark? Don’t worry about
it…it’s past the scope of this class –
just know that they exist.
?
The Atomic Model, cont.
• There is an equal number of
protons and electrons in an atom.
• This causes every atom to be
neutral:
Charge of two protons:
Charge of two neurons:
Charge of two electrons:
Total charge of atom:
+2
0
-2
0
Atomic Model, cont.
• Each energy level can hold only a
certain number of electrons.
–1st energy level can hold up to 2
electrons –
• this level must be completely filled
before putting electrons in the next
level
–2nd level can hold up to 8 electrons
(also must fill this level before filling
next level)
Location of Electrons – The
Problem with Bohr’s Model
• An electron’s exact location cannot be determined.
Imagine the moving blades of a fan – If you were
asked where any one of the blades was located at a
certain instant, you would not be able to give an exact
answer – the blades are moving too quickly!
• It is the same with electrons –the best a scientist can
do is calculate the chance of finding an electron in a
certain place within an atom
• To visually show the likelihood of finding an electron,
shading is used. The darker the shading, the better
the chance of finding an electron. This shaded region
is called an electron cloud.
Orbitals
• Electrons exist in energy
levels, but the regions in an
atom where electrons are
likely to be found are called
orbitals.
• There are four different kinds
of orbitals.
s-orbital
• Simplest orbital
• Shaped like a
sphere.
• Can hold up to
2 electrons
p-orbital
• Shaped like a dumbbell.
• Can be oriented three
different ways – x, y, and z
• Each orientation can hold 2
electrons for a total of 6
electrons in each energy
level
• First found in the 2nd energy
level.
Click to See Shading of Each
Orbital
• http://web.mit.edu/3.091/www/orbs/
d-orbital, f-orbital
• More complex – past the
scope of this class
• There are 5 possible d-orbitals
• There are 7 possible f-orbitals