Dec. 5 - The atom Download

Transcript
December 5, 2016
The Atom
Why are there so many different materials in
the world?
Today’s Keywords
(chemical) Element, molecule, atom;
Electron, nucleus, Bohr atom;
Photon, spectrum, spectroscopy
Contents
n  Introduction
n  The atomic structure
n  When matter meets light
n  The laser – practical application
1. Introduction
- Historical review on discovering the
smallest pieces
Introduction
The smallest pieces by Democritus
n  Democritus, in about 530 BC,
philosophically argued about a
piece that could not be divided
further, and named the
smallest piece the “atom”.
n  The atom defined by
Democritus: “uncuttable”,
eternal and unchanging
Introduction
The smallest pieces by Dalton
n  In 18th century many chemists knew most materials
can be broken down into simpler chemicals, and
finally John Dalton (1766-1844) recognized that there
are a few materials could not be broken down into
other substance by any chemical means
è The chemical materials called elements
Example of (chemical) elements:
Iron
Gold
Sodium
Magnesium
.
.
.
Introduction
The smallest pieces by Dalton n  In 18th century many chemists knew most materials
can be broken down into simpler chemicals, and
finally John Dalton (1766-1844) recognized that there
are a few materials could not be broken down into
other substance by any chemical means
è The chemical materials called elements.
n  Dalton suggested that for each chemical element
there was a corresponding species of individual
objects
Example of atoms:
è atoms
Iron atom
Gold atom
Sodium atom
Magnesium atom
…
Introduction
The smallest pieces by Dalton n  In 18th century many chemists knew most materials
can be broken down into simpler chemicals, and
finally John Dalton (1766-1844) recognized that there
are a few materials could not be broken down into
other substance by any chemical means
è The chemical materials called elements.
n  Dalton suggested that for each chemical element
there was a corresponding species of individual
objects
è atoms
n  Two or more atoms can be stuck together to make up
most of the different kinds of material we see around
us è molecules
Introduction
Element, molecule, and atom with an example: water
n  Water: a kind of material we can see around us;
a chemical compound composed of two kinds
elements hydrogen, oxygen:
fundamental chemical elements
n  Hydrogen atom (H), oxygen atom (O): atoms
n  Water molecule: H2O (2 Hydrogen atoms + 1 oxygen
atom) Introduction
n  However,
No matter how persuasively argued, philosophical
speculations on the existence of atoms were just
speculations
guess,
assumption,
suggestion
Introduction
Discovering chemical elements
n  In 19th century, a new process using electrical
current, called electrolysis, was invented by Volta to
break molecules down into atoms
<Simple example of electrolysis>
Electrolysis of water
2H2O(liquid) → 2H2(gas) + O2(gas)
Water molecule
Hydrogen gas
molecule
Oxygen gas
molecule
Introduction
Discovering chemical elements (cont’d)
n  After the new process was developed, Dmitri
Mendeleev invented a periodic table of the
elements in 1869 for several dozen elements on the
basis of their weights and by groups with distinctive
chemical properties, although he could not know
much about the structure of atoms
The periodic table of the elements
Period
Group
The colors represent different
categories of elements. Please
refer to wikipedia.
Introduction
Discovering chemical elements (cont’d)
n  Today, the periodic table lists 118 elements, of 92 appear
in nature and the rest produced artificially
n  Many natural systems are constructed from just a few
99% of Earth’s solid mass: oxygen, silicon,
magnesium, iron, aluminum, and calcium
Most of human body atoms: hydrogen, carbon,
oxygen, or nitrogen
Most stars: the lightest atom, “hydrogen”
The periodic table of the elements
n  The pattern of the elements in the periodic table
displays a concentric arrangement of electrons into
shells
Period 1: One shell
Period 4: four shells
The periodic table of the elements
n  The pattern of the elements in the periodic table
displays a concentric arrangement of electrons into
shells
Period 1: One shell
Period 4: four shells
** Details will be
discussed in “Atoms
in combination”
2. The structure of the atom
Structure of atom
Atomic nucleus
n  In 1897, Joseph J. Thomson identified a much smaller
and lighter particle than even the smallest atom known,
which has a negative electrical charge è electron
n  Thomson argued that atoms are not the fundamental
building blocks of matter, but rather are made up of things
that are smaller and more fundamental, because except
there is no place where the electron could come other
than inside the atom
Structure of atom
Atomic nucleus – Rutherford exp.
n  In 1911, Ernest Rutherford (1871-1937) discovered the
structure of atom using experiment started with a piece of
radioactive material
** Radioactive material:
matter that sends out energetic particles
Structure of atom
Rutherford’s atom experiment
n  Rutherford experiment
Rays from emitter
Deflected rays
(alpha-particle)
Radioactive
material
(lead)
Structure of atom
Atomic nucleus – Rutherford exp.
n  Rutherford found that a large part of each atom’s mass is
located in a very small, compact object at the center.
è nucleus
n  Later on, it was discovered that the nucleus itself is made
up primarily of two different kinds of particles.
è electrically positively charged proton, neutral neutron
nucleons
Structure of atom
Rutherford atom
•  Rutherford’s atom model
Electrons
in orbits
Structure of atom
Rutherford atom
•  Rutherford’s atom model
n  The atom’s structure by Rutherford:
A small dense positively charged nucleus is sitting at
the atom’s center, with light negatively charged electrons
“in orbit” circling it, like planets orbiting the Sun.
Structure of atom
Rutherford atom
•  Rutherford’s atom model
n  The atom’s structure by Rutherford:
l
a
e
r
esitting at
A small dense positively charged nucleus
is
h
t
n
i
a
l
p
x
e
t
’
n
the atom’s center, witholight
negatively
charged electrons
d
l
u
c
l
e
d
!
o
m
m
o
“in orbit”
circling
it,
like
planets
orbiting
the Sun.
s
t
i
a
h
e
T
h
t
è
f
o
s
r
o
i
v
a
h
e
b
Structure of atom
Bohr’s atom model
n  In 1913, Niels Bohr (1885 -1962) produced a very
strange theoretical model of the atom, which doesn’t
match with our intuition about the real world, but
it explained all the behaviors of atoms!
Structure of atom
Bohr’s atom model (cont’d)
n  His insight began with the fact that hot hydrogen atoms
give off light in several separate wavelengths, rather than
a continuous range of wavelengths.
à He realized that circling electrons around the nucleus
could not maintain their orbits at just any distance
from the center.
èHe suggested there were certain orbits in which an
electron could exist for long periods of time without
giving off radiation:
electron energy levels or electron shells
** How to read element symbol
n  Element, element symbol, atomic number
Helium: 2He
# of nucleons:
atomic mass (A)
4
2
He
# of protons: identify a chemical element = atomic number (Z)
Atomic mass A = atomic number Z + neutron number N of the atom
Structure of hydrogen atom:
1 proton in nucleus + 1 electron
Hydrogen atom:
1
-
H
+
Structure of hydrogen atom in Rutherford’s model
Hydrogen atom:
1
-
H
+
Rutherford’s model
Structure of hydrogen atom in Bohr’s model
Hydrogen atom:
1
H
Possible energy levels
(energy shells)
-
+
Bohr’s model
Structure of atom
Bohr atom
n  Ground state: (a)
lowest energy level
which electrons can occupy
n  Excited states:(b) and (c)
all energy levels above
the ground level
3. When matter meets light
n  We have learned,
light = electromagnetic waves
n  In this section we will learn another side of
light,
light = particles (photons)
When matter meets light
Photons: Particles of light
Assume that an electron is in an exited state. What will
happen to extra energy when the electron moves to the
lowest state?
n  The left over energy when the electrically charged
electron moves from a high state to a lower state is
emitted in the form of a single packet of
electromagnetic radiation è a photon
When matter meets light
Photons: Particles of light (cont’d)
n  Whenever an electron jumps from a high to a lower
energy level, a photon emits at the speed of light. (Go
back to the figure of Bohr model. Electrons can exist in
energy state1(=shell1), state2(shell 2), and so on.)
When matter meets light
Photons: Particles of light (cont’d)
**Absorption of light is something like a mirror image of
light emission:
If a photon, with just the right amount of energy, meets
matter, the photon can be absorbed and the electron will
be pushed up to an excited state.
When matter meets light
Quantum leap Two key ideas embedded in Bohr’s atom model
n  When an electron moves from one allowed state to
another, it cannot be at any place in between
è quantum jump or quantum leap
n  The energies emitted in the two different jumps will
generally be different from each other, but the sum of
the two energies will equal that of the single large jump
Structure of atom
Bohr atom
n  Ground state:
(a) Lowest energy level
which electrons can occupy
n  Excited states:
(b) Quantum leap to energy
Level 3 by absorbing a photon
(c) Quantum leap to energy
level 2 by emitting a photon
When matter meets light
Quantum leap: an example *Quantum leaps are very much in evidence in our
everyday life. (HW)
*An example of quantum leaps:
In fluorescence,
n  the atom absorbs a higher-energy photon of
ultraviolet radiation.
n  Then, the atom emits two lower-energy photons, at
least one of which is in the visible range.
n  Consequently, by shinning ultraviolet “black light” on
the fluorescent material, it glows with a bright color.
When matter meets light
Spectrum
Different atoms give off different characteristic photons
n  Different nuclei have different numbers of protons,
so electrons circling them are in different energy
levels
When matter meets light
Spectrum (cont’d) n  Each chemical element emits a distinct set of
characteristic photons
à The total collection of photons emitted by a given
atom is called its spectrum
à Each possible quantum jump corresponds to light
at a specific wavelength, so each type of atom
produces a set of lines
When matter meets light
Different atoms give off different characteristic
photons
Each possible quantum jump corresponds to light at a
specific wavelength, so each type of atom produces a set
of lines
Sodium 11Na
Lithium 3Li When matter meets light
Different atoms give off different characteristic
photons
•  Elements impart distinctive colors to a flame
Sodium 11Na
Copper 29Cu
Lithium 3Li When matter meets light
Spectroscopy
Spectroscopy
n  The study of forming and looking at spectra using
devices such as spectrometers and spectroscopes
n  Astronomers use emission spectra to find the
chemical composition of distant stars.
n  Spectroscopic analysis is used in manufacturing to
search for impurities on production lines.
4. The laser – practical application of
Bohr atom model
The laser
The laser
n  LASER:
Light Amplification by Stimulated Emission of Radiation
n  Bohr atom provides an excellent way of understanding
the workings of the laser
The laser
The process of stimulated emission:
A.  stimulated emission
B.  Light emitted by another
atom
C.  In a laser, cascade
stimulations happen
very quickly
The crests of all
waves line up exactly
C
www.daviddarling.info/encyclopedia/
The laser
The laser
The process of stimulated emission:
A. If a single photon of the correct frequency enters the
system of excited states of atoms, it will pass the first
atom and stimulate the emission of a second atom
A
Atom in excited
state
B
Photon with the correct frequency stimulate the second
atom to emit a photon with the same frequency
The laser
The laser
The process of stimulated emission:
B. The two photons with the same frequency of the
excited atom state, then, will encounter other atoms,
then will stimulate emission
A
Atom in excited
state
B
Photon with the correct frequency stimulate the second
atom to emit a photon with the same frequency
The laser
The laser
The process of stimulated emission:
C. This cascade stimulation happens very quickly, then,
soon there is a flood of photons in one direction
C
A flood of photons with same frequency in one direction
The laser
Applications of lasers (HW)
n  Low-power lasers
- optical scanners, light pointers, light measures
n  Finely focused laser beams
- eye surgery (LASIK, LASEK, …)
n  Powerful lasers
- cutting tools in factory, futuristic energy beam
weapons (Star Wars laser sword?)
, and so on.
Next topic is,
Quantum mechanics: chapter 5
www.sci.hokudai.ac.jp/~epark/ekpark/jos.html