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

December 5, 2016

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

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

Introduction

Example of (chemical) elements: Iron Gold Sodium Magnesium . . .

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

Introduction

Example of atoms: Iron atom Gold atom Sodium atom Magnesium atom …

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

Introduction

guess, assumption, suggestion

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)

Introduction

Water molecule Hydrogen gas molecule

Oxygen gas molecule

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

Introduction

The periodic table of the elements

The colors represent different categories of elements. Please refer to wikipedia. Group

Per

iod

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”

Introduction

Discovering chemical elements (cont’d)

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

The periodic table of the elements

** Details will be discussed in “Atoms in combination”

2. The structure of the 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

Structure of atom

** Radioactive material: matter that sends out energetic particles

Rutherford’s atom experiment

n  Rutherford experiment

Structure of atom

Radioactive material (lead)

Rays from emitter

Deflected rays (alpha-particle)

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

Structure of atom

nucleons

Rutherford atom

•  Rutherford’s atom model

Structure of atom

Electrons in orbits

Rutherford atom

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.

•  Rutherford’s atom model

Structure of atom

Rutherford atom

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.

•  Rutherford’s atom model

Structure of atom

è This model couldn’t explain the real

behaviors of the 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

Structure of atom

** How to read element symbol

n Element, element symbol, atomic number Helium: 2He

He 2

4

# of protons: identify a chemical element = atomic number (Z)

# of nucleons: atomic mass (A)

Atomic mass A = atomic number Z + neutron number N of the atom

Hydrogen atom:

Structure of hydrogen atom: 1 proton in nucleus + 1 electron

H1

+ - -

Hydrogen atom: H1

+ - - Rutherford’s model

Structure of hydrogen atom in Rutherford’s model

Hydrogen atom: H1

+ - -

Possible energy levels (energy shells)

Bohr’s model

Structure of hydrogen atom in Bohr’s model

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

Structure of atom

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)

Photons: Particles of lightAssume 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

When matter meets light

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

Structure of atom

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

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

Sodium 11Na Copper 29Cu Lithium 3Li

•  Elements impart distinctive colors to a flame

When matter meets light

Different atoms give off different characteristic photons

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.

When matter meets light

4. The laser – practical application of Bohr atom model

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:

C

www.daviddarling.info/encyclopedia/

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

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

The laser

B

Atom in excited state

Photon with the correct frequency stimulate the second atom to emit a photon with the same frequency

A

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

The laser

A

Atom in excited state

Photon with the correct frequency stimulate the second atom to emit a photon with the same frequency

B

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

The laser

A flood of photons with same frequency in one direction

C

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.

The laser

Next topic is, Quantum mechanics: chapter 5

www.sci.hokudai.ac.jp/~epark/ekpark/jos.html