The Early Atom

Chapter 22

Journey in the microscopic world Division of matter: can it be done indefinitely? What are the basic constituents of matter? 19th century: atoms are considered as basic entities many type of atoms: H, He, Li, C, O, Al, Fe,……. the known atoms combine to create complex molecules the great variety of substances are a result of different combination of

a few type of atoms Ordering the atoms after their weight (Dmitri Mendeleev: periodic table of the elements) - periodic properties of atoms - atoms can be arranged in a table, where each column has similar chemical and physical properties. Existence of periodicity: a clue that there might be an underlying structure to atoms ….

Atomic Spectra When electric current passes through a gas, the gas gives off a

characteristic color - passing the light through a prism to spread out the colors, the light

splits up into a pattern of discrete colored lines - this lines are always the same for a given gas (and different from the

patterns produced by other gases): called emission spectrum Spectral lines also appear when white light passes through a cool gas - dark lines in the continuous rainbow spectrum - the particular wavelengths are absorbed: absorption spectra lines in the absorption and emission spectra coincide (more lines in the emission spectrum) Origin of the characteristic spectra unexplained !!!

Cathode Rays: the Electron High voltage applied across two electrodes in a partially evacuated

glass tube produces an electric current and light in the tube. If we use good vacuum a yellowish green glow is visible from one end

of the glass tube indications that rays were coming from the negative electrode

(cathode -rays)

- deflected by electric and magnetic field as negatively charged particles

- fluorescent screen glowed wherever it was struck by the cathode rays - if no electric or magnetic field is applied, thet travel in straight lines J.J. Thomson studies quantitatively the deflection of the cathode rays - measures the charge-to-mass ratio (independent of the gas and

electrodes) 1800 times smaller than the one for the known ions!!! - It should be a new particle! Thomson propose that is the constituent

of the atoms. Named: ELECTRON - Thomson proposes that it has the charge of the Hydrogen ion, and a

mass 1800 smaller than that

Charge of the Electron Measured by R. Millikan (1910) Millikan produced varying charges on droplets (by cathode

rays) and studies there movement in electric field. finds that charges on each droplet were a multiple of an

identical chunk or quanta: 1.6 x 10-19 C, the charge given by an electron to the droplet

mass of the electron: 9.11 x 10-31 kg (1800 smaller than the Hydrogen atom)

First model of the atom:Thomson’s model Thomson proposed that atoms have internal structure What does an atom look like? Atoms has electrons inside Atoms should be electrically neutral --> it should have also

a positively charged part. Thomson proposed the “plum-pudding” model - the atom consisting of some sort of positively charged material with electrons stuck in it, like plums in a pudding - cannot explain the discrete emission and absorption spectra, and the repetitive characteristics in the periodic table

Rutherford’s Model An experiment that radically changed our view of the atom - Bombarding materials with alpha particles (He++) (determining the

shape of an object…) - The target was a thin gold plate - expected that the deflection of the alpha particles is small, since

electrons have 7300 times smaller mass, and the positive charge is uniformly distributed

Surprise! A small number of alpha particles were scattered in backward direction (like hitting a wall….)

To explain the experimental results Rutherford proposed a new model (the solar system model of the atoms)

- positive charge located in a very tinny place: the nucleus - electrons orbit the nucleus like Planets the Sun The model have the same problems as the one proposed by Thomson! + electrons in circular motion should constantly radiate electromagnetic waves and should quickly loose its energy!

Radiating objects Germination of the next model: from the properties of the radiation

emitted by hot objects all objects glow, emit electromagnetic radiation at low and normal temperatures this radiation is invisible all objects at temperatures different from zero emit a continuous

spectra of electromagnetic radiation Distribution of the intensity of the electromagnetic radiation versus the

wavelength --> spectral distribution curve Total intensity of the radiation ~ T4 (Stefan’s law) The position of the maximum in the spectra, changes with

temperature: maxT=const (Wien’s displacement law) The spectral distribution curve does not depend on the radiating

material (if we use a cavity) only on T --> universal shape

no theory could describe the whole shape of the spectral distribution curve--> crisis in classical physics

1900 Max Planck: a solution which describes well the distribution BUT!!! Planck uses a hypothesis in total contradiction with classical

physics

- Atomic oscillators (whose vibration produce the radiation) can vibrate only with certain discrete energies!

- Electromagnetic radiation caries this energy also in bundles, or quanta, this energy quanta is: E=hf (h: the famous Planck constant,

f: the frequency) h=6.63 x 10-34 Js

- every atomic oscillator can have an energy which is a multiple of this quanta (if it oscillates with frequency: f)

Discreteness of energy in disagreement with classical physics Electromagnetic waves caries energy in discrete units --> back to the

particle model for light

The Photoelectric effect Another unexplained phenomenon in 1900 When light is shined on metallic objects, electrons are ejected from

their surface - the electrons come off with a range of energies up to a maximum energy, which depends on the color of the light - the intensity of the light does not determine the maximum energy of the out-coming electrons, it determines only their number - experimental results in contradiction with the wave-model of light Conflict solved by A. Einstein, using Planck’s hypothesis - light carries energy in bundles, called photons - the energy of a photon for an electromagnetic wave with

frequency f is: E=h f - a photon can give either no energy or gives its entire energy to an

atom, which can transfer this energy to an electron New conflict: light is BOTH particle and wave Old conflict: still no good model for the light

Bohr’s atomic model 1913 Niels Bohr, proposes a new model, incorporating Planck’s discrete

energies and Einstein’s photon Bohr’s model accept Rutherford’s picture + new postulates Extra postulates: 1. The angular momentum of the electron is quantized. The possible angular momentum is given as: Consequence: - restriction on the possible radii and speeds (r1=5.3 x 10-11 m is the smallest radius, and n an integer called quantum number) - the kinetic, potential and total energy of the electrons is also quantized2. The electron does not radiate when it is on one of the allowed orbits3. The atom can receive and emit only photons with well defined energies. The energy of the photon equals the energy differences between the

possible states of the electrons. - emitting and absorbing photons

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Explaining the atomic spectra Bohr’s model accounted for the existence of spectral lines drawing energy-level diagrams exciting the electron and return to a ground state explaining the emission and absorption spectra

The Periodic Table Bohr’s theory give a fairy picture of the Hydrogen atom It can not explain completely, however, the atoms with more than one

electron (complications due to the interactions between electrons) Occupancy of the energy levels by the electrons - first level: max. 2 electrons - second level: max. 8 electrons - third level: max. 8 electrons - …..(will learn how on….) atoms in the periodic table arranged after the number of the electrons chemical properties of atoms are determined by the electrons in their

outermost shells atoms “like” and tend to fill up the outermost shell completely with

electrons --> this is the driving force for making molecules noble elements: the outer shell completely filled with electrons - inert, no chemical activity (group 8 in the periodic table) atoms from group one (alkaline metals) are very active (tend to give up the single electron in the outer shell) atoms from group 7 (also very active) (tend to receive one electron to fill up completely the outermost shell)

X Rays 1895 Wilhelm Roentgen: new radiation from cathode-ray tubes this radiation had big penetration depth, very different from cathode-

rays, it is invisible, observable only from their effect on substances it can be produced when cathode-rays enter in different substances

with high energy it cannot be deviated by magnetic or electric fields (no charge) X rays --> due to their unknown nature in 1895 X rays are electromagnetic waves, with very high frequency important applications in medicine --> can reveal internal structures an X ray spectrum typically consist of two parts: - a continuous spectrum (produced by the deceleration of the bombarding electrons) - a set of discrete spikes (X-rays photons emmited by the bombarded material) energy of the X ray photons increases as the atomic number of the bombarded element is increasing

Summary emission and absorption spectrum cathode rays are electrons; charge of an electron: 1.6 x 10 -19 C mass of the electron: 9.11 x 10-31 kg cathode rays led to the discovery of X rays (electromagnetic waves) Thomson’s atomic model: the “plum-pudding” model Rutherford shows that the positive charge of an atom is concentrated in a

very tinny place: the nucleus (100,000 times smaller than the atom) Rutherford’s model “solar system” model of the atom all objects emit electromagnetic radiation; characteristic spectral

distribution curves; Wien and Stefan’s law Planck’s theory about this heat radiation--> first clues that energies at the

atomic level are quantized The energy quanta: E= h f (h=6.63 X 10-34 Js, the Planck constant) Electromagnetic waves (and light also) carries energy in quanta, called photons. Energy of a photon with frequency f: E= h f Einstein explains the photoelectric effect Bohr’s model for the H atom incorporates Planck.s discrete energies and

Einstein’s photons into the Rutherford’s model; it is based on three extra postulates

The Bohr model was succecsfull in accounting for many atomic observations, especially the emission and absorption spectra for hydrogen and ordering of the chemical elements

Home-work assignment Part I. 575/1-10; 576/11-15,17-19,21-24;

578/1-12 Part II. 576/26,28; 577/30,32-38,40-42

577/45-51,54-55,57; 578/58/60,14-18,21,23