HISTORY OF STRUCTURE OF THE ATOM

Early models of the atom

• Century 5th BC, Democritus used the word “atomos.” He said that the differences between substances were the direct result of differences in

the size & shape of tiny, uniform, uncuttable particles• Aristotle & other C 4th BC scholars rejected

Democritus’s idea

• The Greeks said there were 4 elements:

• Air, fire, earth, water

• The properties were : hot, dry, moist, cold

ALCHEMISTS – MIDDLE AGESEurpoean alchemists tried to change lead into gold •Alchemists

experimental work involved separation of metals from ores and many types of distillations

• Ideas were based on philosophy

DALTON’S ATOMIC THEORYIn 1908 Dalton re-introduced the idea of the atom & supported his theory by experimentation. His theory summarized:

• Elements are made up of extremely small particles called atoms

• Atoms of an element are all identical & different types of atoms have different properties

• Each compound is unique & consists of a particular combination of specific types of atoms put together in a distinctive way

• Chemical reactions involve the reshuffling of atoms in a compound to make new compounds. The new compounds are made from the same atoms which were present in the original compounds• Dalton’s model of the atom was an

indivisible, uniformly dense, solid sphere that entered into chemical reactions but was left unchanged by the reaction• Dalton’s theory lead to laws like the

Law of Conservation of Mass (Total mass of reactants = total mass of products)

DALTON’S MODEL OF THE ATOM

Dalton describe atoms as uniformly dense solid spheres

SIR WILLIAM CROOKES’ CRT EXPERIMENTSCrooks used a Cathode Ray Tube & found the atom to be electrical

• The CRT is a tube containing a gas at low pressure with two electrodes implanted which are connected to an electric power source

• The negative electrode is a cathode & the positive electrode is the anode

• With power on, a green glowing beam was emitted from the negative electrode

• Crooks concluded that the beam was

composed of particles & that the beam must be negative, since it came from the

negative electrode

CATHODE RAY TUBE. TV’S & COMPUTER SCREENS ARE CRTS

JJ THOMSOM USED CRT • 1897 Thomson conducted further experiments using cathode

ray tubes• Thomson used a CRT with a

fluorescent screen at the end. The screen would glow whenever stuck by a beam of charged particles.

• Thomson positioned a magnet close to the CRT

• He found that the cathode rays were deflected in the same direction by the magnetic field as negatively charged particles would be

PLUM PUDDING MODEL

• Later these particles were identified as ELECTRONS• Thomson’s model of the atom was

called the PLUM PUDDING model because he used a sphere with subatomic particles embedded: negatively charged electrons & positively charged protons dispersed to give an overall neutral charge• By 1920 Thomson ionized

hydrogen into H+ & e-. The positive proton in the nucleus was known

JJ THOMSON’S PLUM PUDDING MODEL OF THE ATOM

BECQUEREL DISCOVERED RADIOACTIVITY BY ACCIDENT

1896 Becquerel investigated the nature of X rays, which he knew had the ability to fog a photographic plate even when the plate was covered by black paper

• He coated a some black paper with a uranium compound & put on a photographic plate. When exposed to sunlight the plate showed the outline of the compound from the paper.

• He put the experimental plates away in a dark closet, to wait for another sunny day to continue experimenting.

• He decided to develop the plates, even without sun exposure. The outline of the uranium compound was there.

• He concluded that the uranium itself must have given off some kind of rays on its own without the outside influence of sunlight

EARNEST RUTHERFORD DISCOVERED 3 TYPES OF RADIOACTIVE EMISSIONSGold foil experiments were designed to bombard thin metal foil (gold) with alpha particles. When some particles were deflected, instead of passing straight through, he concluded that they must have an electrical charge

•Rutherford realized that the centre of the atom must have a positive charge. He called the centre the nucleus•It was the positive nucleus of atoms in the gold foil that cause the positive alpha particles to deflect

•An alpha particle is a high speed positively charged particle

that has a mass = to a helium nucleus•A beta particle is a high speed electron•Gamma rays have no mass, but are high energy rays, with more energy than X-rays.•All three types of radioactive emissions come out of the unstable nucleus of elements that are radioactive, like uranium

RUTHERFORD’S GOLD FOIL EXPERIMENTRutherford expected the result in the top red picture. Instead he found deflection, showing that some of the + charged alpha particles hit some of the nuclei of the gold atoms & were deflected.

ROBERT MILLIKAN1909

Millikan successfully measured the charge on an electron using a falling, negatively charged oil drop

• Oil drops were sprayed into a chamber & exposed to X rays. The chamber has a positively charged upper plate & a negatively charged bottom plate

• A falling, negatively charged oil drop is made to rise between the plates because of exposure to X rays, which ionized some of the drops. There were positive and negative ions

• He calculated the charge on the drop by knowing the rate at which the drop was rising, the strength of the electrical field, & the weight of the oil drop

• Millikan used his experiments & Thomson’s to calculate the mass of the electron to be 9.11 X 10 (-28th ) grams

MILLIKAN’S OIL DROP EXPERIMENT

The ionized oil drops that were negatively charged actually were repelled by the negative bottom plate & rose up.

MARIE & PIERRE CURRIE• Curries followed up Becquerel’s discovery of radioactivity &

discovered the two radioactive elements Radium & Polonium

• Curries coined the term “radioactive decay” for unstable elements that emit radiation from their nuclei

• Their daughter Irene Joliot-Currie & her husband Frederic did further research

• They bombarded beryllium (Be) with alpha particle (He nucleus +2 charge) & a beam of high penetrating power was formed

• JAMES CHADWICK demonstrated that the beam was made of particles with the same mass of Protons, and it was NOT affected by electric or magnetic fields

• Chadwick was credited with discovering the neutron. Neutrons were named so because they did not possess a charge (neutral)

NEILS BOHR1913 Bohr expanded on Rutherford’s model of the atom

•Bohr’s model has a defined nucleus in the centre with positive protons & neutral neutrons, surrounded by electrons in

“energy levels” or “orbits”

•In a neutral atom the # of protons (+) = the # of electron (-)•The “rules” for filling the energy levels with electrons: first orbit has room for 2 electrons, second & third orbit has room for 8 electrons. “Stable octet” is the term used to describe a full shell. In models, we fill orbits with electrons this way: 2, 8, 8, 18, 18, 32… Then it becomes more complicated

HISTORY OF ATOMIC MODELSSketch the atomic models. We will use Bohr models to draw on flat paper, but be aware that all atoms are 3D, like the most current model depicts