EARLY Atomic theories

democritus The Greek philosopher Democritus (460-370BC) was the first person to propose the idea that matter was not infinitely divisible. He believed that matter was made up of tiny individual particles called atomos, from which the English word atom is derived. Democritus believed that atoms could not be created, destroyed, or further divided. Democritus also believed that matter is composed of empty space through which atoms move.

ARISTOTLE The Greek philosopher Aristotle (384-322BC) rejected Democritus’ atomic “theory” because he did not like the idea that atoms moved through empty space. He did not believe that the “nothingness” of empty space could exist. Aristotle was able to gain wide acceptance for his ideas and his denial of the existence of atoms went largely unchallenged for 2000 years.

Dalton The English schoolteacher John Dalton (1766-1844) began the development of modern atomic theory. Dalton revived and revised Democritus’ ideas based upon the results of his scientific research.

Dalton’s ATOMIC THEORY Elements are made of tiny particles called atoms. All atoms of a given element are identical. The atoms of a given element are different from those of any other element. Atoms of one element can combine with atoms of other elements to form compounds. A given compound always has the same relative numbers and types of atoms (law of constant composition). Atoms are indivisible in chemical processes. That is, atoms are not created nor destroyed in chemical reactions. A chemical reaction simply changes the way the atoms are grouped together.

Dalton’s ATOMIC THEORY Was Dalton’s atomic theory a huge step toward our current atomic model of matter? YES Was all of Dalton’s theory accurate? NO Dalton’s theory had to be revised as additional information was learned that could not be explained by his theory.

Dalton’s ATOMIC THEORY You will soon learn that Dalton was wrong about atoms being indivisible (they are divisible into several subatomic particles – the proton, neutron, and electron). all atoms of a given element are identical (atoms of an element may have slightly different masses – isotopes)

J.J. Thomson The English physicist J.J. Thomson (1856-1940) discovered the first subatomic particle known as the ELECTRON. He did this by using a cathode ray tube and he concluded that the mass of the charged particle (the electron) was much less than that of a Hydrogen atom, the lightest known atom. The conclusion was shocking because it meant there were particles smaller than the atom.

J.J. Thomson J.J. Thomson proved Dalton’s Atomic Theory incorrect. Dalton said that atoms were indivisible But, J.J. Thomson showed that atoms were divisible into smaller subatomic particles.

ROBERT MILLIKAN The American physicist Robert Millikan (1868-1953) discovered the charge of the electron. So good was Millikan’s experimental setup and technique that the charge measured almost 100 years ago is within 1% of the currently accepted value.

J.J. Thomson After the electron’s charge was discovered, two questions arose: If electrons are part of all matter and they possess a negative charge, how is it that all matter is neutral? If the mass of an electron is so small, what accounts for the rest of the mass in an atom? J.J. Thomson attempted to answer these questions…

J.J. Thomson He proposed a model known as the Plum Pudding Model of the atom, which showed a spherically shaped atom that was positively charged with electrons evenly distributed throughout, like raisins distributed in a plum pudding. The Plum Pudding was quickly proved incorrect.

ERNEST RUTHERFORD The New Zealand born Ernest Rutherford (1871-1937) was a student of J.J. Thomson at Cambridge University in England. Rutherford came up with The Gold Foil Experiment. The Gold Foil Experiment was setup to shoot a beam of alpha particles at a piece of gold foil that was surrounded by a zinc sulfide screen.

ERNEST RUTHERFORD If the Plum Pudding Model were correct, Rutherford would see only minor deflections as the alpha particles passed through.

ERNEST RUTHERFORD Rutherford saw some of the alpha particles go straight through the foil without any deflection, but many of the alpha particles hit the gold foil and deflected off at large angles.

ERNEST RUTHERFORD This is because the alpha particles, which are positively charged, are hitting the dense positively charged core in the middle of the atom and because positive repels positive, they are being deflected. The Plum Pudding Model was shown to be incorrect.

ERNEST RUTHERFORD Rutherford finalized his experiment concluding that there was a tiny, dense region, which he called the NUCLEUS, centrally located within the atom that contained all of the atom’s positive charge and almost all of the atom’s mass.

ERNEST RUTHERFORD His new model also showed that most of an atom consists of electrons moving rapidly through empty space. The electrons are held within the atom by their attraction to the positively charged nucleus. The volume of space through which the electrons move is huge compared to the volume of the nucleus.

ERNEST RUTHERFORD Later, Rutherford did further research to conclude that the positively charged particle was to be called a PROTON and that they have a charge equal to but opposite of an electron.

James chadwick Rutherford’s student, an English physicist named James Chadwick (1891-1974) discovered the final subatomic particle, the NEUTRON, which is present in the nucleus, has a neutral charge, and is about equal in mass to the proton.