Studying Atoms 4.1 Seeing the Invisible

The Big Debate Can matter be divided into smaller and smaller pieces forever? YES! NO! AristotleDemocritus

Atomic Models: Greek Democritus ( B.C.) “Matter can not be divided forever” Smallest piece = “atom” (Greek “atomos” = “uncuttable”) He had no way of knowing what atoms looked like! Strictly philosophy – no scientific evidence Most followed Aristotle

Gold Gold The word “atom” comes from a Greek word that means “uncuttable” or “indivisible” Imagine you had a piece of gold that you then cut in half… …and then you cut one of these smaller pieces in half… … and you kept on cutting the leftover piece in half…

…and kept going… Eventually you would have 1 piece of gold left. If you cut it in half, you wouldn’t have gold any more. This tiny, tiny single piece of gold is called an atom of gold. An atom is the smallest particle of an element that acts like the element. An atom of gold

Democritus He hypothesized that atoms were: –Small & Solid –Different in shape & size –Infinite –Always moving –Capable of joining 4 Greek Elements 7:29

Time Goes By… Dalton –1803 English chemist and school teacher Studied gasses and how they exert pressure in air. Concluded they are made of tiny particles proposed his atomic theory

Dalton’s Atomic Theory: 1. All elements composed of atoms 2. Atoms of the same element are exactly alike 3. Compounds contain atoms of more than one element 4. In a given compound, atoms of different elements always combine in the same way (i.e. Calcium Chloride CaCl 2 – always has 1 Calcium atom for every 2 Chlorine atoms)

Atomic Models: J.J. Thomson 1897 Passed electricity through vacuum tube –Glowing beam attracted to positively charged plate –Beam was NEGATIVELY charged –Negative charges came from inside atom! Electron –Thomson discovered the Electron –Evidence: different metal plates produced same result –First experiment to prove atoms made of smaller particles! JJ Thomson’s Cathode Ray 2:49

But wait… We know atoms are neutral, or all matter would be repelling all other matter. How can atoms be NEUTRAL if they are full of negatively charged electrons?

Atomic Models: J.J. Thomson + charges must be present to balance charge of electron + & - lumped in a cluster he said looked like “plum pudding” Dalton’s Model Thomson’s Model

Atomic Models: Rutherford 1911 Passed + Charged alpha particles thru gold foil –Most passed right through –Some bounced off at odd angles –Very few bounced straight back Rutherford’s Gold Foil Experiment 4:06

Rutherford’s Gold Foil Experiment

Rutherford’s Work

Rutherford’s Conclusion the results showed that something + was repelling +alpha particles More direct alpha particles hit mysterious + charge, greater deflection + charge concentrated in small central area - Rutherford called nucleus Atom is mostly empty space! The atom 1:15

Ok, so here’s what we know so far….. The atom is made up of + particles in nucleus, and negative electrons. Where are these electrons found in the atom? The Atom Song 3:28

Atomic Models: Bohr Electrons are found in specific energy levels –Like planets around the sun

Atomic Models: Wave Electrons move so fast that it is impossible to determine their location Move in all directions around nucleus Atoms song - Mark Rosengarten