Grade 9 Chemistry, Lesson 4 - The History of Atomic Theory - Free Educational videos for Students in K-12

Grade 9 Chemistry, Lesson 4 - The History of Atomic Theory - By Lumos Learning

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00:00	all right . This is Mr Lee Han teaches you
00:02	stuff . Grade nine Chemistry lesson for the history of
00:05	atomic theory . So we're gonna start off with ancient
00:09	theories . But before we do , I just wanted
00:12	to go over what we mean by a theory .
00:14	Now , in everyday life , a theory usually just
00:16	means a good guess about something like , Oh ,
00:18	I have a theory about where I left my keys
00:20	or I have a theory about what Harry Potter's parents
00:23	liked on their pizza . But these theories are not
00:27	the same as scientific theories . A scientific theory is
00:30	a model based on observations from experiments and scientific reasoning
00:37	. Um , that helps explain the world . It's
00:39	a framework by which we understand some aspect of the
00:43	world . And we use these theories to help us
00:47	, uh , make predictions and design experiments to help
00:51	us better understand that world . So anyways , the
00:56	Greeks had a theory about matter . The Greeks thought
00:59	that all matter was composed of four elements earth ,
01:02	air , fire and water . And these four elements
01:06	could combine in different ratios to produce the variety of
01:08	substances we see today . So , for instance ,
01:12	the Greeks thought that wood was made from some earth
01:18	, some air and some fire . And when you
01:22	set the wood on fire , the fire came out
01:24	and the smoke came out and that was aired coming
01:26	out . And then you had ash left over ,
01:28	and that was the dirt . So that's what they
01:31	thought . All the stuff on Earth was made from
01:33	those four elements , very similarly in ancient China .
01:39	The elements they thought made everything up was firewater ,
01:44	wood , metal and earth . Now you may think
01:47	it's a little silly that these ancient cultures thought there
01:49	was only four or five elements . Uh , but
01:53	really , if you think about it , all the
01:55	elements that we know of today are made of three
01:57	basic subatomic particles the proton , the neutron and electron
02:03	. So really , everything is made of only three
02:04	things . So the idea that everything might be made
02:07	of four or five things , you know , isn't
02:09	that far fetched . But as far as elements go
02:13	, we now know that there are over 100 elements
02:15	that make up all the matter around us . So
02:19	our first stop at a scientist happens at 4 40
02:22	b . C . With Democrats and Democrats came up
02:26	with a theory that as you broke materials down into
02:28	smaller and smaller particles , Eventually you get particles so
02:32	small that you could not break it down any further
02:34	. He called this particle an atom , and that's
02:38	still a pretty good explanation of what an atom is
02:40	today . So smart guy . Next up , we
02:44	have John Dalton . Dalton pictured atoms as very small
02:48	spheres with different properties , and he created a table
02:52	of elements with symbols to represent each one . So
02:56	this is his table of elements here . Now we
02:58	have a periodic table of elements today . It's not
03:02	based on his table , but good idea to have
03:05	a table with symbols on it . Now . What
03:07	John Dalton was really known for was his four point
03:10	theory to describe the nature of matter . So he
03:13	said that all matter is made of atoms . All
03:17	atoms of a certain element are identical and properties atoms
03:21	of different elements have different properties , and Adams can
03:25	combine to create new substances so somewhat similar to the
03:28	particle theory . But John Dalton came up with these
03:31	long time ago , and they all hold true today
03:33	. So smart guy , So next up on our
03:36	scientific tour through history , is J . J .
03:38	Thompson . It's got a nice moustache , uh ,
03:42	using something called a cathode ray to he caused elements
03:45	to give off a stream of negatively charged particles that
03:49	he called electrons . And he figured out there are
03:51	negatively charged because in his cathode ray tube , he
03:56	had an electric field generated , and when that electric
04:00	field was turned on , it deflected the stream of
04:02	particles , so he knew that it must therefore be
04:05	negatively charged . So since he knew that there was
04:09	negatively charged particles and atoms , he knew that there
04:14	must also be positive charges in there as well ,
04:17	because elements don't have an overall charge by themselves ,
04:21	right ? Atoms don't have an overall charge . Usually
04:24	so , Thompson figured , there must be positive particles
04:26	in the atoms as well . So he came up
04:29	with the idea that an atom is a positive mass
04:33	with negative electrons in it . And this is the
04:36	raisin bun model . So you're electrons are negatively charged
04:40	raisins in a positively charged button . So the whole
04:45	the whole atom was thought to be positively charged ,
04:47	with little specks of electrons scattered throughout . And then
04:53	along came Ernest Rutherford , who had an awesome moustache
04:56	as well . Um , and he discovered that the
04:58	protons are located in a central nucleus , so instead
05:01	of having the entire atom is sort of a bun
05:04	of positive energy , Rutherford found that all of the
05:08	positive protons are located in a central nucleus . So
05:13	this is the apparatus that Ernest Rutherford used to figure
05:16	this out . So he has an alpha particle emitter
05:19	, which actually shoots protons . That's what alpha particles
05:22	are , and it shoots the protons at a piece
05:25	of gold foil , and it's very , very thin
05:27	gold foil . So it's just a very , very
05:30	thin piece of gold , and the protons were supposed
05:32	to fly right through it . And then there's the
05:36	detecting screen , this green screen here , which will
05:39	light up when it gets hit by the protons .
05:42	So they fired the alpha particles through the screen ,
05:45	and it hits the back , just like they expected
05:48	. But unexpectedly , some some of the beams I
05:52	deflected and off to the side . They had bits
05:55	lighting up , which was very surprising . They didn't
05:58	expect this at all , but most surprising of all
06:02	was that some of the beams actually turned right around
06:04	and did like a 1 80 came right back at
06:06	them and lit up the detecting screen on the other
06:09	side . That was totally unexpected . In fact ,
06:14	Ernest Rutherford said , it was like firing a cannonball
06:17	at a piece of tissue paper and then having the
06:20	cannonball bounced back and hit you in the face .
06:22	That's how surprising it was . So , based on
06:27	what they knew at the time the raisin Bond model
06:31	, you had these negative electrons and a mostly positive
06:36	Adam . So if you fired these positively charged alpha
06:41	particles or protons Adam Adam , they should just fly
06:45	right through , because there's nothing especially positive anywhere or
06:48	especially negative to deflect it anyway . It should .
06:52	It should all balance out . So this is what
06:55	Rutherford expected based on the raisin bun model . So
06:58	that's why they expected to just fly straight through .
07:01	But what they saw was that most things went straight
07:03	through , but some deflected , and then some shot
07:07	right back at them . So based on this ,
07:11	Rutherford concluded that the atoms must have all the positive
07:14	charges clumped together in the middle of the atom ,
07:17	and that's the nucleus . So the next problem with
07:22	scientists had to figure out was why Adams had too
07:24	much mass , so they had discovered the atoms weighed
07:27	more than they should if there was only protons and
07:30	electrons in there . There are also isotopes of elements
07:34	that weighed different amounts but had the same charges .
07:37	A regular atom , so lead to four and lead
07:40	to six were both lead , but one of them
07:43	weighed slightly more than the other one , and they
07:44	didn't know why . Um , some scientists thought that
07:48	they the nucleus , held some extra protons , but
07:51	also extra electrons , and they canceled each other out
07:55	. That wasn't right . A guy by the name
07:57	of James Chadwick . I figured out that there was
08:00	another subatomic particle called a neutron , and the neutron
08:04	added weight to the atom but had no charge at
08:06	all . That's why it's called a neutron , because
08:08	it's neutral . So scientists then knew that there were
08:12	protons and neutrons in the nucleus with electrons orbiting around
08:16	it . So the model of the Adam at this
08:20	point looks something like this . So you've got a
08:24	central nucleus there , with electrons in orbit around these
08:27	protons and neutrons in the middle . But there were
08:32	still mysteries that scientists needed to figure out scientists knew
08:35	that when certain elements gained energy , they would release
08:38	that energy only in very specific amounts . And Niels
08:41	Bohr was the one that figured this all out .
08:44	So he figured out that the electrons could exist only
08:46	in very specific orbits around the nucleus . So this
08:51	Adam here on the left , this is neon ,
08:54	and the electrons orbiting the nucleus and neon cannot exist
08:58	in between the orbits . They can't sort of slowly
09:01	move up to the next orbit . They make jumps
09:05	. It's more like an elevator where you can get
09:06	off at one floor or the next floor or the
09:09	next floor . Electrons are the same way they can
09:11	be in one orbit or the next orbit or the
09:14	next orbit . They can't get off in between floors
09:17	. They have to go right up to the next
09:18	floor , right up to the next orbit . So
09:21	this led to the Bohr model of the atom .
09:22	That's the Bohr model on the left there , and
09:25	this helps explain why electrons or why atoms will give
09:30	off very specific amounts of energy , because when energy
09:34	comes in , it causes those electrons to jump up
09:38	to the next floor or the next orbit , and
09:42	then one of the energy goes away . Those electrons
09:45	will fall right back down , and they'll release a
09:47	very specific amount of energy , just the amount of
09:49	energy needed to make it go up to the next
09:52	level . So that explains why electric or why atoms
09:57	will give up very specific amounts of energy . So
10:01	the most current model we have is the quantum mechanical
10:03	model . And in this model of the atom ,
10:05	scientists have agreed that we don't know , and we
10:07	can't know where exactly an electron is or what momentum
10:11	it has , because if we try to figure that
10:15	out , we use some instrument , then will probably
10:17	alter where that electron is . And we have nothing
10:21	precise enough to figure out exactly where it is without
10:23	changing it . So you can kind of think of
10:26	it as a You had a room with several soccer
10:29	balls in it , and it was pitch black .
10:31	The only way you can figure out where those soccer
10:33	balls were , because if you walked around kicking stuff
10:37	well , eventually you run into one of those soccer
10:39	balls and you kick it and you'd say , Oh
10:41	, this is where it was . But you wouldn't
10:43	know where it is anymore because you've already kicked it
10:46	. So the same is sort of true of trying
10:48	to figure out where an atom or where an electron
10:50	is in an atom , because if you ever figure
10:54	out where it is , you've probably kicked it ,
10:56	and it's not there anymore . However , although scientists
11:00	have agreed that they don't know where electron is ,
11:03	they still want to do work using electron locations .
11:08	So to continue using equations based on their model of
11:11	the atom , scientists have come up with the idea
11:14	of a cloud of possible locations for the electrons electron
11:17	clouds . And these are based on the probability of
11:20	an electron being in any one spot . And it's
11:23	not that the electron exists as a cloud itself .
11:26	It's just that they think of the electron as a
11:30	cloud of probability so that they can continue to make
11:34	predictions . So that is it . For history of
11:38	atomic theory . Tune in for the next video elements
11:41	and the structure of the atom

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