Grade 9 Chemistry, Lesson 8 - Bohr diagrams - Free Educational videos for Students in K-12

Grade 9 Chemistry, Lesson 8 - Bohr diagrams - By Lumos Learning

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00:00	Hey there . And welcome to Mr Lee Han teaches
00:02	you stuff . This is grade nine chemistry lesson eight
00:05	board diagrams . So if you're looking to make a
00:08	board diagram of an element , the first thing you
00:10	need to do is draw a circle , and this
00:12	circle represents the nucleus in that circle . You're going
00:16	to write the number of protons and neutrons that that
00:18	element has . So let's choose an element . We're
00:22	gonna take sulfur . So here's the box for sulfur
00:26	and up at the top . Left says it has
00:29	an atomic number of 16 , which means it has
00:32	16 protons , so we'll write 16 p in our
00:36	circle . Next , we need to figure out the
00:38	neutrons , so we'll take our big number . Subtract
00:42	our little number . We get 30 to minus 16
00:45	is 16 neutrons , so he put 16 n .
00:50	So the next thing we need to do is add
00:52	the electrons . Now you need to remember that the
00:55	number of electrons is equal to the number of protons
00:58	. So since sulfur has 16 protons , we're gonna
01:01	have to draw in 16 electrons and we put them
01:06	in orbits or shells , and the first three shells
01:08	can hold 28 and eight electrons and each shell must
01:15	be filled before another shell can be started . So
01:19	we need to start with the first two in the
01:21	first shell and then at eight in the next shell
01:24	, etcetera , and one last point . When we
01:27	add electrons , we have to pair them up after
01:29	half filling the shell . So let's get started .
01:33	The first one can hold two , and we pair
01:36	them up because it's more than half full . So
01:39	we've got two electrons out of the way . The
01:40	next shell can hold eight . So we put eight
01:45	in the second orbit there and notice that they're all
01:49	paired up because it's more than half full , mhm
01:53	and then the third orbit we're already at . What
01:55	is that ? 10 electrons . We need six more
01:57	to make 16 . So we add six more electrons
02:02	to that third orbit or that third shell and two
02:06	of which are paired up . And then we have
02:07	to before I guess , are paired up and then
02:10	to sort of lonely , uncared electrons over there .
02:14	So that's it . That's the board diagram for sulfur
02:18	. So let's try a couple more . So now
02:20	let's do a board diagram of lithium . So first
02:23	we draw the nucleus , figure out how many protons
02:26	and neutrons lithium has so little has got an atomic
02:30	number of three . So we know it has three
02:31	protons . We can write that in and next we
02:35	need to figure out how many neutrons it has .
02:38	It has an atomic mass , which is protons plus
02:41	neutrons 6.94 So we round that up to seven .
02:46	So it has seven minus three new drug seven minus
02:49	three new drones , which is four . So long
02:52	story short . Take the big number . Subtract a
02:54	small number . You get the number of neutrons .
02:58	Four . So next up is the electrons . How
03:03	many electrons is gonna have ? It's gonna have the
03:04	exact same number of electrons protons , so it's gonna
03:07	have three electrons . So first we add up the
03:12	first shell , and then we add more to the
03:14	second . The first one can only hold too .
03:16	So we put two in there , and then we
03:19	only need one more . So we add one more
03:21	in the powder shell and we are done . And
03:26	remember , when we add electrons , we have to
03:27	pair them up after half filling the shell But we
03:31	don't need to do that yet because there was only
03:33	one in that outer outer shell . So that's it
03:36	for the board diagram of lithium . It's done .
03:39	So now I want to show you a few tricks
03:40	that you can use with the periodic table to help
03:44	you create board diagrams . Um , so first off
03:49	, the period the element is in will correspond to
03:51	the number of orbits the board diagram will have .
03:55	So if your element that you're creating is in the
03:59	first period right here , it will have one orbit
04:05	. If it is in the second period , anywhere
04:08	in this highlighted yellow area , it will have two
04:12	orbits or two shells . If it's in the third
04:17	period , it will have three orbits and if it
04:21	is in the fourth shell , and I only highlighted
04:24	the first two because we were only doing board diagrams
04:27	for the 1st 20 elements . But if it's in
04:30	this fourth period , it will have four orbits ,
04:35	so that's just a quick little way to figure out
04:37	how many orbits it's going to have . You can
04:39	draw them in . Another fun fact is that if
04:42	you count the number of elements in each period ,
04:45	it will match how many electrons are in that shell
04:49	, so the first period has two elements in it
04:54	, so there's only two electrons that will fill up
04:57	that shell . For the second one . There's eight
05:02	. So to fill up that shells you used eight
05:04	electrons . The third one is also ate , so
05:08	the third shell there has eight electrons in it .
05:12	And then the fourth one I mentioned before , we're
05:15	only doing the first two elements , so you don't
05:18	ever have to put in two electrons in that fourth
05:22	shell . So we'll give a demonstration of how this
05:26	works . We're going to choose an element , and
05:29	then we'll figure out what the board diagram would look
05:30	like for that element . But unlike before , we're
05:33	not going to use any information . We're just going
05:35	to use its location on the periodic table to create
05:39	a board diagram . So here we go . We're
05:41	gonna pick this element , not even going to say
05:44	what this element is . It doesn't matter . So
05:47	let's figure out how to draw this element . It
05:51	is in the third period , right ? It's three
05:54	rows down , so we already know it has three
05:58	shells or three orbits . Now in the first shell
06:05	, it's going to have to electrons , so we'll
06:08	put those in two electrons in the second shell .
06:12	It has eight electrons , so we dry in eight
06:15	electrons , and then in the third shell it has
06:20	12345 And then we stopped there because that's the element
06:24	we're at . So we dry in five electrons ,
06:30	and if we count up all the electrons here ,
06:33	we'll see that there is 15 of them . So
06:36	we know that it must have 15 protons as well
06:39	. Um , now , without the periodic table ,
06:41	Uh , and the atomic mass . We can't really
06:45	say how many neutrons are in here , but we
06:47	got fairly far with this four diagram by just looking
06:52	at a location on the periodic table . So I
06:55	just wanted to show you how helpful it can be
06:56	to have a periodic table around when you're creating board
07:00	diagrams . So let's do a board diagram of oxygen
07:07	. Oxygen has an atomic number of eight , so
07:09	it has eight protons and electrons , and it has
07:13	an atomic mass of 16 . So it has 16
07:16	minus eight , which is eight new tribes . So
07:20	here's our nucleus eight p . Eight n If we
07:24	look at our periodic table , there's oxygen . It's
07:27	in the second row , which means , well ,
07:31	the second period . Rather so it has two orbits
07:35	, two shells . The first shell is going to
07:40	have two electrons in it , and we need to
07:42	add up to eight . So we need six more
07:45	, and we add those extra six in the next
07:48	orbit and we are done . There is a board
07:50	diagram of oxygen now . Valence shells and valence electrons
07:57	are very important in chemistry . The outermost shell ,
08:00	or orbit of an atom is called the Valence shell
08:04	, and the electrons in it are called valence electrons
08:07	. So this right here is the valence shell ,
08:09	the outer shell or the outer orbit . I've been
08:11	calling them orbits . That's the valence shell , and
08:15	any electrons in that shell are valence electrons . And
08:20	they're important because these are the ones that react with
08:23	other atoms . So when there's a chemical reaction of
08:27	some sort , the reason that chemical reaction is happening
08:30	is because the valid shells of an atom are interacting
08:35	. Now let's take a look at the patterns of
08:38	the valence electrons in the periodic table . So if
08:41	we look at the board diagrams of a period on
08:44	the periodic table . We can see that the number
08:46	of Valence electrons increases by one each step to the
08:50	right . So in the first row are the first
08:55	period we have one and then two Valence electrons .
08:58	In the second period we have one and then two
09:02	and then 345678 electrons in the Valence shell . And
09:10	then in the next period we have 12345678 electrons in
09:16	the valence shell . And then in the fourth period
09:20	, we only have one and then two . And
09:22	then we stopped there because we stop at 20 .
09:25	So we can also see from looking at these board
09:28	diagrams that the number of Valence electrons is the same
09:32	for every element in a group or column . So
09:36	for the first column there , the first group ,
09:38	those are the alkali metals . There is one Valence
09:43	electron for the second group , the alkaline earth metals
09:49	there is too failing to electrons , and that's the
09:52	same for that whole group . And the pattern keeps
09:55	going from there goes 34567 and then a full valence
10:01	shell . We say it's eight , but that top
10:04	one there only has to , uh , but that's
10:07	a full valence shell for the last column there .
10:11	Now It's important because the number of valence electrons determines
10:16	the way the elements combined to create compounds . And
10:19	remember that each one of the columns in the periodic
10:22	table all have elements that behave very similarly chemically ,
10:27	and those two things are linked . So the reason
10:29	why they behave similarly in a chemical reaction is because
10:33	they have the same number of valence electrons . Now
10:37	, if we take a look at the noble gases
10:39	on the far right of the table , each of
10:43	these noble gases has the maximum number of electrons in
10:46	their valence shell , and this is what makes them
10:48	very un reactive . So if you take a look
10:52	, they can't fit any more electrons into their outer
10:54	shell , and this is why they don't react with
10:59	anything . The whole reason elements react with each other
11:02	is so that they can fill up their valence shell
11:06	. These guys already have full valence shells , so
11:09	they have no interest in interacting with any other elements
11:12	at all . So that's why they run reactive .
11:16	All right , so that's it for this video .
11:18	Tune in for the next video on isotopes and ions

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