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Hello tiny scientists! In this video you're going to learn how to make binary compounds which are

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compounds of two elements from the periodic table. I need you to remember the octet rule

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because we're going to use it to explain how atoms combine with others making compounds.

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Finally you will learn how to write the formula of a compound in the right order. The octet rule

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is a very simple way to explain how atoms combine. Basically it says that every atom from the predict

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table wants to have eight electrons in the last shell becoming like the closest noble gas.

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Notice that all noble gases except helium which is too small all noble gases have eight electrons

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in their last shell. There are some exceptions. Let's apply the octet rule to some elements to

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see if we can predict the behavior of atoms when they combine for example chlorine which is the

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closest let's apply the octet rule to some elements to see if we can predict the behavior

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of atoms when they combine for example chlorine which is the previous element to argon has

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seven electrons in the last shell and eight electrons in the previous one so if chlorine

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wants to become a noble gas if he wants to have eight electrons in the last shell

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he can do two things he can lose these seven electrons to become neon if you take

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these electrons from chlorine then it would be exactly like neon or he can obtain

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one extra electron from another atom and then become argon as you can imagine

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it's much easier for chlorine to accept to gain one extra electron than to lose seven of them

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so the general tendency of chlorine is to obtain one extra electron which is represented by its

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oxidation number negative one means that he wants to accept one electron and this tendency

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is the same for all halogens in the same group that than chlorine now let's see what happens to

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sodium for example sodium has one electron in the last shell and eight electrons in the previous one

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so if he wants to become a noble gas he can do again two things he can lose this electron and

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remain with the eight in the previous cell or he can accept seven electrons to complete the last

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cell as you can imagine again it's much easier to lose one electron than to obtain seven so the

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tendency of sodium is always to give this last electron and that's why he has

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positive one as an oxidation number and this is the tendency of all the first

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group in the predictable all of them wants to give this last electron that

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they have except for the hydrogen which is a exception that has oxidation

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numbers of positive one and negative one which we're going to explain now one

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last example. Let's see what happens to magnesium. Magnesium has two electrons in

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the last cell and eight electrons in the previous one. Again to become a noble

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gas he can lose these two electrons and remain with these eight in the last cell

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or he can obtain or accept six electrons from other atoms. It's again much easier

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to lose two electrons than to accept six so the tendency of magnesium always is to lose these two

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electrons in the last shell to become a noble gas and this is the same tendency as the as for all

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the elements in the second group they all want to lose these two electrons that have in the last

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shell. That's why they have all oxidation number of positive 2. Notice that there

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are some elements that don't have a clear tendency. For example we have

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spoken about hydrogen which has positive 1 and negative 1 as oxidation number.

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This means that hydrogen can lose the only electron that it has

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or he can accept one electron to have two electrons in the last shell because

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the closest noble gas to hydrogen is helium which has two electrons in the

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last shell so hydrogen depending on which it combines two it can lose the

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electron or accept one electron now let's try to understand what happens

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when two atoms of different elements combine. In this first video we're going to study elements

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with hydrogen called metallic hydrates or hidruro-metallicos in Spanish. They are the

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combination of hydrogen with a metal from the periodic table. Remember the metals in the

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periodic table are the ones to the left of this line. All metals in the periodic table have

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something in common they have positive oxidation numbers which means that they prefer to give

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electrons than to accept them okay so all of the metals of the project table wants to give

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electrons because of their positive oxidation numbers in metallic hydrates hydrogen uses the

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oxidation number of negative one always because he's combining with metals that always have

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positive oxidation numbers so if metals want to give electrons then hydrogen chooses to accept

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them the general rule to combine atoms is that electrons must not be free this is if an element

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wants to give one electron there must be another element to accept it. The sum of

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given and accepted electrons must be zero. Let's see an example. Imagine we

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want to combine hydrogen with lithium. We have learned that lithium wants to give

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one electron and that's why he has positive one as oxidation number.

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remember that hydrogen uses its negative oxidation number which means that

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hydrogen wants to accept one electron so if lithium wants to give one electron

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and hydrogen wants to accept it then that's it we have one

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electron one given electron and one accepted electron and we sum up these

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two electrons then it's equal zero and then we can write the formula for the

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combination of hydrogen lithium which is lithium hydride or either the lithium

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let's see another example now we want to combine hydrogen with beryllium for

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example from the oxidation numbers we know that beryllium wants to give to

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electrons is that what what the oxidation number of plus 2 means and we

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said that hydrogen we in metallic hydrates in idrurus metallicus always

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acts with negative 1 that's it that's it hydrogen always want to obtain or to

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accept one electron now we have a problem because we told that no electrons can be

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free so if a beryllium wants to give two electrons we cannot write beryllium and

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hydrogen like this because one accepted electron and two given electrons have

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assumed are assumed like a not equal to zero so this cannot be like this instead

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if beryllium needs to give two electrons but hydrogen only can accept one then we

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need two hydrogens and then one of the hydrogen accept one of the electron and

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the other hydrogen accept another electron so this hydrogen accept one the

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beryllium gives two and this hydrogen except one and they sum all zero so the formula for the

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compound with hydrogen beryllium must be beryllium hydrogen two because we need two hydrogens two

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atoms of hydrogen for each one of the atoms of beryllium because of beryllium wants to give

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two electrons and hydrogen can only accept one. So this is the formula for

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beryllium hydride or hydrurode beryllium. In our last example we're going to learn

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what happens when an element has more than one oxidation number. Let's try for

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example to combine hydrogen with iron. As you already know iron has two possible

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oxidation numbers plus two or plus three this means that iron wants to give

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electrons but he can give two electrons or it can give three electrons depending

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on the conditions so if iron wants to give two electrons then we need two

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hydrogens to accept these two electrons of course so one is for this

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hydrogen and this goes to this hydrogen and this way we have accepted

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an electron, two given electrons and one accepted which is a sum of zero and so

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we have a compound with two atoms of hydrogen for each atom of iron but we

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can have another possibility which is that iron gives three electrons so in

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this case we need three hydrogens three atoms of hydrogen to accept these three

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electrons and then three electrons plus one plus minus one and plus negative one

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then it's equal to zero so this compound is iron hydride three which is

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hydrido de hierro as well as this one but they are different compounds with

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different properties in nature we can found these two compounds of course they

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have different formula because they have different proportions of hydrogen and

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iron and this is all caused by the two possibilities of the oxidation numbers

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of iron which are plus two and plus three the last thing we're going to

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learn in this video is to write the symbols in a formula in the correct

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order. To write the formula for binary compound we compare the positions of

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both elements in this diagram. The leftmost element is written first and

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then the rightmost. For example, in the previous examples we wrote the

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combination of iron and hydrogen. We had two possibilities due to the two

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oxidation numbers of iron but in both of the cases in both the compounds that we

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got we wrote first the iron and then the hydrogen and not first the hydrogen and

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then the iron this is incorrect and this is incorrect why do we do this because

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when we combine iron with hydrogen in this table in this diagram we find iron

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to the left of hydrogen then we write first high iron and then hydrogen

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another example if we want to for example write the formula for water

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which is a combination of hydrogen and oxygen as we find hydrogen to the left

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of oxygen then we have to write first hydrogen and then oxygen and not oxygen

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and then hydrogen this is incorrect okay another example for example if we want

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to write the formula of ammonia ammonia co in Spanish which is a combination of

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nitrogen and hydrogen again we write first nitrogen which is at the left and

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then hydrogen with the corresponding number according to the proportions in

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which nitrogen and hydrogen combines this is correct and the other possibility to

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write first the nitrogen and then the nitrogen is incorrect okay this is the

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correct order in which we write symbols and we can apply this general rule to

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any combination of two elements in this diagram for example let's say we want to

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combine potassium with sulfur then if potassium is on the left we have to

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write first potassium and then sulfur and then the number corresponding to the

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proportions in this case to number two in potassium okay this is correct and

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the other possibility to write first sulfur and then potassium is incorrect