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• - [Instructor] We are told that the first five

• ionization energies for a third period element

• are shown below.

• What is the identity of the element?

• So pause this video

• and see if you can figure it out on your own

• and it'll probably be handy

• to have a periodic table of elements.

• So before I even look at a periodic table of elements,

• let's make sure we understand what this table

• is telling us.

• This is telling us that if we start with a neutral atom

• of this mystery element,

• it would take 578 kilojoules per mole

• to remove that first electron

• to turn that atom into an ion

• with a plus one positive charge.

• And then, it would take another 1,817 kilojoules per mole

• to remove a second electron.

• So to make that ion even more positive.

• And then after that it would take another

• 2,745 kilajoules per mole to remove the third electron.

• And then to remove the fourth electron,

• it takes a way larger amount of energy.

• It takes 11,578 kilojoules per mole.

• And then the fifth electron takes even more,

• 14,842 kilojoules per mole.

• And so, for the first, second, and third

• you do have an increase in ionization energy,

• but when you go to the fourth the energy required

• to remove those is way higher.

• So to me, these look like you're removing valence electrons

• and these look like you're removing core electrons.

• So one way to think about it is

• let's look on our periodic table of elements

• and look for a third period element

• that has three valence electrons.

• So we have our periodic table of elements.

• We want a third period element,

• so it's gonna be in this third row

• and which of these has three valence electrons?

• Well, sodium has one valence electron,

• magnesium has two valence electrons,

• aluminum has three valence electrons.

• So one way to think about it is

• that first electron, it's a reasonable ionization energy.

• Then the second one, a little higher.

• Then the third, a little bit higher than after that,

• but then the fourth, you're starting to go into the core.

• You're going to have to take an electron

• out of that full second energy shell,

• which takes a lot of energy.

• And so this is pretty clearly aluminum

• that is being described.

- [Instructor] We are told that the first five

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# 実践例。連続する電離エネルギーから元素を同定する｜カーンアカデミー (Worked example: Identifying an element from successive ionization energies | Khan Academy)

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林宜悉 に公開 2021 年 01 月 14 日