How to can we know the last shell for an atom ? Watch

Sara_Acton
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How to can we know the last shell for an atom ? (For lewis structure and electron configurations)

Any help please ??
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Pigster
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(Original post by Sara_Acton)
How to can we know the last shell for an atom ? (For lewis structure and electron configurations)

Any help please ??
What level are you? KS3? GCSE? A level? Degree?
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Sara_Acton
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(Original post by Pigster)
What level are you? KS3? GCSE? A level? Degree?
Why u're asking
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Plantagenet Crown
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(Original post by Sara_Acton)
Why u're asking
I assume for the purposes of explaining. If someone is only at GCSE level for example it might not be the best course of action to explain electron shell arrangement by using s, p, d and f shells, as well as bringing up principal quantum numbers, Pauli exclusion principle, Hund's rules etc.
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Sara_Acton
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(Original post by Plantagenet Crown)
I assume for the purposes of explaining. If someone is only at GCSE level for example it might not be the best course of action to explain electron shell arrangement by using s, p, d and f shells, as well as bringing up principal quantum numbers, Pauli exclusion principle, Hund's rules etc.
Oh alright. Well it's for A-levels
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Plantagenet Crown
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(Original post by Sara_Acton)
Oh alright. Well it's for A-levels
Can you be more specific in your question? Do you need to know the number of valence electrons a compound has, or specifically the shell the last electrons are in, such as 2p, 3s etc.?
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Sara_Acton
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(Original post by Plantagenet Crown)
Can you be more specific in your question? Do you need to know the number of valence electrons a compound has, or specifically the shell the last electrons are in, such as 2p, 3s etc.?
Well i need to know how to know how many electrons are in the last shell in order to draw lewis structure for elements and compounds.

And I want to know how to determine the valence are core electrons in an element ( for example, P has the electrons configeration of 1s2, 2s2, 2p6, 3s2,3p3. How many valence and core electrons does it have?)

Many thanks in advance
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Pigster
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(Original post by Sara_Acton)
Well i need to know how to know how many electrons are in the last shell in order to draw lewis structure for elements and compounds.

And I want to know how to determine the valence are core electrons in an element ( for example, P has the electrons configeration of 1s2, 2s2, 2p6, 3s2,3p3. How many valence and core electrons does it have?)

Many thanks in advance
They have the same number of valence e- as their group number (using the old 1-8 way of doing it, rather then the new fangled IUPAC 1-18).

For Lewis structure, you don't need to know how many core e-. But 2,8,18 should do you.
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Plantagenet Crown
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(Original post by Sara_Acton)
Well i need to know how to know how many electrons are in the last shell in order to draw lewis structure for elements and compounds.

And I want to know how to determine the valence are core electrons in an element ( for example, P has the electrons configeration of 1s2, 2s2, 2p6, 3s2,3p3. How many valence and core electrons does it have?)

Many thanks in advance
In general, the valence electrons of an element can be determined from its group (column) in the periodic table. H is in group 1 ergo it has 1 valence electron, Be is in group 2 ergo it has 2 valence electrons. The transition elements can be ignored for these purposes, meaning we can consider the group beginning with B as the 3rd group, meaning carbon is in the 4th group, and we all know carbon has 4 valence electrons. Phosphorus is in group 5 (following this convention) and it therefore has 5 valence electrons: the 2 in 3s2 + the 3 in 3p3.

Core electrons are just total electrons minus valence electrons. In the case of phosphorus, it has a total of 15 electrons, 5 of them being valence, therefore the remaining 10 are core electrons.
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Sara_Acton
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(Original post by Plantagenet Crown)
In general, the valence electrons of an element can be determined from its group (column) in the periodic table. H is in group 1 ergo it has 1 valence electron, Be is in group 2 ergo it has 2 valence electrons. The transition elements can be ignored for these purposes, meaning we can consider the group beginning with B as the 3rd group, meaning carbon is in the 4th group, and we all know carbon has 4 valence electrons. Phosphorus is in group 5 (following this convention) and it therefore has 5 valence electrons: the 2 in 3s2 + the 3 in 3p3.

Core electrons are just total electrons minus valence electrons. In the case of phosphorus, it has a total of 15 electrons, 5 of them being valence, therefore the remaining 10 are core electrons.
So why do we consider the 3s2 and 3p3 as the valence electrons for phosphorus? Is that because they are in the outer shell ?
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Plantagenet Crown
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(Original post by Sara_Acton)
So why do we consider the 3s2 and 3p3 as the valence electrons for phosphorus? Is that because they are in the outer shell ?
From the rules just explained. Phosphorus must have 5 valence electrons because it's in group 5, and therefore we look at its shell filling and count the 5 outermost electrons: 3s2 + 3p3. Valence electrons are outer shell.
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Sara_Acton
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So what does the -3 (the valence of Phosphorus) refer to ?
I am confused because -3 and 5 are named as valence electrons for Phosphorus
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Sara_Acton
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(Original post by Plantagenet Crown)
From the rules just explained. Phosphorus must have 5 valence electrons because it's in group 5, and therefore we look at its shell filling and count the 5 outermost electrons: 3s2 + 3p3. Valence electrons are outer shell.
So what does the -3 (the valence of Phosphorus) refer to ?
I am confused because -3 and 5 are named as valence electrons for Phosphorus
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Plantagenet Crown
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(Original post by Sara_Acton)
So what does the -3 (the valence of Phosphorus) refer to ?
I am confused because -3 and 5 are named as valence electrons for Phosphorus
I see why you’re getting confused. The explanation is thus:

1) Neutral phosphorus always has 5 valence electrons.

2) The term valency is normally used when referring to an element within a compound (I personally dislike the term and don’t find it very useful. It wasn’t mentioned once during my chemistry degree). So for example, in PCl3, phosphorus has a valency of 3 because it has shared 3 of its 5 electrons to make 3 bonds.

3) However, phosphorus also has the ability to share all its 5 valence electrons, such as in the compound PCl5 for example. In this case P has a valency of 5.
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Sara_Acton
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(Original post by Plantagenet Crown)
I see why you’re getting confused. The explanation is thus:

1) Neutral phosphorus always has 5 valence electrons.

2) The term valency is normally used when referring to an element within a compound (I personally dislike the term and don’t find it very useful. It wasn’t mentioned once during my chemistry degree). So for example, in PCl3, phosphorus has a valency of 3 because it has shared 3 of its 5 electrons to make 3 bonds.

3) However, phosphorus also has the ability to share all its 5 valence electrons, such as in the compound PCl5 for example. In this case P has a valency of 5.
Perfect, I got it 👍🏼
Is there any other name for the -3 rather than valency ?
Thanks !x
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Sara_Acton
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(Original post by Plantagenet Crown)
I see why you’re getting confused. The explanation is thus:

1) Neutral phosphorus always has 5 valence electrons.

2) The term valency is normally used when referring to an element within a compound (I personally dislike the term and don’t find it very useful. It wasn’t mentioned once during my chemistry degree). So for example, in PCl3, phosphorus has a valency of 3 because it has shared 3 of its 5 electrons to make 3 bonds.

3) However, phosphorus also has the ability to share all its 5 valence electrons, such as in the compound PCl5 for example. In this case P has a valency of 5.
Can I ask another question please?
Why does Hund rule NOT apply on Mg and does apply on N ?
I found that in both of the elements, electrons ocupy the sub-shell as single before pairing begins
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Plantagenet Crown
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(Original post by Sara_Acton)
Can I ask another question please?
Why does Hund rule NOT apply on Mg and does apply on N ?
I found that in both of the elements, electrons ocupy the sub-shell as single before pairing begins
Both follow Hund's rules when doing simple electron filling. Mg has 12 electrons and is thus 1s2 2s2 2p6 3s2. I've never come across material that says Mg doesn't follow Hund's rules, but if it doesn't then I guess that's more to do with the behaviour of bulk metals rather a single Mg atom. And the chemistry of bulk and surface materials can be very complex and is not my specialty.
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Sara_Acton
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(Original post by Plantagenet Crown)
Both follow Hund's rules when doing simple electron filling. Mg has 12 electrons and is thus 1s2 2s2 2p6 3s2. I've never come across material that says Mg doesn't follow Hund's rules, but if it doesn't then I guess that's more to do with the behaviour of bulk metals rather a single Mg atom. And the chemistry of bulk and surface materials can be very complex and is not my specialty.
Yh That's why I struggled in this point as I see that all of the elements given do obey Hunds's rule but the teacher said Mg and K ,for example, don't
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