Lower Secondary Science

[Dr.033430Daniel]

Verysuperkiasu:

Ds has a chemistry question.....


He said he knows how to get valency of simple ions - like potassium K has a valency of 1+ due to its electronic configuration of 2.8.1 hence need to get rid of 1 electron.

But he doesn't know how the valency of polyatomic ions like CO3 and NH3, etc are derived. In my sec sch days, I just memorized the valencies of these polyatomic ions. Now when my ds asks me, I don't know how to explain.

Anyone can help to explain?

Sometimes the easiest thing is to memorize the charges on these ions, for quick recollection. But it is logical how they form and its good to be able to understand this. Here is an explanation.

See if he can make sense out of why the CO2 molecule forms. Carbon has 4 valence electrons and would like to share 4 more to get to 8. Each Oxygen has 6 valence electrons and would like to share 2 more to get to 8. So when the Carbon shares 2 electrons with each oxygen everybody is happy, they all have 8.

Now Carbonate or CO3 has a third oxygen atom. This third Oxygen atom needs to find two more electrons to get to a valency of 8. So what happens is the CO3 molecule can steal 2 electrons from another source to compensate for the need of the third Oxygen atom to have two more electrons. When it does this it becomes CO3 with a 2- charge.

For Nitrate or NO3, the Nitrogen has one more valence electron than Carbon, so NO3 needs to steal one less electron so that all the atoms in the molecule have 8 shared electrons. So the NO3 ion has a -1 charge.

Another easy one to understand is OH- or Hydroxide. Oxygen has 6 valence electrons and would like to share two more to get to 8. So H2O is formed for that reason. The Oxygen shares one electron from each Hydrogen. With only one hydrogen attached to an Oxygen, the molecule steals one more electron, so it becomes OH with a -1 charge.

The understanding of Sulfate and Phosphate is a bit more difficult, but if you follow similar logic you will get there.

[Verysuperkiasu]

Ah…so! Now I see it. Will try to explain this to my ds. Thanks so much, Dr Daniel, for your comprehensive and detailed explanation. Really appreciate that!!


Just one more question…Carbon has an a valence electron of 4. So, can we say carbon is only found in covalent molecules? That is, it can’t transfer electrons like metals can to form positive ions? And carbon can only share electrons, right?

[Dr.033430Daniel]

Carbon mostly forms covalent bonds. The terms covalent and ionic really are relative terms. Most bonds are somewhere in between equal sharing of electrons between two atoms and electrons completely taken from one atom. Later on in chemistry they will introduce the concept of electronegativity which gets into the degree to which an atom or molecule attracts electrons to itself.


But in chemistry you do sometimes see carbon involved in ionic bonding. The compound Calcium Carbide or CaC2 is usually described as having ionic bonds within it.

But the main point about Carbon that is usually stressed in Lower Secondary is that it is very well suited to form covalent bonds because it has exactly half of the 8 valence electrons. Atoms that have one extra electron have a strong desire to give one away. Atoms that are one short have a strong desire to gain one. Carbon and Silicon, in the middle, are best suited for sharing electrons.

Happy NEW YEAR !! - Hope everyone has a good 2012 !
We can toast the new year with C2H6O

[Verysuperkiasu]

Dr.Daniel:

But the main point about Carbon that is usually stressed in Lower Secondary is that it is very well suited to form covalent bonds because it has exactly half of the 8 valence electrons. Atoms that have one extra electron have a strong desire to give one away. Atoms that are one short have a strong desire to gain one. Carbon and Silicon, in the middle, are best suited for sharing electrons.

Happy NEW YEAR !! - Hope everyone has a good 2012 !
We can toast the new year with C2H6O

:thankyou:
Happy a great year ahead too.. :rahrah:

[Koky462]

My DS has a physics question. It is on the topic of light.


Converging lens A & B have the same focal length, but B is only half the diameter of A. Both lenses are used to form the images of distant objects on a screen. Which of the statement is correct?

A. For both lenses, the distances from the lens to the screen is the same.
B. For both lenses, the images are of the same brightness.
C. For both lenses, the images are the same size as the object.
D. Lens B gives a smaller image than A.

The correct answer is C, but my DS still do not understand. May anyone enlighten us?

[mum_sugoku]

Koky462:

My DS has a physics question. It is on the topic of light.


Converging lens A & B have the same focal length, but B is only half the diameter of A. Both lenses are used to form the images of distant objects on a screen. Which of the statement is correct?

A. For both lenses, the distances from the lens to the screen is the same.
B. For both lenses, the images are of the same brightness.
C. For both lenses, the images are the same size as the object.
D. Lens B gives a smaller image than A.

The correct answer is C, but my DS still do not understand. May anyone enlighten us?

Think the correct answer is wrong. Should be A instead.

[mum_sugoku]

Dr.Daniel:

Yes, C is correct. It is the curvature of a lens that determines the magnification. Just take a magnifying glass and cover half of it. When you look through, you still see the same magnification. But not as much light makes it through. So the image from a full lens will be brighter than an image from half of the lens. This might not be very apparent at normal lighting conditions. But the magnification depends on how much the light bends – and that is due to the curvature of the lens, not how big the lens is.


With telescopes you can have two telescopes that have the same curvature to the lens, or the same magnification. But one with a big lens collects more light and you can generally see a sharper image. I was out looking at the lunar eclipse with the Singapore astronomers last month. One of the best views was a guy who had a huge set of binoculars. The magnification wasn’t that much but the image was very clear because he was collecting so much light due to the size of the lenses.

At O-Level there are lots of problems where you move an object closer to and further from a lens and the image focal point changes and so does the magnification. Doing those kind of problems (and some experiments with lenses) help students get a feel for it.

Agree that it is the curvature of a lens that determines the magnification.. However, did you notice that the question already specified that both A & B have the same focal length?

My understanding is that for image to have exactly the same size as the object, the object should be positioned at exactly 2x the focal length (2F) away from the (centre of) lens.. However, for this particular question, we are looking at images formed by distant objects (>>2F away), and based on what I know, image formed by distant object should be smaller (ie diminished) - instead of of the same size as object - and with an image distance (location where you can get a sharp image) at exactly 1F away from the lens..

Which is why I believe that C is incorrect and it should be A instead lor.

[Dr.033430Daniel]

Oh yes - good point. You are right - A looks like the right choice.


It says the images are the same size as the object, not necessarily true. Its true that the images are the same size as each other, but not the same size as the object.

Thanks

[mum_sugoku]

Dr.Daniel:

Oh yes - good point. You are right - A looks like the right choice.


It says the images are the same size as the object, not necessarily true. Its true that the images are the same size as each other, but not the same size as the object.

Thanks

:oops: actually, I had made exactly the same mistake as you - failed to notice that \"same focal length\" thingy and had also thought \"C\" was the right answers :slapshead: . (Alas nowadays, some questions can be so tricky that we'll have to read and re-read before we could correctly figure out what they are asking ya )

[Koky462]

Thanks everyone for contributing their replies!


My doubts are cleared for now!