What does below chemistry rebus means?

I am Huge on Sunday and Saturday.
I am small from Tuesday to Thursday.
I do not exist on Monday and Friday.
Who am I?

You are given four tennis balls and asked to arrange those balls in a manner that the distance between each one of them is exactly equal. How will you do it?

I have one of the three numbers: 1, 2, or 3 in my mind. I speak only truth. You can ask me just one question for which I will only reply in yes or no or don't know. What question will you ask from me so that you are able to know the number?

What runs around the whole yard without moving?

Look at the below sequence of letters and tell us what the next letter should be:

O T T F F S S ?

What's the best place to go shopping in the ocean?

I am the black child of a white father, a wingless bird, flying even to the clouds of heaven. I give birth to tears of mourning in pupils that meet me, even though there is no cause for grief, and at once on my birth, I am dissolved into air. What am I?

Which Number is the odd one out?

A) 7145
B) 2716
C) 5321
D) 4135

Find the missing letter in the last circle logically in the given below picture.