What does below chemistry rebus means?

Eight of us go fourth, not back, to protect our king from an attack. What are we?

How many bricks does it take to complete a building that is 15 feet wide, 30 feet deep, and 12 feet tall made of brick?

Do you know who was buried in John's tomb?

Until it is measured, not known. Yet how you miss it when it has gone. What is it?

Two guards were guarding the camp.Guard-1 was looking towards the south to make sure no threat is coming from the road.Guard-2 was looking at the north to make sure no threat is coming from the top. Suddenly Guard-1 ask the Guard-2 why he is smiling?How Guard-1 knows that Guard-2 is smiling?

I am a five-letter word and comprises two different vowels and three same consonants. I am so strong that I can spoil your entire work.

Can you find out who am I?

There are three boxes. One is labeled "APPLES" another is labeled "ORANGES". The last one is labeled "APPLES AND ORANGES". You know that each is labeled incorrectly. You may ask me to pick one fruit from one box which you choose.

How can you label the boxes correctly?

Can you complete the sequence puzzle?

AR TA GE CA LE VI LI _

I possess a head

I flaunt a tail

But I don't have mouth or legs

Can you find out who I am?