# What kind of sequence is this?

1

1 1

2 1

2 1 1 8

2 1 2 8

?

Hint 1:

See the new tag

Hint 2:

Look and say with Periodic Table

Hint 3

Fourth is Hydrogen Peroxide

• I always like sequences that start like other sequences and then veer off dramatically Oct 12, 2021 at 13:34
• For me a sequence is a one dimensional array of numbers. Here I see some numbers arranged in a two-dimensional-ish grid and a single 1 lying outside the grid. This is very unconventional sequence ... Oct 12, 2021 at 14:28
• One number is out of the blockquote. On purpose? Oct 12, 2021 at 14:37
• Not particularly. Once the answer is in it will make sense.
– DrD
Oct 12, 2021 at 19:36
• The only thought i had i put in answer. Any new hints? Nov 3, 2021 at 7:08

I think the next in the sequence is:

Trioxidane (H2O3)

The first line 1 1 should be read as:

1 atom of element 1 (i.e., Hydrogen)

Then the second line 2 1 is read as:

1 atoms of element 1 = H2 (Hydrogen Gas)

For 2 1 1 8 we get:

2 element 1s and 1 element 8 = H20 (Water)

Then we progress to 2 1 2 8:

2 element 1s and 2 element 8s = H202 (Hydrogen Peroxide)

If we kept extending we would:

follow H2O3 with H2O4, then H2O5. Following the Hydrogen polyoxides in other words.

• I still don't understand what this sequence is. I mean I can see how it extends from the third term but why are the first two terms there? Amoz also seems to have made this point in the comments. Dec 7, 2021 at 17:00

It looks like next row should be

2 1 3 8

Explanation:

These are electron count of the elements in periodic table. Hydrogen have 1 electron, Helium have 2. Then new period starts. It build on helium (2 electrons in first orbit), and have 1,2,3...8 in second.

So next ones will be

2 1 3 8
2 1 4 8
2 1 5 8
and so on till 2 1 8 8

• I had a very similar thought about this, but why have '1' in the second position and not '2' in the fourth, or '8' in the fourth position and not '2' in the second...?
– Stiv
Oct 24, 2021 at 9:35