second led to the blink led circuit

hello, i am beginner in arduino. i have knowledge in electric circuit and associated parts. my question is when i want to connect second led as serial to existing led , leds dont illuminate. could you please explain why they dont work? do i need to reduce ohm? here is my circuit subparts:

thanks,

1 ea resistor 333ohm,
1 ea led
5V digital output from ardunio

The output on Arduino pins has limits of 40 mA.
You need to always consider that aspect when hooking anything up to an Arduino.

Even some LEDS (unknown type from your post) can draw more than an Arduino can supply.

Might I suggest you read THIS POST and add any additional info.

Bob.