When you're looking for LEDs to blink (LED stands for "Light Emitting Diodes"), you run right into the topic of resistors, which gets into impedance and related issues.

A distracting detour from the thrill of lighting up an LED. You can figure out impedance later. All the time in the world. 

I advise skipping that hurdle altogether by choosing an LED that has the same power requirements as the Particle Photon: 3.5 volts. So no resistors required.

An added benefit: these LEDs are the brightest LEDs you can use out of the box. Why not go big?

Adafruit sells 25 of these super bright LEDs for $8.

And with Adafruit, you get your products with a side of content: a bunch of convenient explanations on how these things work.

I love this level: so simple, everything makes sense. Easy to get the concept and the tiny real-world exercise.

On the Internet:

• All About LEDs
• LED Origami