Most commonly, we start off by learning about electrons as being negative particles that speed around a positive nucleus in an erratic manner - we say that they form an 'electron cloud' around the nucleus. This is often depicted like the image below, with the more saturated colour near the nucleus indicating the increased likelihood of finding an electron there at any one point in time.
We know from various observations that electrons exist in distinct energy levels in an atom. We can depict these energy levels as spherical shells, and it's common to say that electrons orbit the nucleus in these shells. Each shell is numbered off, starting from the inside out.
What you may not realise is that each shell can also be referred to by a letter. For example:
- the first shell (n=1) is also known as the K shell,
- the second shell (n=2) is also known as the L shell,
- the third shell (n=3) is also known as the M shell,
- the fourth shell (n=4) is also known as the N shell.