When we talk about electron configuration, we are essentially referring to the arrangement of electrons in an atom's electron shells or energy levels. It is important to note that electrons occupy these energy levels in a specific order. The filling order is determined by the Aufbau principle, which states that lower energy levels are filled before higher energy levels.

Let's take the element oxygen (O) as an example. Oxygen has 8 electrons in total. The first energy level, closest to the nucleus, can hold a maximum of 2 electrons. The second energy level can hold up to 8 electrons. Following the Aufbau principle, we place 2 electrons in the first energy level and the remaining 6 electrons in the second energy level of oxygen.

To represent the electron configuration, we use a shorthand notation called the noble gas notation. This notation involves using the symbol of the noble gas that comes before the element in the periodic table, followed by the additional electron configuration. For oxygen, the noble gas before it is helium (He), which has an electron configuration of 1s². Thus, the noble gas notation for oxygen becomes [He] 2s² 2p⁴.