0620 · 8.5Noble gases

Full outer shells and reactivity

0:00Many elements react by forming bonds with other atoms. Noble gases are different because their atoms already have a particularly stable electronic configuration. They usually remain as separate atoms rather than joining together. By the end of this video, you will understand how their electrons explain both their low reactivity and their monatomic nature. The noble gases occupy Group VIII, written with Roman numerals in the Cambridge syllabus. Helium, neon and argon are examples of this family.

0:26They are unreactive, monatomic gases. Each part of that description matters, but the explanation begins with the arrangement of electrons inside their atoms. Electrons occupy shells around the nucleus. An electronic configuration tells us how those electrons are distributed between the shells. The outer shell is especially important because its electrons are involved in chemical bonding. Atoms of many elements lose, gain or share electrons when they react. Helium has two electrons, both in its first shell.

0:54That shell is full because the first shell can hold only two electrons. Neon has the electronic configuration two, eight, so its outer shell contains eight electrons. In argon the configuration is two, eight, eight, so its outermost shell holds eight as well. The important similarity is not that these atoms have the same total number of electrons. Instead, each has a full outer shell. Helium achieves this with two electrons, while neon and argon have eight in their outermost shell.

1:20This gives each atom a stable electronic configuration. A full outer shell means these atoms do not need to lose, gain or share electrons to achieve a stable arrangement. They therefore have little tendency to form chemical bonds under ordinary conditions. This is why noble gases are described as unreactive. Their electron arrangement explains their behaviour rather than merely accompanying it. You may hear that atoms want a full outer shell, but atoms do not make decisions.

1:47That phrase is only shorthand for the stability of particular electron arrangements. Noble gases already have this stable configuration. For an explanation, connect the full outer shell directly to the lack of a need to lose, gain or share electrons.

Monatomic gases and argon

2:00Monatomic means existing as single atoms rather than molecules. In a sample of neon gas, each particle is a single atom. Because its atoms do not normally share electrons to form bonds, they remain separate. The same description applies to helium and argon. Monatomic does not mean that an entire container holds only one atom. A sample contains many atoms, but they are not chemically bonded together. The word gas describes the physical state, with particles far apart and moving randomly.

2:27Monatomic tells us what each particle consists of, so the two words refer to different features. Consider a neutral atom with eighteen electrons. We need to determine its electronic configuration and explain whether it has the characteristic behaviour of a noble gas. First, place two electrons in the first shell, leaving sixteen. Next, place eight in the second shell, leaving eight for the third. The resulting configuration is two, eight, eight, which identifies this atom as argon.

2:55Its outer shell is full, so it has a stable electronic configuration. It does not need to lose, gain or share electrons, making it unreactive under ordinary conditions. Argon therefore exists as separate atoms and is described as monatomic.

Common mistakes and exam answers

3:09A common mistake is to say that every noble gas has eight outer electrons. Helium shows why that statement is wrong, because its only shell is full with two. Another mistake is to say that noble gases have no electrons available. They do have electrons, but their full outer shells explain their low reactivity. An exam may ask you to describe the noble gases and explain their lack of reactivity in terms of electronic configuration.

3:31Describe means give their characteristics, while explain requires the reason behind them. A suitable answer is that noble gases are unreactive, monatomic gases. Their atoms have a full outer shell of electrons. They do not need to lose, gain or share electrons. The key connection is between electronic configuration and chemical behaviour. A full outer shell gives noble gas atoms a stable arrangement, explaining why they are unreactive. Their particles are individual atoms, which is what monatomic means.

3:59Remember that helium has two electrons, neon has two, eight and argon has two, eight, eight.