number of valence electrons periodic table

The reason your rule begins to break down at krypton is because the filling of the d subshell adds an extra 10 electrons to the core electrons. Krypton's valence electrons are the 4s and 4p electrons (again in total eight) which means the remaining 28 electrons are core electrons. So your rule does work, but only for noble gases up to argon.

Neon's valence electrons are the 2s and 2p electrons (in total eight) and the two 1s electrons in the lower shell are the core electrons.Īrgon also has eight valence electrons, but this time the 3s and 3p electrons meaning the remaining ten in the lower first and seconds shells are core electrons. There isn't a shell below the first electron shell so it has no core electrons. All other electrons would be considered core electrons.įor helium it only has two 1s electrons which constitute the valence shell. For noble gases the valence electrons are the s and p electrons of the highest electron shell. The core electrons for any atom are all the electrons of the atom which aren't valence electrons. Hope that starts to clarify things a little more. Electrons are always partially in the nucleus. All electron states overlap with the nucleus, so the concept of an electron 'crashing into' the nucleus does not really make sense. The states with more energy are more spread out. An electron in an atom spreads out according to its energy. In fact, electrons in the s orbitals tend to peak at the nucleus. And using this understanding of the electron, electrons in the atom do enter the nucleus. Under quantum mechanics the electron is a quantized wave function which occupies certain probable regions around the nucleus called orbitals. To accurately describe the nature of electrons, we must resort to quantum mechanics. This is using classical mechanics to describe electrons when in reality they behave much more differently. So the issue with thinking electrons could crash into the nucleus is that it assumes electrons are solid particles orbiting the nucleus according to Coulomb's Law much like how planets orbit the sun because of gravity.

Number of valence electrons periodic table