Advanced electron shells
Electron shells and valency
Earlier the electron shell model, a three-dimensional way of depicting the electrons surrounding the nucleus in an atom was introduced. The number and arrangement of electrons determines the behaviour of an atom. The positive charge of the protons in the nucleus attracts the negative charge of the surrounding electrons. The strength of this attraction depends on the distance of a shell from the nucleus. Electrons in the same shell are of a similar distance from the nucleus and possess about the same level of attraction to the nucleus.
Electrons of the outer shell are called valence electrons and that valency is an atom's ability to lose, gain or share electrons. An element's valency corresponds with its stability and also its reactivity - its ability to form compounds with other elements.
See animation 1.
Elemental properties at a sub-atomic level
Group I elements have one electron on their outer shell. This electron is outside a filled shell, which weakens the pull of the protons in the nucleus. Group I elements are prone to losing their lone valence electron and are therefore highly reactive. See image 1.
The halogens of group VII are also highly reactive, although they possess seven electrons on their outer shell. The halogens try to fill their outer shells to become stable and so they react strongly with other elements to try and gain the extra electron. See image 2.
With its single electron, hydrogen sometimes acts as a group I element during reactions. Occasionally, because hydrogen also needs only one more electron to fill its shell to capacity, it acts as a group VII element.
Transition metals have up to two electrons in their outermost shell, which means their valency varies from element to element. They also fill their other shells in an irregular manner, which makes their behaviour hard to predict in chemical reactions. Copper and gold both have one electron on their outer shell and occupy the same column on the periodic table, but copper reacts easily while gold is comparably stable.
Most metals occupy the first few groups of the periodic table, indicating that they have one, two or three valence electrons. They participate in ionic reactions and tend to give away their electrons to form positive cations.
Non-metals have more valence electrons than metals and tend to gain electrons in order to stabilise their outer shells, forming negative anions in an ionic chemical reaction. Non-metals also bond covalently, where both elements stabilise their outer shells by sharing electrons.
The properties of group IV elements vary. Carbon is the only non-metal in the group and tends to form covalent molecules with itself and other elements. The elements become more metallic as the atomic number increases down the group, therefore elements like tin and lead tend to donate electrons to form cations in chemical reactions. Group IV elements have valencies of ±4 to reflect this variation and it is necessary to be careful about the valency when balancing equations involving these elements.
Different atomic models
For every rule in chemistry there is an exception. This is because chemists still do not know everything there is to know about atomic behaviour and much of what is learnt at this level is introductory.
Quantum mechanics is the study of the behaviour of matter, in particular sub-atomic particles, fusing chemistry and physics into one branch of science. For more complicated models of atoms beyond the electron shell model, quantum mechanics will explain more about atomic structure.
