Chemistry
Alkali Metals

Why are alkali metals considered to have the lowest ionization energy?

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10/03/2007

The Ionisation energy of an atom is the amount of energy (usually electrical) required for one mole of electrons to be taken away from one mole of the atom (in its gaseous form). Ionisation Energy is usually shortened to IE. As you should know, metals always form positive ions easily. So usually all metal have low ionisation energies; if doesn't take a lot of energy to ionise an atom that will probably end up losing electrons anyway. Atoms are usually stable when they have a complete out shell of electrons (if not complete, then half complete). So it is quite easy for the group 1 atom to lose one electron to become stable (in GCSE chemistry terms). For Advanced level chemistry pupils; you should know that the electrons are in rings (well...kind of) around the nucleus with electrons filling rings in an order depending on the ring number and its energy level. Electrons, as their number increases, should appear further away from the nucleus in their respective rings. What I mean is that some electrons will appear near the nucleus and some will appear a little further away. The distance of the electron from its nucleus relates to the ionisation energy. As any group 1 metal has just one electron in its outermost shell (which is further away than all the other electrons in previous shells), it is quite easy to pick it off. As you go across the row in the periodic table (across the period), the other metals should lower in ionisation energy because the distance of the last electron shouldn't have changed (its in the same shell, so it will orbit the nucleus like the ones before it). In fact the distance of that last electron to its nucleus has decreased (it got closer), making it harder for it to be removed. The reason it gets closer is because of the electrons using up the same shell, so it won't go further away. However, the force of positive attraction from the nucleus draws all the electrons in closer, which in turn makes it harder to pick off. I wrote this answer assuming you are studying chemistry at advanced level, so there may be some words you won't understand or some knowledge that you wouldn't have been taught, so if you find it misleading, confusing or hard to follow send a message and I'll try to re-word it for you. Hope I helped