Perhaps my thread title is a little confusing, but here is my actual question. (It is a bit long though, sorry! ) A single atom of flourine has 9 protons and 9 electrons. The number of electrons is exactly balanced by the number of protons. But flourine has a huge tendency of gaining one electron. A single atom of sodium has 11 protons and 11 electrons. Number of electrons is equal to number of protons here too. In this case, the atom has a huge tendency to lose an electron. A single atom of argon has 10 electrons and 10 protons. The number of electrons here is also equal to number of protons. But argon has almost zero tendency to gain/lose electron. Why is this so? The most common answer is : Atoms follow the octet rule. To complete their octet, they do these things. We know that noble gases have eight electrons in their outermost shell, so we assume that all other elements try to attain this stability by gaining or losing electrons. But helium is also a noble gas despite not having an octet. So octet rule cannot be the right answer. So, we can modify the octet 'rule' to say that: whenever an atom has a single electron in its orbital, it has a tendency to get a fully filled orbital. Especially where the outermost orbital is half filled, the atom is highly reactive (or unstable). It tries to gain or lose one electron so that the orbital has two electrons, so as to achieve stability. Whether it will lose or gain the electron to attain stability depends on the distance of electron from the nucleus i.e. if it is far away, it will prefer losing it (metallic nature), otherwise it will prefer gaining it (non metals). It also depends on the number of electrons required. For example, nitrogen needs 3 electrons to fill-up all its three half-filled orbitals (1 electron in each orbital). But 7+3=10 electrons cannot be attracted by 7 protons, therefore it shares electrons with other atoms. Why does a singly-filled outermost orbital make the atom so eager to gain/lose electrons? Because the atom has an opportunity to have its energy still more lowered if its outermost orbital is fully filled (i.e. it contains two electrons). But exactly how/why does the energy of an atom decrease if its orbital is fully filled? Secondly, in case of non metals, where does the attraction to hold the extra electron(s) gained come from(since n number of protons are already holding n number of electrons)? My own guess for first question: The energy possessed by an electron is equal to the work done on it by the nucleus. Suppose the nucleus did x joule work on an electron, then this energy of electron (-x joule) manifests itself in the form of angular momentum (both due to its 'revolution' around the nucleus <speaking in classical terms>, as well as its spin on its axis). When another electron with opposite spin is introduced in the orbital, it cancels the ang.moment. due to spin of the other electron, thus lowering the energy of the atom. But I don't know how the nucleus attracts the electron it has gained to attain this stability, unless it is due to the fact that the size of atom decreases with increase in non-metallic character (as we move along a period), thus decreasing the distance between nucleus and the electron to be gained, resulting in stronger attraction on the electron by the nucleus. If this is true, then the opposite is true for the metals. That is, very weak attraction exists between valence electron(s) of a metal atom and its nucleus, resulting in easy loss of the valence electron, which leads to higher stability. But one may ask a question here: if a metal/non metal is more stable in its ionic form, why does it exist as an atom in the first place. My answer here would be that a non metal rarely exists in its monoatomic form. It is always in search of an electron to become more stable. Even if a mono-atom is prepared artificially, it immediately reacts with an atom(s) of its own kind or of different kind to form diatomic molecules or ionic/covalent compounds. Similarly, suppose we have a metal atom. For this atom, losing its electron will make it stabler. But the problem is that if it loses its electron, it immediately develops a positive charge, which starts attracting nearby negative charges. Thus, the atom cannot leave its electron even if it wants to. It can only do so, if there is some external charge to keep the electron from coming back to the ion (for example non metals like chlorine, flourine etc.). So a metallic ion in a salt (like NaCl) is in its most stable form. Am I right? I will be grateful for your replies. Thanks.