Actually, you have hit on one of the rare cases when theory outpaced observation in spontaneous natural radioactive decay (as opposed to nuclear physics where this is almost always the case). In 1980 it was predicted that nuclei could in fact decay by other modes, namely cluster decay (i.e. emitting a particle heavier than an alpha, we will get to neutrons and protons is a second). This should not be confused with spontaneous fission, which produces neutrons in addition to two relatively equal daughters. But in both cases, this is basically a quantum tunneling phenomena of the particle through a barrier described by Gamow (of Alpher, Bethe, Gamow fame. The story goes that BETHE was only added to the author list for effect. :-) ) in 1928 by solving the Schrödinger equation with the nuclear potential (usually a modified Yakawa). Alpha decay is actually just a subset of the other modes of cluster decay (11 observed to date), many predicted. The branching ratio determines which mode of decay is most likely with the branches for the larger clusters being much smaller than the alpha branch in the 1e-5 to 1e-16 range. So by nature we observe the alphas most easily, and experimentally it is not always easy to differentiate multiple alphas (pileups) from the heavier clusters like C14. Quantum tunneling theory explains “well” the decay of heavy nuclei, using models like analytical superasymmetric fission and the liquid drop macroscopic-microscopic model. Now back to protons and neutrons, basically the issues is that for large nuclei to exist you need lots of the strong nuclear force to keep the electro-magnetic force (+protons) from repelling each other, and that means extra neutrons. The strong force can keep the nucleus intact as long as it is basically symmentric (round) because the strong force only acts over 2.5 femtometers but the Coulomb force acts over 1/r^2, so superdeformed nuclie can undergo spontaneous fission. These superdeformed states can be energetically possible due to high-spin states (adding inertia) and Coulomb repulsion in high-Z nuclei giving rise to asymmetries. In general all the longer lived natural heavy isotopes are extremely neutron rich, such that proton decay is highly unlikely, since you need a proton rich nuclei for that to be probabilistically favorable, which does happen (predicted in 1960 by Goldansky) in lutetium-151 and thulium-147, or double proton decay in iron-45. About 25 isotopes are proton emitters, essentially excited nuclei (beta-delayed). Pure neutron emitters are also rare, but exist, like beryllium-13 and helium-5. Be-13 is the basis for small neutron generators (AmBe or PuBe sources), as can be the isotope californium-252. But heavy nuclei are neutron rich in general so the tunneling probability favors alpha emission, spontaneous fission, and cluster emission over simple proton or neutron emission.

More on this theory can be found here (by the author of the 1980 predictive paper on cluster decay): Dorin N Poenaru, Walter Greiner (2011, Ed. C. Beck). Cluster Radioactivity, Chapter 1 in Clusters in Nuclei, Vol 1. Lecture Notes in Physics 818. Springer, Berlin. pp. 1–56.

http://books.google.com/books?id=7bI4S_07Yn0C&printsec=frontcover&dq=isbn:9783642138980&hl=en&sa=X&ei=pjHSUbzZCa3IywH2yIG4Bw&ved=0CC8Q6AEwAA#v=onepage&q&f=false

Theory of cluster decay: Sandulescu, A., Poenaru, D. N. and Greiner W. "New type of decay of heavy nuclei intermediate between fission and alpha-decay". Sov. J. Part. Nucl. 11, pp. 528–541 (1980)

Nature paper, discovery of cluster decay: Rose, H. J. and Jones, G. A., "A new kind of natural radioactivity", Nature 307 (5948), pp. 245–247 (1984) http://www.nature.com/nature/journal/v307/n5948/pdf/307245a0.pdf

Thank you sir for your elaborate answer.

Since mankind started wondering around and searching to explore the universe, two essential questions have been the challenge:

What are things made of? And how do things work?

After the discovery of the radioactivity by Becquerel and the achievement of Curie in separating the radioactive material, the first understanding was achieved by Rutherford by proposing that the atom consists of a massive, positively charged nucleus. When Chadwick discovered the neutron, Heisenberg in his hypothesis, stated that nuclei consist of protons and neutrons. Later, experimental attempts were made to understand the nuclear forces and to determine the nuclear properties, and to study the nuclear structure. Nuclear theory also has been through great developments in order to establish a complete unified model which could describe the nucleus. Since nucleus can not be treated statistically and there is no central force which can dominate the system and then we treat the forces between nucleons as small perturbations. Therefore nuclear physicists have recourse to the method of nuclear model which resembles a nucleus. In this way the nucleus has been treated ‘as if‘, it is gas, a liquid drop, and several other things. No single model ca account for all the known facts about nuclei. We have to mention the shell model and the magic numbers which give us the idea about the stability, the spherical shape of nuclei and the classification of the single particle states. The collective nuclear motion led to the theory of nuclear deformation which based on the liquid- drop model.

The spherical equilibrium shape for nuclei for nuclei with a few nucleons outside closed shells may be due to the pairing force, and the collective motion in this case is vibrational.

As the number of valence nucleons increases, the quadrupole interaction of the extra nucleons may become important, and the spherical shape becomes unstable and a permanent deformation is required. These two motions were unified by Nilsson model, in which shell model orbits are setup in a deformed potential which is semi- empirical. However later further models have been proposed to overcome some of the shortages in explanations such as PPQ, DDM and Interacting boson model IBM. In spite that the first two models are geometrical and third is phenomenological, still the main contributors to the shape and most of the dynamic properties of the nuclei are due to the valance nucleons which are outside the closed shell, by which can the nuclei be stable, spherical if they are closed shell or with few nucleons outside the shell. But when this number is increase the nuclei are heading to have deformed shape and no longer is their excitation vibrational, it goes to be rotational.

The regions of deformation are their almost in the mid shells, and we have two important regions, the rare earth nuclei and the actinides.

In the semi empirical mass formula of nuclei, we have many terms: volume term, surface term, pairing term, coulomb term and shell effect term. These terms not only governor the binding energy of the nuclei but also influencing some other properties. Gathering these ideas and remembering that the construction of nuclei is not numerical collection of nucleons but a complicated combinations of interacting forces, so then no longer the proton or the neuron is free to come in or go out with small fee of energy. I do believe that in these heavy nuclei were the alpha decay mostly take place, the main parameter is the shape and the distribution of the charge and matter and most likely the phenomena of liquid drop is proper image of the process which is just like the way of forming the fission process when the U-235 capture thermal neutron, and the shape and many other properties were no longer in equilibrium, and the effect of the short range saturated nuclear force can not keep the nuclei stable, there the surface tension , electrostatic , volume and pairing forces will rearrange the system by the separation of particles which have to obey the same concepts of the semi empirical formula of mass, since alpha itself stable nucleus with pairing effect closed shells in proton and neutron. Further explanation can be approached by remembering the cluster model it seems that in heavy nuclei particles may arrange in the form of clusters and there are models which treat the nuclear structure in this way i.e Cluster model and the nuclear vibron model.

However, some time and/or always the alpha decay will followed by beta or positron decay, why is that? It is self reforming to achieve great stability and equilibrium in charge and matter distribution. You see again nuclei can not just ejecting proton or neutron to get rest. When there are more protons, one of them is changed to neutron and ejecting positron, and when there are more neutrons, one of them is changed to proton and ejecting electron. Here the role of what we call it the weak force is governing the beta decays.

I hope by this lengthy argument, some reasonable ideas have been gathered to explain the subject.

In Heavy nucleous alpha particle is pre-formed,during the formation of alpha particle high KE(BE of alpha particle is less than its constituent nucleons) is released.This KE is sufficient to escape from the Nucleous.where as For Neutron and Protan we have to supply KE from out side the nucleous,which is not possible.Hence Nuclides with higher mass no decay through alpha and not through neutron or protan.

Why alpha decay of higher mass no? It has been observed that there are neutron:proton ratios that result in more stable nuclides. When the ratio n/p is not optimal, the nuclide decays so that its n/p ratio tries improve that ratio to be in the stable zone. If the number of protons is too high compared with the number of neutrons (n/p ratio not good), the nuclide tries to correct that by loosing an alpha particle (equivalent to 2 protons in charge) so that the ratio neutron:proton improve so that the nuclide enters in the stability zone.