For heavier nuclei, the number of neutrons exceeds the number of protons in stable nuclei. Whats the reasons?
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For all elements found in nature, we can identify mainly 3 components: a nucleus formed by protons and neutrons (both of them nucleons), and a core of electrons orbiting around these nucleons. As you may know, protons have a positive electric charge, electrons have negative charge and neutrons are electrically neutral.\n\nThe thing here is that protons, neutrons and electrons are different types of particles: protons and neutrons are made out of quarks, making them susceptible to the influence of the Strong force, the Weak force and the Electromagnetic force; in the other hand, electrons only respond to the Electromagnetic force and the Weak force (Of course, Gravity is always present for all of them).\n\nThe Strong force, compared to the Electromagnetic, acts with a much higher strength; however, it is a very short-ranged force: it's effects fall much more rapidly with increasing distance compared to EM interactions.\n\nIn any type of nuclei, protons are all "put" together within very small distances. Because of these small distances, the Electromagnetic force reaches very high magnitudes, making these particles tending to fly away from each other. However, because of this small distances, the Strong force plays a major roll keeping these particles together. Neutrons also contribute to the total strong interaction, increasing the total magnitude of this attractive type of forces among the nucleons. For light nuclei, the ratio of protons to neutrons is about 1:1. This ratio keeps balanced the attractive forces against the EM repulsion, giving us a stable nucleus. \n\nWhen we have heavy nuclei, just putting the protons together creates a nucleus that is larger in size than the effective range of the Strong force; in other words, the strong interaction between two protons in opposite sides of the nucleus is much weaker compared with the Electromagnetic repulsion. Thus, a lot of neutrons have to be added to this nucleus, in order to build up an attractive force that would keep protons in the outer sections of the nucleus in place. \n\nThat's why, in order to get a stable heavy nucleus we need many more neutrons to get over the effect of Electromagnetic repulsion, compared to lighter nuclei.\n\nI hope it helps.
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The assembled nucleus has less energy than the collective nucleons. If it did not\n\na) it would not hold together with the collection of + charged protons, and\n\nb) it would not require added energy in the form of colliding particles to make it come apart.
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When the nucei of elements form, they are more or less stable combinations of protons and neutrons.  Unstable groupings decay usually in microseconds, or dont form at all.  For any given element, there is usually more than one stable number of neutrons for any given number of protons (the number of protons determine which element it is, the total number of protons and neutrons determine the molecular weight, and thus the isotope).  Often, one type of isotope is much much more stable or more energetically easy to form than others.  So, for example, usually oxygen has 8 protons and 8 neutrons-but sometimes it has 9 neutrons.  During the original formation process, which for larger elements usually occur during a violent nova or supernova explosion of a star, there are lots of free protons and neutrons, or naked nuclei of smaller atoms, being energetically smashed together.  So, just by chance, most will be the most stable or most easly formed nuclei, but a few will be slightly different.
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Isotopes are atoms with the same amount of protons, but different numbers of neutrons.
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This is because the nuclear attraction over the electrons increases.\nRemember Coulombs law of electro_nuclear forces.
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B: Some mass is converted into energy in order to split the unstable nucleus into two, smaller, stable ones.\n\nC: Their nuclei are too big to remain in one lump of mass. Particles (alpha, beta, gamma, etc) start shooting off in all directions, to bring the nucleus down to a smaler size. It's all about the smaller size.\nAlso, no atom can have more than one nuclues, because then it would be two atoms. More or less elctrons make an atom into a charged ion, not an isotope. Neutrons are what make isotopes.
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The atomic number is equal to the number of protons.\nThe mass number is equal to protons plus neutrons.\n\nThe difference between those two numbers would equal the number of neutrons.
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The total number of protons in a water molecule is fixed. It has 10 protons.\nHowever, because there are many isotopes of hydrogen and oxygen, so the total number of neutrons in a water molecule is not fixed. For the water molecule formed by protium (no neutrons) and oxygen-16(eight neutrons), it has only 8 neutrons. For the water molecule formed by deuterium (one neutron) and oxygen-16, it has 9 neutrons. For the water molecule formed by tritium (two neutrons) and oxygen-16, it has 10 neutrons.
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The Strong Nuclear Force is the force that binds the protons and neutrons in the nucleus together.  Otherwise the protons would be likely to repel each other away.  The Strong Nuclear Force has a very limited range which is less than the length of an atomic nucleus
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A carbon atom is one that has 6 proton in the nucleus. If it is uncharged, there will also be 6 electrons. The number of neutrons varies depending on the isotope. The most common isotope has 6 neutrons. There is another stable isotope with 7 neutrons and a radioactive one with 8.
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Well that's not exactly correct: Atomic radius decreases in the same period of the periodic table as we go from left to right, so it decreases even though the mass number increases. That is because the heavier atom has more protons in nucleus that exert stronger forces to the electrons of the outer shell.\n\nBut the radius increases in the same group as we go from top to bottom, so a heavier atom has a bigger radius.
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The strong force holds quarks together in combinations that make protons and neutrons.  It also holds the protons and neutrons together, but in a more subtle way, because protons and neutrons are color neutral.  This means that protons and neutrons don't have excess color charge (which determines the strength of the strong interaction), and by this logic shouldnt be attracted to each other.  The quarks are not in a point-like configuration, though, so each nucleon has a "dipole" and higher order moments. If you've never heard of this kind of thing, it's the same idea as the electric force holding electrically neutral atoms together to make chemical compounds.\n\nThe differences in mass between atoms and the sum of their particles is a consequence of the strong force... the strong force is not caused by the mass difference.  In fact, atoms above iron have a mass which is larger than the sum of their nucleons, while atoms below iron have mass that is smaller than the the sum of their nucleons.  That's why you can only break large atoms for energetic nuclear fission.
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The atomic number is representative of the number of protons in the nucleus.  The atomic mass is representative of the number of total particles in the nucleus, which is made up solely of protons and neutrons.  That means that (35 total particles)-(17 protons)=18 neutrons.
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Protons and neutrons.
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Protons and neutrons build a nucleus.\nThe number of protons determines the kind of element.\nThe number of protons and neutrons determines the mass of an atom. The electrons outside the nucleus hardly contribute to the mass.\nThe number of protons equals the number of electrons. That is why an atom is electrically neutral.
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They are unstable because they can not hold together, they have too many or too few neutrons.\n\nThe are called radio-isotopes.\n\nThe disintegrate by emitting a particle or an atom into another element.  The new element may be a radio-isotope too.
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Simple:\n\nN-16  - beta- ==========> O-16\n\nIn releasing a beta particle, one of the neutrons in the nucleus changes to a proton.  That moves the atomic number up by one, changing the element from nitrogen to oxygen, but does not effect the atomic weight, which remains 16.  So Nitrogen 16 decays into the stable form of Oxygen 16.\n\nEasy, no?