GROUP 4 Firdiana Sanjaya ( ) Ana Alina ( )

The Nucleus  The nucleus of an atom is comprised of protons and neutrons; it is therefore positively charged. The number of protons within the nucleus of a given atom is equal to the atomic number of the corresponding element, which can be found on the periodic table. For example, the atomic number of helium is two. Therefore, the number of protons is also two. The number of neutrons within the nucleus of a given atom can be found by subtracting the atomic number from the atomic mass

The mass number is the sum of protons and neutrons. Atomic Mass Number = Number of Protons + Number of Neutrons To find the number of neutrons, subtract the atomic number, the number of protons, from the mass number. Notation of a specific element follows this format: where E is a specific element, A is mass number, Z is the atomic number, and C is the charge. For helium, the notation is as follows: Helium has 2 protons, 2 neutrons and a charge of zero.

 How big is a nucleus? We know that atoms are a few angstroms, but most of the atom is empty space. The nucleus is much smaller than the atom, and is typically a few femtometers. The nucleus can be thought of as a bunch of balls (the protons and neutrons) packed into a sphere, with the radius of the sphere being approximately:

Problem Complete the chart below

Answer

The strong nuclear force What holds the nucleus together? The nucleus is tiny, so the protons are all very close together. The gravitational force attracting them to each other is much smaller than the electric force repelling them, so there must be another force keeping them together. This other force is known as the strong nuclear force; it works only at small distances. The strong nuclear force is a very strong attractive force for protons and neutrons separated by a few femtometers, but is basically negligible for larger distances.

 The tug-of-war between the attractive force of the strong nuclear force and the repulsive electrostatic force between protons has interesting implications for the stability of a nucleus. Atoms with very low atomic numbers have about the same number of neutrons and protons; as Z gets larger, however, stable nuclei will have more neutrons than protons. Eventually, a point is reached beyond which there are no stable nuclei: the bismuth nucleus with 83 protons and 126 neutrons is the largest stable nucleus. Nuclei with more than 83 protons are all unstable, and will eventually break up into smaller pieces; this is known as radioactivity.

Nuclear binding energy and the mass defect A neutron has a slightly larger mass than the proton. These are often given in terms of an atomic mass unit, where one atomic mass unit (u) is defined as 1/12th the mass of a carbon-12 atom.

The mass of the nucleus is a little less than the mass of the individual neutrons and protons. This missing mass is known as the mass defect, and is essentially the equivalent mass of the binding energy. Einstein's famous equation relates energy and mass: In any nucleus there is some binding energy, the energy you would need to put in to split the nucleus into individual protons and neutrons. To find the binding energy, then, all you need to do is to add up the mass of the individual protons and neutrons and subtract the mass of the nucleus: The binding energy is then:

Radioactive decay Radioactive decay, also known as nuclear decay or radioactivity, is the process by which a nucleus of an unstable atom loses energy by emitting ionizing radiation During radioactive decay, principles of conservation apply. Some of these we've looked at already, but the last is a new one:  conservation of energy  conservation of momentum (linear and angular)  conservation of charge  conservation of nucleon number

Alpha decay  In alpha decay, the nucleus emits an alpha particle; an alpha particle is essentially a helium nucleus, so it's a group of two protons and two neutrons. A helium nucleus is very stable.  An example of an alpha decay involves uranium-238:  The process of transforming one element to another is known as transmutation.  Alpha particles do not travel far in air before being absorbed; this makes them very safe for use in smoke detectors, a common household item.

 Beta decay  A beta particle is often an electron, but can also be a positron, a positively- charged particle that is the anti-matter equivalent of the electron. If an electron is involved, the number of neutrons in the nucleus decreases by one and the number of protons increases by one. An example of such a process is:  In terms of safety, beta particles are much more penetrating than alpha particles, but much less than gamma particles.

Gamma decay  The third class of radioactive decay is gamma decay, in which the nucleus changes from a higher-level energy state to a lower level. Similar to the energy levels for electrons in the atom, the nucleus has energy levels. The concepts of shells, and more stable nuclei having filled shells, apply to the nucleus as well.  When an electron changes levels, the energy involved is usually a few eV, so a visible or ultraviolet photon is emitted. In the nucleus, energy differences between levels are much larger, typically a few hundred keV, so the photon emitted is a gamma ray.  Gamma rays are very penetrating; they can be most efficiently absorbed by a relatively thick layer of high-density material such as lead.

Radioactivity Making a precise prediction of when an individual nucleus will decay is not possible; however, radioactive decay is governed by statistics, so it is very easy to predict the decay pattern of a large number of radioactive nuclei. The rate at which nuclei decay is proportional to N, the number of nuclei there are: is known as the decay constant. Whenever the rate at which something occurs is proportional to the number of objects, the number of objects will follow an exponential decay. In other words, the equation telling you how many objects there are at a particular time looks like this:

 Where N o is the original number of particles.  The decay constant is closely related to the half-life, which is the time it takes for half of the material to decay. Using the radioactive decay equation, it's easy to show that the half-life and the decay constant are related by:

Radioactive dating  Radioactivity is often used in determining how old something is; this is known as radioactive dating. When carbon-14 is used, as is often the case, the process is called radiocarbon dating, but radioactive dating can involve other radioactive nuclei. The trick is to use an appropriate half-life; for best results, the half-life should be on the order of, or somewhat smaller than, the age of the object.

Example