Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Level 1- Recap, The Atom + Proton N Neutron - Electron The Bits Protons and Neutrons are in the nucleus. Electrons are in orbit outside the nucleus. The electrons are arranged in shells around the nucleus Each shell can contain only a fixed number of electrons: The 1st shell can hold up to two electrons, the 2nd shell can hold up to eight electrons, the 3rd shell can hold up to 18, and 4th shell can hold up to 32 and so on. Since electrons are electrically attracted to the nucleus, an atom's electrons will generally occupy outer shells only if the more inner shells have already been completely filled by other electrons (usually). The electrons in the outermost occupied shell (or shells) determine the chemical properties of the atom; it is called the valence shell. Elements A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Periodic Table of Elements Simplified, showing stable elements. Notice how in each case there is the same number of protons(+) and electrons(-). The overall electrical charge is ZERO. Keep that in mind for later! The complete table Don't get stressed. At this stage you only need to know that elements are divided into groups This simplified table shows only the number of protons in each element It tells you how the table is, roughly, organised. It's actually a lot more complicated. You will Learn about that in year 11 and 12. Isotopes Isotopes are variants of a particular chemical element such that, while all isotopes of a given element share the same number of protons and electrons, each isotope differs from the others in its number of neutrons. For example, carbon-12, carbon-13 and carbon-14 are three isotopes of the element carbon with mass numbers 12, 13 and 14 respectively. The atomic number of carbon is 6, which means that every carbon atom has 6 protons, so that the neutron numbers of these isotopes are 6, 7 and 8 respectively. About atomic weight Look at the atomic weights of a few different elements on your periodic table. Do you notice that very few of the elements have atomic weights that are close to being nice whole numbers? Do you know why this is? After all, for our purposes, the mass of both the proton and the neutron are almost exactly 1, and in chemistry we usually ignore the mass of the electron because it is so very small. Why then, if the mass of the atom comes mainly from the protons and neutrons it contains, don’t the atomic weights of the all come out to be nice whole numbers? The reason is this; the atomic weights given on your tables are “weighted averages” of the weights of the different naturally occurring isotopes of the element. Let’s look at an example. Approximately 75% of the chlorine atoms found in nature have a mass of 35. The other 25% have a mass of 37. What should we report as the atomic weight for chlorine? What we do is to take the “weighted average” of these isotopes. We multiply 75% times 35 and then add that to 25% times 37... [(.75)(35)] + [(.25)(37)] = 26.25 + 9.25 = 35.5 Level 2, Molecules A molecule is an electrically neutral group of two or more atoms held together by bonds formed in the sharing of electrons Shell Love In nature, usually atoms don't like to have vacancies in their shells, so most of the time they will try to fill these spaces by pairing up. The single atoms will attempt to bond and form a molecule. There are a number of different types of bonds. Different types of bonds Covalent bonds Ionic bonds Metallic bonds Different groups of elements bond in different ways. What they are all trying to do is to arrange themselves in to some type of stable structure. It's all about the electrical charges between the nucleus and the electrons in the shells. Covalent Chemical Bonds. In the simplest view of a so-called 'covalent' bond, one or more electrons (often a pair of electrons) are drawn into the space between the two atomic nuclei. Here the negatively charged electrons are attracted to the positive charges of both nuclei, instead of just their own. The oxygen atom has space in it's outer shell for two more electrons. The hydrogen atom has space in it's single shell for one more electrons. By sharing electrons each atom fills up the spaces in it's shell. Ionic Bonds In a simplified view of an ionic bond, the bonding electron is not shared at all, but transferred. In this type of bond, the outer atomic orbital of one atom has a vacancy which allows addition of one or more electrons. This transfer causes one atom to assume a net positive charge, and the other to assume a net negative charge. The bond then results from electrostatic attraction between atoms, and the atoms become positive or negatively charged ions. A quick note about Ions An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving the atom a net positive or negative electrical charge. Ions love to bond and get stable (have a net neutral charge). They are very reactive. Metallic Bonds A less often mentioned type of bonding is the metallic bond. In this type of bonding, each atom in a metal donates one or more electrons to a "sea" of electrons that reside between many metal atoms. Metals are just party animals. The electrons are jumping into and out of the shells of each atom. Bit like a big car park. While they are parked they stabilise the nucleus. When they move out another one jumps in. Because there are free electrons whizzing around they can be made to move. Electricity is just moving electrons in a metal. But that's another topic. But how can we know this! We have the photos Chemical bonds and molecular structure have proven experimentally accurate in organic chemistry research labs all over the world, but few suspected the models would look so close to reality. A team of researchers from The Department of Energy’s Berkeley Lab have acquired images of bonds breaking and forming during a chemical reaction, and the real molecules look like they’re copied out of the textbook diagrams. The images published in Science show several molecules, starting with oligo(phenylene-1,2-ethynylenes), which is a cluster of three benzene rings linked with carbon chains. Placed next to the skeletal diagram, it’s a stunningly good match. After initiating a temperature-dependant organic reaction, the experiment was imaged again. The reaction products were present, and they too looked exactly as predicted by the models.