Chem Unit: Part II The Periodic Table of Elements

Transcription

1 Chem Unit: Part II The Periodic Table of Elements You and I are made of atoms, as is the chair in which you're sitting, the air, the Sun and Earth. Atoms can connect together into molecules: in simple ways like the oxygen molecules in the air, or complicated ways, like the vast molecules that are the building blocks of our bodies. Atoms can also connect together into large lattices; this is the way they're connected together in a solid piece of metal.

2 mass (amu) electric charge (Coulombs) proton neutron electron Atoms themselves are composed of 3 types of smaller particles: protons, neutrons, and electrons. Protons and neutrons are similar to each other in mass. The masses of all 3 are given in the table at the left. The unit of mass here is the atomic mass unit, or amu. You can see that both the proton and neutron are just a little heavier than one amu each. One amu is a very small unit of mass, about one six hundredth of a millionth of a billionth of a billionth of a kilogram. Physicists use this very small unit of mass because atoms are so small, and it would be awkward to always express their masses in kilograms. (1) How many times heavier is a proton than an electron? The other property listed is the amount of electric charge, the property that gives the particle the ability to interact through the EM force. Protons and neutrons make up the nuclei of atoms, when held together by the Strong Nuclear Force, one of the four known fundamental forces of nature. The tables below describe the direction of the force between particles types for 2 of the four forces. EM electron proton neutron electron repulsive attractive none proton attractive repulsive none neutron none none none Strong (very short range) electron proton neutron electron none none none proton none attractive attractive neutron none attractive attractive You can see from the table that electrons don't feel the Strong Force at all, but this force causes neutrons to attract each other, protons to attract each other, and protons and neutrons to attract each other. It's the strongest known force of nature (hence the name), but it's only felt over a very very short range and so two of these particles must be extremely close to each other to be attracted. The other table describes the interaction of the EM Force, or Electromagnetic Force. This is the force that causes electrons to orbit the nuclei of atoms: they're attracted to the protons there. It also holds atoms together into molecules because electrons can be attracted to protons in the nuclei of other atoms.

3 So, these two forces create atoms from the 3 particle types. All 90 naturally occurring elements are composed this way, and the only difference between them is the numbers of the 3 types. Most important is the number of protons in each nucleus: hydrogen has one in each nucleus, helium has 2, lithium 3...and uranium has 92. Here is a list of the elements in order of increasing number of nuclear protons. Because of the EM Force, the number of nuclear protons determines the number of orbiting electrons, and as we'll see in the next section, the number of electrons determines how the atom connects to other atoms. That makes all these elements chemically different from each other. Metals & Nonmetals Most of the elements are metals. A piece of a metallic element tends to be hard, have a lusterous surface, and conducts electricity and heat. In the periodic table that you have (the link in the title of this exercise), there is a dark line around a set of elements. These are metalloids. Elements to the right of these are nonmetals and elements to the left of them are metals. Hydrogen is a nonmetal, so there is a dark line separating it from the metals. (2) Find the names of ten metals in the list of elements that you have heard of and list them. Elemental Composition of Materials Every object or substance is made of one or more elements. Find the composition of the following materials. You can use the internet to do this. (3) Cement (4) Steel (5) Air (6) Seawater (7) Quartz (a very abundant mineral) (8) gasoline (9) glass

4 Abundances of the Elements Some elements are far more abundant than others. The universe consists largely of the two simplest, hydrogen and helium, as you can see in these tables. You'll construct two pie charts displaying element abundances. Each group should produce one set of charts. (10) Use the data in the table with abundances in the human body to create a pie chart to display these amounts (just down to Iodine or so). The box below describes how to use Microsoft Excel to make a pie chart without too much trouble. (11) Make a similar pie chart for abundances in the Earth's crust. Making a Pie Chart With Microsoft Excel 1. Fire up Excel and also display this page in a Microsoft Internet Explorer window (there are problems with the way this works when using Mozilla). 2. Highlight all the columns of data in the table (just one table). Put the cursor over part of the highlighted area and right click, then click on copy. 3. Click into a square in the Excel spreadsheet. Then right click and click on paste. The data should appear in individual cells like on the webpage. 4. In the Excel file, highlight only the columns with element names and their percentage abundances. Then click on insert then on Pie, and choose (12) Three quarters of the mass of Earth's crust is comprised of which two elements? (13) What's the most abundant element in the human body? (14) This element is so abundant in the human body because it's the main componant of what compound? (15) What's the second most abundant element in the human body? (16) Why is this element so abundant in the body? your Excel file to the instructor. Molar Mass The instructor handed out a version of the periodic table. In each square is the symbol for the element, its atomic number, and its Molar Mass. The molar mass is the mass

5 in grams of one mole of atoms of that element. One mole is atoms. But the molar mass is also the mass of a single atom in amu. In other words, 1 gram is amu. Since one amu is about the mass of a proton or a neutron, the molar mass of an element is approximately the number of nucleons (protons & neutrons) in each nucleus. (17) What's the molar mass of oxygen? (18) How many moles of oxygen atoms are in 453 grams of oxygen? (this is one pound) (19) How many atoms is this? Chemists also use the concept of a mole for molecules. One mole of water molecules (H2O), has oxygen atoms and twice that many hydrogen atoms. (20) What's the molar mass of water? (21) What's the molar mass of tyrosine, C9H11NO3?