Chem Unit: Part I Atoms and the EM Force

Transcription

1 Chem Unit: Part I Atoms and the EM Force I. Electric Charge & the EM Force of Attraction and Repulsion Some of the fundamental particles that make up the STUFF in this universe have a property called electric charge. If a particle has this property, it can feel the EM force. Electric charge comes in two types: what we call negative and positive charge. They're called this because particles with negative charge have the opposite effect on other charged particles than do particles with positive charge. The effects are summarized in the statements below. 1. Two particles with electric charge of the same sign repel each other. 2. Two particles with electric charge of oppostie sign attract. 3. The amount of force is proportional to the amount of charge that the particle has.

2 In the first frame above, two positively charged particles are shown repelling each other. (1) For the other frames, draw arrows to indicate the forces between the particles. You can draw more than one arrow on a particle to indicate that it is being forced by more than one other particle. Remember that every pair of particles has an interaction, so each particle should have an arrow for every other particle in the frame. (2) Now in each frame, draw a single arrow on each particle (in another color) to represent the direction of the total force due to the other particles. Now click here. You can download a game that illustrates the EM force. The object of the game is to get the black positively charged particle into the goal. You can take charged particles from the boxes to the upper right and place them on the playing field. When you click Start, the black particle will respond to the forces of the other particles. Try the following: (3) Use one positively charged particle to push the black particle into the goal. How can it be done? (4) Use one negatively charged particle to do the same (hint: the black particle will go through the others without hitting them). How did you do it? (5) Use at least one positively charged and one negatively charged particle to get the black particle into the goal. How did you do it? II. Protons and Electrons

3 The particles in nature that have electric charge are protons and electrons, two of the constituent particles of atoms. Protons have a positive charge, and electrons have a negative charge. In each of the following, circle the appropriate word in parentheses. (6) Protons (attract, repel) each other. (7) Electrons (attract, repel) each other. (8) A proton and an electron (attract, repel) each other. The amount of electric charge on a proton is small because protons are small. The amount is about Coulombs (the Coulomb being the unit of electric charge used in the SI unit system). The amount of electric charge on an electron is Coulombs, exactly the negative of that on a proton. Thus the repulsive force of an electron on another electron is the same as the attractive force of the electron on a proton. In the figure above, the electron to the right is being attracted to the proton but repelled equally by the electron, and so the net force is zero (actually it would only be exactly zero if the two at the left were in exactly the same spot). The point is this: atoms have equal numbers of protons and electrons, and so have no net charge. Thus, the molecules of air in this room, for example, don't attract or repel each other. Matter is normally electrically neutral. (9) Try the above configuration with the game. First, click reset and clear, and uncheck the box labeled Puck is Positive (this will make the black particle negatively charged). Next place one positive and one negative particle in the same spot near the black particle. When you start it, what happens? (10) Now click reset and put another positively charged particle in the same spot as the other two. What happens when you start it? Choose one answer in parentheses for each of the following. (11) Protons have a (positive, negative) electric charge.

4 (12) Electrons have a (positive, negative) electric charge. (13) If you put a bunch of particles in the same spot, the negatively charged black particle will only be attracted in if you have more (positively, negatively) charged particles. Another point to be made is this: Although protons and neutrons are much heavier than electrons, the spaces in which they reside are much smaller in size (this is how things work in the world of elementary particles). And so a drawing that is somewhat closer to reality of an electron and a proton together would look like the one below. The proton appears to be inside the electron. And this is basically the structure of a hydrogen atom. Below is a simple depiction of some hydrogen atoms near each other. They don't attract or repel each other since the attractive and repulsive forces are balanced out.

5 III. Atomic Structure What if an atom has 2 protons in the nucleus instead of one, as in the figure below? Now there is twice as much attraction as repulsion for an additional electron, which will be attracted in (below).

6 With every additional proton, there is enough attractive force to attract another electron, but once the number of electrons is equal to the number of protons, the atom once again becomes neutral. You can see why atoms have equal numbers of protons and electrons. Below is a depiction of a nucleus of 3 protons. Another electron has joined the first two. We'll see later that this third electron will reside in a region further from the nucleus (the electrons' residential regions are called shells).

7 (14) This is an atom of Lithium, the element with 3 protons in each nucleus. The outer electron is being attracted by these 3 protons, but repelled by 2 electrons, so the net attraction is as though the number of nuclear protons is.