CEM 141 Chapter 1 Learning Objectives Questions with well explained answers

Identify and explain the similarities and differences between atoms, molecules, elements, and compounds - Atoms: the smallest unit of ordinary matter that forms a chemical element Molecules: electrically neutral group of two or more atoms held together by chemical bonds. Elements: consists only of the same atoms, with the same number of protons Compounds: a chemical substance composed of atoms from 2+ elements held together by chemical bonds Make an argument(claim, evidence, reasoning) for the existence of atoms - Claim: atoms are the smallest unit of matter that forms an element and make up everything in the world Evidence: an example is water, which is a liquid we drink and makes up more than half our body. Reasoning: water is made up of hydrogen and oxygen atoms, we know this because they can be boiled and evaporate into the air and can be separated/added to each other. Make an argument(claim, evidence, reasoning) for the existence of electrons - Claim: electrons are subatomic particles within an atom which hold a negative charge Evidence: JJ Thompson used a cathode ray tube to prove the existence of electrons. This CRT was connected to metal discs coated with a chemical that responds to electric and has wires embedded inside. Reasoning: with his CRT, he found that the rays were negatively charged, emerged from one end and moved to the other, can be deflected by magnetic fields, carried an electric charged which bent the rays present in the CRT, and found the metal did not affect the ray behavior. He concluded the charge must come from inside the atoms with this data and developed the plum pudding model with it. Make an argument(claim, evidence, reasoning) for the existence of the existence of a small, massive, and positively charged nucleus - Claim: a nucleus is a subatomic particle inside an atom. Evidence: Rutherfords experiment using helium atoms which he projected onto a gold foil. Reasoning: most of the particles(He atoms) went straight through the foil. However, some atoms were deflected off the foil and an even smaller amount bounced right back to where they were initially projected from. This is because of the bigger nuclei inside gold atoms which the He at times would come into contact with, causing them to deflect. The nucleus must exist or else the He atoms would not have been deflected, and all of them would have gone straight through. (nucleus positive because electrons of He repel it, it is small because MOST went through)How does a scientific theory differ from everyday use of the term "theory" - In everyday use, the word "theory" often means an untested hunch, or a guess without supporting evidence. But for scientists, a theory has nearly the opposite meaning. ... A theory not only explains known facts; it also allows scientists to make predictions of what they should observe if a theory is true. compare the parts of the various atomic theories that stayed the same over time and those that changed - same: small nucleus in the middle, have electrons with a negative charge, different elements have different atoms different: atoms not embedded inside the atom(plum pudding model) but they surround nucleus in an electron cloud. electrons do not orbit the nucleus(planetary model) but again surround and move randomly in the electron cloud. atoms are also not indivisible: they are made of protons, neutrons, and electrons. All atoms are also not the same because of the existence of isotopes compare and constant gravitational and electrostatic forces - gravitational: mediated by fields, increase as mass increases, require presence of 2+ objects, cannot be associated with a single object, only attractive, decrease as distance increases, increases as distance decreases, MASS electrostatic: mediated by fields, both attractive AND repulsive, need 2+ objects, decreases as distance between objects increases, increases as the distance between objects decrease, increase as the charge of objects increase, CHARGE both have a very similar formula and are inversely proportional construct an atomic level explanation for why two isolated atoms would attract each other as they approach, and why they would repel if they get too close together - Attraction between two atoms is caused by London dispersion forces created by the random and chaotic movement of electrons. This is because the electrons in an electron cloud is not always proportional, sometimes because of their chaotic movement, electrons can bunch up more on one side of the nucleus than the other, creating an instantaneous dipole. This creates a PARTIAL +/- charge within the atom. This causes the electron cloud of a neighboring atom in the isolated system to be attracted to the partial + charge. This is the induced dipole. This attracts the two atoms and they approach each other. However, as they get extremely close, the electron clouds of the two atoms begin to overlap, so the electrons of both atoms are repelled by each other because they are both negative. As they move even closer, the repulsive force will overcome the attractive one because the close electrons want to move away, causing them eventually to pull ct and explain the changes in the potential energy, the kinetic energy, and the total energy as two isolated atoms approach each other - total energy: stays the same PE: decreases at first as they are pulled together, but increase when the electron clouds overlap, causing the repulsive force to overcome the attractive force making them move apart KE: increases as the two atoms attract each other b/c as they attract, they speed up to get to each other, increasing the velocity. but they decrease when they begin to repel/move apart