2: Atoms
- Page ID
- 118779
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- 2.1: Cutting Alumimun until you get Atoms
- Take some aluminum foil. Cut it in half. Now you have two smaller pieces of aluminum foil. Cut one of the pieces in half again. Cut one of those smaller pieces in half again. Continue cutting, making smaller and smaller pieces of aluminum foil.
- 2.2: Indivisible: The Atomic Theory
- You learned earlier how all matter in the universe is made out of tiny building blocks called atoms. All modern scientists accept the concept of the atom, but when the concept of the atom was first proposed about 2,500 years ago, ancient philosophers laughed at the idea. It has always been difficult to convince people of the existence of things that are too small to see. We will spend some time considering the evidence (observations) that convince scientists of the existence of atoms.
- 2.3: The Properties of Protons, Neutrons, and Electrons
- Electrons are extremely small. The mass of an electron is only about 1/2000 the mass of a proton or neutron, so electrons contribute virtually nothing to the total mass of an atom. Electrons have an electric charge of −1, which is equal but opposite to the charge of a proton, which is +1. All atoms have the same number of electrons as protons, so the positive and negative charges "cancel out", making atoms electrically neutral.
- 2.4: Elements: Defined by Their Number of Protons
- Scientists distinguish between different elements by counting the number of protons in the nucleus. Since an atom of one element can be distinguished from an atom of another element by the number of protons in its nucleus, scientists are always interested in this number, and how this number differs between different elements. The number of protons in an atom is called its atomic number (Z). This number is very important because it is unique for atoms of a given element.
- 2.5: Counting Nails by the Pound
- The size of molecule is so small that it is physically difficult if not impossible to directly count out molecules. However, we can count them indirectly by using a common trick of "counting by weighing".
- 2.6: Counting Atoms by the Gram
- In chemistry, it is impossible to deal with a single atom or molecule because we can't see them or count them or weigh them. Chemists have selected a number of particles with which to work that is convenient. Since molecules are extremely small, you may suspect this number is going to be very large and you are right. The number of particles in this group is Avagadro's number and the name of this group is the mole.