THE PHYSICAL UNIVERSE MatterEnergy Einstein: E=mc 2. Energy and matter are interconvertible. Has mass Takes up space = Has volume The capacity to move matter through space = ability to do work

Matter Pure Substance (1 chemical entity) Mixture (more than 1 chemical entity) Nothing to separateCan be separated by chemical or physical means into individual substances

Types of Pure Substances Element Compound Contains only 1 kind of atom (similar to letter of alphabet) Contains more than 1 kind of atom (similar to a “word”)

Do Atoms Have Parts? Or, What’s Inside an Atom? Some experimental observations: 1.Atoms are overall electrically neutral 2.The atom of one element is different chemically & physically from another element’s atoms 3.Atoms can be decomposed into 3 types of subatomic particles: protons (+ charge), electrons (- charge), and neutrons (0 charge) 4.Protons from any source are all alike 5.Electrons from any source are all alike 6.Neutrons from any source are all alike

SO: How can atoms of different elements be made from the same 3 particles? How can an atom be made from charged particles, but itself have no charge? How are these particles arranged in an atom? What makes an element unique? What does this have to do with chemical behavior?

ANSWERS to these questions came slowly after much work by many people. Skipping a lot of history: 1.The elements are composed from the same 3 particles, but the number if the of particles in each element is different. 2.Atoms contain an equal number of + particles (protons) and – particles (electrons), so the overall charge would be 0. 3.Each element is unique in its atomic number = # protons = # electrons. 4. Chemical compounds form and react in processes involving gain, loss, or sharing of electrons. The nucleus is never involved in a chemical reaction, only in a nuclear reaction (radioactive decay, a nuclear bomb, or power plant).

How are the subatomic particles arranged in an atom? In the late 19 th and early 20 th century scientists discovered that all the protons and any neutrons present in an atom are scrunched into an incredibly small volume of space called the nucleus. The electrons in an atom are found in an enormous volume of space surrounding the nucleus. For comparison purposes, if the nucleus fills a volume the size of a baseball, the electrons would be found in a sphere of space about a mile in diameter centered on the nucleus! (see section 2.4F in Bettelheim)

The strangeness continues… One atom does not weigh very much -- so little in fact that it can’t be weighed directly. It happens that the mass of a proton and neutron are practically the same. Rather than use gram or kilogram units, we can say the mass of 1 proton or 1 neutron is about 1 atomic mass unit, or 1 amu. But it would take almost 2,000 electrons to have the same mass as 1 proton! Since an atom has the same number of protons and electrons, this means: effectively all the mass of an atom is packed into the nucleus. Most of the atom is otherwise “empty” space, containing some extremely lightweight electrons!

Some more terms and symbols… Already defined atomic number = # protons Atomic mass number = (# protons + # neutrons) Each element could be called by its atomic number, but names were given long before subatomic particles were known. Elements are usually symbolized by one or two letters from the name in English (more on this later). Suppose the element symbol is X. The standard way to report the atomic mass number, atomic number and symbol is: yy = atomic mass number as superscript zz = atomic number as subscript X element symbol

Example: C 14 6 (C is the symbol for carbon.) Atomic number 6 and C or Carbon are synonymous– names are older and easier for many purposes. The symbol is read as “carbon 14.” The number of neutrons in an atom = mass number – atomic number. So, this atom of carbon has 8 neutrons. Originally (and logically) it was proposed that all atoms of an element are the same. When the neutron was discovered, scientists learned that all atoms of an element have the same number of protons, but atoms of the same element can have different numbers of neutrons.