Chapter 3

Brief History of Electricity 1. Ancient Greeks discovered that if a piece of amber were rubbed with fur- pieces attracted each other. Later attributed to opposite charges (+ and -) – called static electricity 2. Benjamin Franklin – mid 1700’s – showed that lightning was a form of static electricity

Alessandro Volta – 1800 – First battery G. Johnstone Stoney – 1874 – coined the word “electron” for the basic unit of electricity

By the end of the 19 th Century scientists had discovered that the atom was not solid and indivisible as Dalton had proposed. Several important experiments led to these discoveries. Let’s look at 2 of them: 1. J.J. Thompson and the Crookes Tube Experiment

A) Led to discovery of electrons as basic unit of negative charge, found randomly inside atom. B) Plum-Pudding Model of the atom Cathode Rays are electrons

2. Ernest Rutherford and discovery of the nuclear atom A) Natural radioactivity – 3 types – alpha particles (  ), beta particles (  ) and gamma radiation (  ) B) Gold foil Experiment

C) Nuclear Model of the atom Electrons make up only about 1% of the total mass of the atom atom Nucleus: positive charge very dense[very small(less than 1% of the entire space in the atom) but contains almost all the mass of the atom]

C) Nuclear Model of the atom First model After discovery of neutrons

Summary of particles inside atom: ParticleSymbol LocationRelative ChargeRelative Mass (Mass #) Electrone - Outside nucleus ≈ 0 Protonp + Inside nucleus Neutronn 0 Inside nucleus01.000

Atomic Number (Z) – The number of protons in an atom. Also the number of electrons in a neutral atom. Unique for each element. Identifies the element.

Mass Number – The number of protons + the number of neutrons in an atom. For example: An atom with 5 protons and 7 neutrons Mass # = 5p + + 7n 0 = 12 This number is not unique for each element. All but one element have atoms with different numbers of neutrons and therefore different mass numbers. For example, there are 3 different atoms of C found in Nature, all having 6 protons, but one having 6 neutrons, one 7 neutrons and one with 8 neutrons. What are their Mass Numbers? These 3 atoms are called isotopes of C. Isotopes are atoms of the same element with different Mass Numbers. Isotopic Symbols - Mass # Atomic #

Atomic Weight (AW) or better Atomic Mass – The relative weight of an average atom of one element compared to that of another atom chosen to be the standard. The modern standard is the C-12 isotope (arbitrarily assigned a value of amu (atomic mass unit). Actually the AW of an element is the average (taking into account the relative amount of each) mass of all the naturally found isotopes. This is the non-whole # found under each element in the Periodic Table.

Bohr Model – Also called Solar System Model. Nucleus with electrons revolving around in orbits of specific energies (called Energy levels) Niels Bohr – Danish

Each successive orbit has a maximum # of electrons that can exist (2, 8, 18, 32, 50 etc) We will only worry about the first 20 elements. For these the maximum # of electrons is 2 for the first and 8 for the rest (actually 18 in the third, but the last 10 are not important for us in this course). For every atom the electrons fill up the orbits in order beginning with the lowest energy. The closer to the nucleus, the lower the energy. In other words Li has an atomic # of 3, therefore 3 electrons (2 in the first level and 1 in the next).

See the following table. The electrons in the outermost energy level are called Valence electrons and that level is called the Valence Shell.

Dimitri Mendeleev and the Periodic Table Modern Version Names of Families Metals, Non-metals and metalloids and Noble Gases Representative elements and transition metals

1. Metals – Left side - a) Properties – Shiny, malleable, ductile, conduct electricity & heat 2. Non-Metals – Right Side b) Properties – Brittle as solids, non-conductors 3. Metalloids – In between c) Properties – Some of both metals & non-metals 4. Noble Gases – Far Right Column d) Properties – All are gases even at very low temperatures, non-conductors, very little tendency to chemically react

Representative Elements – Those in columns headed by an A (IA, IIA etc). We will spend most of our time dealing with these elements. Transition Metals – Those in columns headed by a B. Inner Transition Elements – Bottom 2 rows