History of the Atom & the Periodic Table

Parts of the Atom Protons = positive charge Neutrons = no charge Electrons = negative charge Nucleus = both protons and neutrons with positive charge

The Nucleus  Nucleus = protons + neutrons  The nucleus occupies only a small fraction of the atom  However, it contains almost all the mass of the atom  One proton or neutron is almost 2000 times greater than the mass of an electron

Is There Anything Smaller than an Atom?  YES!! It is called a quark

How Atomic Models have Changed  Model of the atom has changed over the years  6 very important changes have taken place! 1. Democritus 2. John Dalton 3. J.J. Thompson 4. Ernest Rutherford 5. Niels Bohr 6. Electron Cloud Model

Democritus  400 B.C.  Idea proposed that atoms make up all objects  Aristotle disagreed with Democritus' theory  Aristotle’s theory that matter was the same throughout was accepted  Thought atoms looks like a solid sphere

John Dalton  1800s  Proved atoms existed  Proposed:  1. All substances are made of atoms and cannot be destroyed  2. Atoms of the same element are exactly alike  3. Atoms join other atoms to make substances (like water)  Proved atoms existed by using a cathode tube

*****Not in notes!

Joseph John “JJ” Thompson  1904  Thompson did not know how electrons were arranged He thought they were mixed throughout atom Thought it was a positive sphere with negative electrons on sphere, scattered around  He thought it looked like a ball of chocolate chip cookies!

Ernest Rutherford  1911  A student of JJ Thompson!  Created the Gold Foil Experiment  From the experiment, he learned: 1. The atom is mostly empty space 2. The nucleus is very densely packed 3. The nucleus is positively charged

Niels Bohr  Hypothesized that electrons travel in levels around the nucleus  Levels are called orbitals

Modern Electron Model  Known as the electron cloud model  Electrons travel in “areas” instead of levels  Areas are called clouds

Match the Model with the Correct Theory… 1. Modern Theory 2. Rutherford Model 3. Thompson Model

The Periodic Table I. 1 st Periodic Table was developed by Dmitri Mendeleev II. Elements are organized by increasing atomic number (in other words, increase number of protons in the nucleus) III. Properties in lighter elements could be shown to repeat in heavier elements

Organization of Periodic Table I. Groups (also known as families) A. Vertical columns B. Organized by simil ar properties C. Have same # of electrons in outer energy levels D. Each level can have a maximum # of electrons E. Each row ends when an outer energy level is filled F. You can use electron dot diagrams to represent outer energy level electrons (known as valance electrons) II. Periods A. Horizontal rows B. Organized by increasing # of protons and neutrons C. Classified as metals, nonmetals, and metalloids

Periods… Vertical or horizontal? HORIZONTAL!! Groups… Vertical or horizontal? VERTICAL!!

Valance Electrons (outer electrons)  Valance electrons are electrons in the outer most layer  In general, the number of valance electrons of an element is equal to the group number  *Groups 3-12 are skipped and do not follow the same pattern  Group 18 (Noble Gases)  Stable because outer electron level is full

Atomic Number vs. Mass Number I. Each element has same number of protons, but different number of neutrons II. In a neutral atom, protons (+) and electrons (-) are equal… so they balance out to zero I. Atomic # – number of protons in an atom (also identifies the element) II. Mass # – sum of protons and neutrons in the nucleus (identifies mass of the nucleus)

Using Periodic Table, Complete this chart: Element Atomic # # of Protons / Electrons # of Neutrons Mass # Helium – Carbon – 12 Carbon – 14 Oxygen – 16

Isotopes I. Isotopes – atoms of the same element with different numbers of neutrons A. Different isotopes have different properties B. Name of the element, followed by mass # identifies the isotope 1. Ex) Uranium-235 and Uranium-238