Elements and the Periodic Table

Elements

• Elements are pure substances that cannot be broken down further
• On the periodic table elements are represented by one or two letter symbols.

Periodic Table Organization

• Elements are organized by increasing atomic mass and atomic number.

Groups

• Group 1: Alkali Metals - Highly reactive, low melting point, and low density
• Group 2: Alkaline Earth Metals - Shiny, less reactive than Group 1
• Group 17: Halogens - Highly reactive and create salts
• Group 18: Noble Gases - Not reactive gasses

Metals, Nonmetals, and Metalloids

• Metals: Shiny, conduct heat and electricity, usually solids at room temperature example Mercury (Hg)
• Nonmetals: Dull, brittle, poor conductors of heat and electricity, often good insulators, low densities, and melting points
• Metalloids: Have characteristics of both metals and nonmetals

Atom

• Atoms are the smallest particle of an element
• Any substance is made up of many atoms

Dalton's Atomic Theory

• Atoms are tiny particles of matter
• Atoms of the same element are similar and differ from other elements
• Atoms of two or more elements combine to form compounds
• Atoms are rearranged in chemical reactions
• Conservation of matter: atoms are neither created nor destroyed in chemical reactions

Subatomic Particles

• Atoms contain 3 types of subatomic particles
• Protons: Located in the nucleus, have a positive charge (+1), and have a mass of 1 atomic mass unit (amu)
• Neutrons: Located in the nucleus, have no charge (neutral), and have a mass of 1 amu
• Electrons: Located outside the nucleus, have a negative charge (-1), and have no appreciable mass

J. J. Thomson's Experiments

• Cathode Ray experiment results showed cathode rays contain negatively charged particles
• Thomson determined these particles (electrons) have a much smaller mass than the atom
• Plum Pudding Model describes how electrons are interspersed in a positive "pudding."

Rutherford's Gold Foil Experiment

• The gold foil experiment investigated positively charged particles and discovered the atomic nucleus

Atomic Structure Summary

• Dalton's Atomic Theory: Conservation of matter, atoms are small particles, atoms of the same element are similar, atoms combine to form compounds
• J.J. Thomson's Cathode Ray Experiment: Cathode rays contain small negative particles called electrons
• J.J. Thomson's Plum Pudding Model: Electrons are much smaller than atoms and protons
• Rutherford's Gold Foil Experiment: Atoms are mostly empty space with a small positively charged nucleus

Atom Makeup

• Nucleus containing protons and neutrons that contribute most of the mass.
• Electrons exist in the large surrounding "electron cloud"

Key Terms

• Atomic number: A whole number that is the same for all atoms of an element; equals the number of protons in each atom.
• Mass Number: Represents the mass of the protons + neutrons in an atom

Neutral Atoms

• When atoms are neutral the overall charge of the atom is 0
• The # of protons equals the # of electrons when atoms are neutral

Finding Subatomic Particles

• Protons: The number of protons for atoms of that element is in the atomic number
• Neutrons: mass number - the # of protons = # of neutrons
• Electrons: # of protons = # of electrons

Isotopes

• Isotopes are atoms of the same element but have different mass numbers
• The # of protons is the same for the same element
• Change in the mass number is due to the number of neutrons

Average Atomic Mass

• Elements can have multiple isotopes
• The atomic masses on the periodic table are weighted averages of the isotopes.
• Weighted Average: The average atomic mass taking into account the number of isotopes and their % abundance.
• Total % abundance of all isotopes of a single element equals 100%
• More abundant an isotope is, the greater its weight in the average atomic mass