Chemical Foundations Review-Day 2

Periodic Table The Noble Gases At the start of the 1890s, no one had any idea that there was a separate group of gases in the periodic table, the noble gases. Noble gases are familiar to us from their use in neon signs and helium balloons. By 1900 this whole new group had been identified and isolated. While trying to determine an accurate atomic mass for nitrogen, British physicist Lord Raleigh (1842-1919) discovered that nitrogen prepared from ammonia was noticeably lighter than nitrogen that came from the atmosphere. He and William Ramsay (1852-1916) both studied “atmospheric” nitrogen. By removing the nitrogen from it, they produced a tiny quantity of another gas. Since it did not react with anything they called it argon, from the Greek word for lazy. The discovery of helium followed a year later in 1895. Ramsay and his assistant Morris Travers (1872-1961) then started to search for additional elements in this new group. They attempted this by fractional distillation of large quantities of liquid air and argon. In 1898, their efforts were rewarded; they had prepared krypton, neon, and xenon. Eyewitness Science “Chemistry” , Dr. Ann Newmark, DK Publishing, Inc., 1993, pg 32 Kelter, Carr, Scott, Chemistry A Wolrd of Choices 1999, page 74

The Modern Periodic Table Arranged according to their atomic number Columns are called groups (18) Elements in same group have similar properties Rows are called Periods (7) Metals-most abundant, left side of table mostly solid @ RT, good conductors of heat and electricity Non-Metals-right side of table, mostly gas @RT, do not conduct heat or electricity Metalloids-have properties of both metals & non metals

What does the Table tell us… For each element… Atomic number tells us the number of protons in the atom In a balanced atom, the atomic number is also equal to the number of electrons around the nucleus Mass Number tells us the number of protons and neutrons in the nucleus (round the atomic mass), subtract the atomic number from the atomic mass to get the number of neutrons.

Examples Hydrogen 1 Carbon 6 Oxygen 8 Argon 18 22 Gallium 31 39 Chemical Name # of protons # of neutrons # of electrons Hydrogen 1 Carbon 6 Oxygen 8 Argon 18 22 Gallium 31 39

Models of the Atom

Valence Electrons (what makes an atom happy?) Atoms want to become stable and achieve a valence “octet” (a full outer shell) Recall that valence electrons are only those in its outermost level or “shell” The group number in the periodic table will predict how many electrons in its outer shell (this only works for the main groups) Atoms will bond to get that full octet by transferring electrons (ionic bonds) or sharing electrons (covalent bonds) NOTE: Protons & Neutrons do not move, only Electrons

Recall that electrons are negative particles! Atoms that lose or gain electrons, now become charged. They are now “Ions” Recall that electrons are negative particles! Atoms which lose electrons become Positively charged or Cations (metals) Example: Mg → Mg2+ [loses 2 electrons] Atoms which gain electrons become Negatively charged or Anions (non-metals) Example: O → O2- [gains 2 electrons] Ions have a charge number on upper right

Predict the ionic charge on the following atoms Na (Sodium) group 1 metal Ca (Calcium) group 2 metal Al (Aluminum) group 13 metal N (Nitrogen) group 15 non-metal S (Sulfur) group 16 non-metal Br (Bromine) group 17 non-metal Ne (Neon) group 18 (huh???)

Isotopes Atoms that have the same atomic number but a different atomic mass…Example Magnesium

Meaning of Chemical Formula

Count the atoms in each chemical formula PbI2 Al2O3 Mg(NO3)2 Al2(SO4)3 2Al2(SO4)3