Transcript A.6 –A.8
A.6 –A.8 In which you will learn about: •The history of the periodic table •Grouping elements by properties A.6 The Periodic Table • By the mid-1800s, chemists had identified about 60 elements • Periodic table: an arrangement where items with similar properties are placed near one another in a chart • In 1869, Dmitri Mendeleev published a periodic table of elements • Patterns of properties repeat themselves after every row – EX: Lithium is chemically similar to sodium Properties Used to Determine Groupings of Elements • 1) Atomic mass • 2) “combining capacity” – EX: one K can combine with one Cl & one Cs can combine with on Cl (represented as ECl , one element E with one Cl) --EX: one Mg can combine with two Cl & one Sr can combine with two Cl (represented as ECl2) Modern Periodic Table • In Mendeleev’s periodic table… – Elements with similar chemical properties were placed in the same vertical column (group) – Horizontal arrangements (periods) were based on increasing atomic weight • In the modern periodic table… – Elements with similar chemical properties are placed in the same vertical column (group) – Horizontal arrangements (periods) are based on increasing atomic number [Henry Moseley figured this out] A.7 Grouping the Elements • Alien Activity completed in class A.8 The Pattern of Atomic Numbers • Creators of the early periodic tables were unable to explain WHY neighboring elements had similar properties – Answers weren’t found until 50 years after Mendeleev’s work [Henry Moseley, see below] • Atomic number: the number of protons in an atom. Gives each element its identity. Also is # of electrons for a neutral atom. Neutrons to the Rescue! • Using mass and properties works well for small atoms, but falls apart for larger atoms – Total mass of an atom is the protons AND neutrons (both found in the nucleus) – Mass number: # of protons + # of neutrons. THIS IS NOT THE SAME AS ATOMIC MASS!!! – Electrons are too small to contribute Isotopes • The same element can have different numbers of neutrons = isotope • Mass number – Atomic number = # of neutrons • Isotopes are the major reason for fractional atomic masses found on the periodic table. • More on this tomorrow when we do an activity… HOMEWORK (your hand-made colored periodic table will help you with most of these) • 1) Give another term for each of these features on the periodic table: – A) row b) column • 2) Give the names and symbols of two elements other than lithium in the alkali metal family. • 3) Consider the noble gas family: – A) Where are noble gases located on the periodic table? – B) Name one physical property that noble gases share. – C) Name one chemical property that noble gases share. HOMEWORK SOME MORE • 4) Given a periodic table and the formulas BeCl2 and AlN, predict the formula for a compound containing – A) Mg and F B) Ga and P • 5) The melting points of sodium (Na) and rubidium (Rb) are 98°C and 39°C respectively. Estimate the melting point of potassium (K). MORE HOMEWORK • 6) Complete the following table for each electrically neutral atom: Element Symbol Ca # of Protons # of Neutrons # of Electrons 6 6 6 6 7 6 21 117 U 146 78 FINAL HOMEWORK QUESTION • 7) A student asked to explain the formation of a lead (II) ion (Pb2+) from an electrically neutral lead atom (Pb). The student says that a lead atom must have gained two protons to make the ion. How would you correct this student’s mistaken explanation?