Atomic Mass (Weights)

1. Atomic Mass (Weights) • Dalton’s atomic theory featured the idea that atoms of an element had a characteristic mass • Naturally occurring elements may be a mixture of isotopes • Dalton could not weigh each atom – he could find the average mass of one atom relative to the average mass of another – atomic weights

2. Atomic Mass Unit • Carbon-12 Isotope is the standard and is assigned the mass of exactly 12 atomic mass units • Atomic mass unit (amu) – is a mass unit equal to exactly one-twelfth the mass of a carbon -12 atom • Atomic weight – the average atomic mass for the naturally occurring element, expressed in atomic mass units

3. Atomic Weights • Isotope – atoms whose nuclei have the same atomic number, but different mass numbers • Atomic “weight” – where weight means average atomic mass • Relative abundance – the total weighted average of all the isotopes of an element • Fractional abundance –the fraction of the total number of atoms that is composed of a particular isotope

4. Chromium, Cr has the following isotopic masses and fractional abundances: What is the atomic weight of Cr?

5. Solution: multiply each isotopic mass by its fractional abundance

6. Periodic Table of Elements • Russian Chemist Dmitri Mendeleev (1834-1907) – constructed the periodic table as part of his effort to systematize chemistry • A tabular arrangement of elements in rows and columns – highlighting regular repetition of properties • Period – the elements in any one horizontal row • Groups or Families – the elements in any one column

7. Periods of the Periodic Table • Period 1 = Hydrogen (H) and Helium (He) only • Period 2 = Begins with Lithium (Li) and ends with Neon (Ne) • Period 3 = Begins with Sodium (Na) and ends with Argon (Ar) • Period 4 = Begins with Potassium (K) and ends with Krypton (Kr) • Period 5 = Begins with Rubidium (Rb) and ends with Xenon (Xe) • Period 6 = Begins with Cesium (Cs) and ends with Radon (Rn) it includes the Lanthanides elements 58-71 (in the section below called the inner-transition metals) • Period 7 = Begins with Francium (Fr) and it includes the Actinids (in the section below called the inner-transition metals) elements 90-103

8. Groups of the Periodic Table • Groups are numbered and also include letter • IUPAC convention uses columns which are numbered from 1 to 18 • North American convention show Roman numerals and letters • Group A elements are known as the main group elements • Group B elements are known as the transition elements • Two rows at the bottom are called inner transition metals

9. Groups • Group IA – The Alkali Metals: Li, Na, K, Rb,Cs,Fr – soft silvery – form +1 Ions – most reactive of all metals • Group IIA The Alkaline Earth Metals: Be, Mg, Ca, Sr, Ba, Ra – less reactive than Group IA and harder metal • Group VIIA – The Halogens: F, Cl, Br, I, At, reactive non metals (except Astatine) • Group VIIIA – The Noble Gases – He, Ne, Ar, Kr, Xe, Rn – though to be inert

10. Elements Divided into 3 Categories: • Metals – left and center with the exception of Hydrogen (H) • Good conductors • Nonmetal – right side • Poor conductors • Metalloids – between metals and non metals • Properties intermediate between those of metal and non metals (semi conductors = Si) they are: B, Si, Ge, As, Sb, Te, At