1. Chapter 4: Elements, Atoms, and Ions Chemistry 1020: Interpretive chemistry Andy Aspaas, Instructor

2. Elements All substances on earth are made from a combination of 114 or so elements –88 found in nature, others are man-made Abundance: % found in nature –Oxygen most abundant on earth & in human body (by mass) –Abundances vary in different parts of environment Each element has its own symbol –One or two letters - first is always capitalized

3. Dalton’s atomic theory According to Dalton: –Elements are composed of tiny unbreakable particles called atoms –All atoms of a given element are identical –Atoms of a given element are different from those of any other element –Atoms of one element combine with atoms of other elements to form compounds –Atoms are indivisible by chemical processes

4. Conclusions from Dalton’s atomic theory Law of constant composition –All samples of a pure compound contain the same proportions of the elements –Chemical formulas used to show those proportions Law of conservation of mass –Atoms are never created or destroyed, only rearranged –Atoms of one element cannot change into atoms of another element –All atoms present at the beginning of a reaction are present at the end

5. Compounds and chemical formulas A compound is a pure substance that is composed of atoms of two or more elements Compounds are described by giving the number and type of each atom in the simples unit of the compound Each element represented by letter symbol Quantity of atoms of each element written as subscripts (subscript 1 never written) Polyatomic groups in parentheses if more than one

6. Structure of the atom J.J. Thomson investigated beams known as cathode rays –Made of tiny negatively charged particles called electrons –Smaller than a hydrogen atom! –Atoms of different elements produce the same electrons

7. Thomson’s plum pudding model Atoms are indeed breakable, contrary to Dalton’s theory Electrons are suspended in a positively charged electric field (to balance electrons’ neg. charge) Mass of atom is due mostly to electrons Atom is mostly empty space

8. Rutherford’s gold foil experiment Tried to prove Plum Pudding model Shot “bullets” of alpha particles through thin sheet of gold atoms Expected alpha particles to fire straight through –Most indeed did –But about 2% were deflected by very large angles –Disproved Thomson’s plum pudding model

9. Rutherford’s nuclear model Most of atom’s mass is in a tiny dense center called the nucleus –Positively charged –Only 1/10 trillionth the volume of the atom –Nucleus’s positive charge balances electrons’ negative charge Electrons fly around in the empty space surrounding nucleus

10. The modern atom Nucleus composed of two types of particles –Protons: +1 charge –Neutron: 0 charge, mass similar to proton Electrons: -1 charge, outside of nucleus

11. Isotopes Number of protons defines which element an atom is –Called atomic number, found on periodic table Atoms of an element with different numbers of neutrons are called isotopes All isotopes of an element behave identically in chemical reactions But have different masses Identified by mass number –Mass number = # protons + # neutrons Isotope symbols contain mass number, atomic number, and element symbol

12. Elements Arranged in pattern called Periodic Table Properties can be predicted based on position in table Metals –About 75% of elements –Lustrous, malleable, ductile, conduct heat and electricity Nonmetals –Dull, brittle, insulators Metalloids or semi-metals –Properties of both metals and nonmetals

13. Groups and periods Elements with generally similar chemical and physical properties are in the same column Columns are called groups or families –Different numbering schemes Rows are called periods

14. Regions of the periodic table Main group = representative elements –“A” columns in label Transition elements –All metals Bottom rows: Inner transition elements or Rare earth elements –All metals –Inserted into periodic table after La and Ac

15. A few important groups Group 8A: Noble gases –Colorless gases at room temperature –Non-reactive (inert) –Found in nature as single atoms uncombined Group 7A: Halogens –Very reactive nonmetals –Exist as diatomic molecules in nature (Cl 2, Br 2, etc) –React with metals to form ionic compounds

16. Allotropes Some solid nonmetallic elements can exist in different forms with different physical properties –Allotropes: these different forms Different physical properties are from different arrangements of atoms in the solid Allotropes of carbon: –Diamond –Graphite –Buckminsterfullerene (C 60 )

17. Ions Ions are atoms or groups of atoms that carry an electrical charge Cations: positive charge (loss of electrons) Anions: negative charge (gain of electrons) Unlike charges attract, so cations and anions are attracted to each other Electrolytes: ions that dissolve in water and cause it to conduct electricity Ionic compound: combination of cations and anions that form a pure substance with no total charge

18. Atomic structures of ions Metals always form cations Loss of one electron creates a +1 charge –Na = 11 e -, Na + = 10 e - –Ca = 20 e -, Ca 2+ = 18 e - Cations have same name as uncharged metal Charge can be determined from group number

19. Atomic structures of ions Nonmetals form anions Gain of 1 electron = -1 charge Anions named by changing ending of element name to -ide –Fluorine: F + 1 e - = F - (fluoride ion) –Oxygen: O + 2 e - = O 2- (oxide ion) Charge determined by taking 8 - group number

20. Writing ionic formulas Ionic compound must have no net charge So change number of ions to cancel out positive and negative charges Compound made of Mg 2+ and Cl - –Must have formula MgCl 2