Unveiling the World of Elements: Atoms, Ions, and Compounds

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 that atoms are mostly empty space • 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 (Cl2, Br2, 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 (C60)

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-, Ca2+ = 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- = O2- (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 Mg2+ and Cl- • Must have formula MgCl2