Chapter 4 Elements Atoms and Ions Chemistry 1020

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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)

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

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

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

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 Mg. Cl 2