Elements, Atoms, and Ions

• Dalton’s Atomic Theory

  • Elements are composed of atoms

  -tiny, hard, unbreakable, spheres

  • All atoms of a given element are identical

  -all carbon atoms have the same chemical and physical properties

  • Atoms of a given element are different from those of any other element

  -carbon atoms have different chemical and physical properties than sulfur atoms
• Dalton’s Atomic Theory

  • Atoms of one element combine with atoms of other elements to form compounds.

  -Law of Constant Composition

  -all samples of a compound contain the same proportions (by mass) of the elements

  -Chemical Formulas
• Dalton’s Atomic Theory

  • Atoms are indivisible in a chemical process.

  -all atoms present at beginning are present at the end

  -atoms are not created or destroyed, just rearranged

  -atoms of one element cannot change into atoms of another element

  -cannot turn Lead into Gold by a chemical reaction
• Formulas Describe Compounds

  • a compound is a distinct substance that is composed of atoms of two or more elements
  • describe the compound by describing the number and type of each atom in the simplest unit of the compound

  -molecules or ions

  • each element represented by its letter symbol
  • the number of atoms of each element is written to the right of the element as a subscript

  -if there is only one atom, the 1 subscript is not written

  • polyatomic groups are placed in parentheses

  -if more than one
• Are Atoms Really Unbreakable?

  • J.J. Thomson investigated a beam called a cathode ray
  • he determined that the ray was made of tiny negatively charged particles we call electrons
  • his measurements led him to conclude that these electrons were smaller than a hydrogen atom
  • if electrons are smaller than atoms, they must be pieces of atoms
  • if atoms have pieces, they must be breakable
  • Thomson also found that atoms of different elements all produced these same electrons
• The Electron

  • Tiny, negatively charged particle
  • Very light compared to mass of atom

  -1/1836th the mass of a H atom

  • Move very rapidly within the atom
• Thomson’s Plum Pudding Model

  • Atom is breakable!
  • Atom has structure
  • Electrons suspended in a positively charged electric field

  -must have positive charge to balance negative charge of electrons and make the atom neutral

  • mass of atom due to electrons
  • atom mostly “empty” space

  -compared size of electron to size of atom
• Rutherford’s Gold Foil Expt

  • How can you prove something is empty
  • put something through it

  -use large target atoms

  -use very thin sheets of target so do not absorb “bullet”

  -use very small particle as bullet with very high energy

  -but not so small that electrons will affect it

  • bullet = alpha particles, target atoms = gold foil

  -α particles have a mass of 4 amu & charge of +2 c.u.

  -gold has a mass of 197 amu & is very malleable
• Rutherford’s Results

  • Over 98% of the α particles went straight through
  • About 2% of the α particles went through but were deflected by large angles
  • About 0.01% of the α particles bounced off the gold foil
• Rutherford’s Nuclear Model

  • The atom contains a tiny dense center called the nucleus

  -the volume is about 1/10 trillionth the volume of the atom

  • The nucleus is essentially the entire mass of the atom
  • The nucleus is positively charged

  -the amount of positive charge of the nucleus balances the negative charge of the electrons

  • The electrons move around in the empty space of the atom surrounding the nucleus
• Structure of the Nucleus

  • The nucleus was found to be composed of two kinds of particles
  • Some of these particles are called protons

  -charge = +1

  -mass is about the same as a hydrogen atom

  • Since protons and electrons have the same amount of charge, for the atom to be neutral there must be equal numbers of protons and electrons
  • The other particle is called a neutron

  -has no charge

  -has a mass slightly more than a proton
• The Modern Atom

  • We know atoms are composed of three main pieces - protons, neutrons and electrons
  • The nucleus contains protons and neutrons
  • The nucleus is only about 10^-13 cm in diameter
  • The electrons move outside the nucleus with an average distance of about 10^-8 cm

  -therefore the radius of the atom is about 10^5 times larger than the radius of the nucleus
• Isotopes

  • All atoms of an element have the same number of protons
  • The number of protons in an atom of a given element is the same as the atomic number

  -found on the Periodic Table

  • Atoms of an element with different numbers of neutrons are called isotopes
  • All isotopes of an element are chemically identical

  -undergo the exact same chemical reactions

  • Isotopes of an element have different masses
  • Isotopes are identified by their mass numbers

  -mass number = protons + neutrons
• Elements

  • Arranged in a pattern called the Periodic Table
  • Position on the table allows us to predict properties of the element
  • Metals

  -about 75% of all the elements

  -lustrous, malleable, ductile, conduct heat and electricity

  • Nonmetals

  -dull, brittle, insulators

  • Metalloids

  -also know as semi-metals

  -some properties of both metals & nonmetals
• The Modern Periodic Table

  • Elements with similar chemical and physical properties are in the same column
  • Columns are called Groups or Families
  • Rows are called Periods
  • Each period shows the pattern of properties repeated in the next period
• The Modern Periodic Table

  • Main Group = Representative Elements

  -“A” columns

  • Transition Elements

  -all metals

  • Bottom rows = Inner Transition Elements = Rare Earth Elements

  -metals

  -really belong in Period 6 & 7
• Allotropes

  • Many solid nonmetallic elements can exist in different forms with different physical properties, these are called allotropes
  • the different physical properties arise from the different arrangements of the atoms in the solid
  • Allotropes of Carbon include

  - diamond

  - graphite

  - buckminsterfullerene
• Electrical Nature of Matter

  • Most common pure substances are very poor conductors of electricity

  - with the exception of metals and graphite

  - Water is a very poor electrical conductor

  • Some substances dissolve in water to form a solution that conducts well - these are called electrolytes
  • When dissolved in water, electrolyte compounds break up into component ions

  - ions are atoms or groups of atoms that have an electrical charge
• Ions

  • ions that have a positive charge are called cations

  -form when an atom loses electrons

  • ions that have a negative charge are called anions

  -form when an atom gains electrons

  • ions with opposite charges attract

  -therefore cations and anions attract each other

  • moving ions conduct electricity
  • compound must have no total charge, therefore we must balance the numbers of cations and anions in a compound to get 0 total charge
• Atomic Structures of Ions

  • Metals form cations
  • For each positive charge the ion has 1 less electron than the neutral atom

  -Na = 11 e-, Na+ = 10 e-

  -Ca = 20 e-, Ca+2 = 18 e-

  • Cations are named the same as the metal

  sodium Na → Na+ + 1e- sodium ion

  calcium Ca → Ca+2 + 2e- calcium ion

  • The charge on a cation can be determined from the Group number on the Periodic Table for Groups IA, IIA, IIIA

  -Group 1A ⇒ +1, Group 2A ⇒ +2, (Al, Ga, In) ⇒ +3
• Atomic Structures of Ions

  • Nonmetals form anions
  • For each negative charge the ion has 1 more electron than the neutral atom

  -F = 9 e-, F- = 10 e-

  -P = 15 e-, P3- = 18 e-

  • Anions are named by changing the ending of the name to -ide

  fluorine F + 1e- → F- fluoride ion

  oxygen O + 2e- → O2- oxide ion

  • The charge on an anion can be determined from the Group number on the Periodic Table

  -Group 7A ⇒ -1, Group 6A ⇒ -2