Chapter 4

Chemical Foundations:Elements, Atoms,

and Ions

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Elements

• Over 112 known: 88 found in nature, others are man made.

• Each element has a unique one- or two-letter symbol

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Elements (cont.)

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Elements (cont.)

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Dalton’s Atomic Theory

• Elements are composed of tiny particles called atoms.• 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.

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Dalton’s Atomic Theory

• Atoms of different elements combine to form compounds.

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Dalton’s Atomic Theory

• Law of Constant Composition: all samples of a compound contain the same proportions (by mass) of the elements that form the compound.

• Atoms are indivisible by chemical processes.– All atoms present at beginning are present at the end.– Atoms are not created or destroyed, just rearranged in

chemical reactions.– Atoms of one element cannot change into atoms of

another element.• Cannot turn lead into gold by a chemical reaction• View Early Discoveries

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Formulas Describe Compounds

• A compound is a distinct substance that is composed of atoms of two or more elements.

• Compounds are identified by the number and type of each atom in the simplest unit of the compound.– Molecules or ions

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Writing Formulas of Compounds

• Each element is represented by its symbol.• The number of each type of atom is indicated by a

subscript written to the right of the element symbol.

• If only one atom is present, do not include a subscript.

• If polyatomic groups are present in the molecule, they are written inside parentheses if more than one group is present.

• Examples: H2O, NaCl, KNO3, Mg(NO3)2• View Describing Chemical Formulas

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What Are Atoms Made Of?

• J.J. Thomson investigated a beam called a cathode ray.

• He determined that the ray was made of tiny negatively charged particles we now call electrons.

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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• Atoms of different elements all produce

electrons; therefore electrons are a fundamental unit of atoms.

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The Proton

• If negative electrons exist, must also have positive particles to balance the charge and make the atom electrically neutral

• Assume “Plum Pudding Model” - electrons suspended in a positively charged electric field

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Rutherford’s Nuclear Model

• The atom contains a tiny dense center calledthe nucleus.

– The volume is about 1/10 trillionth the volume of the atom.

• The nucleus is essentially the entire massof 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.

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Components of an Atom

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The Modern Atom

• Atoms are composed of three main pieces: protons, neutrons, and electrons.

• The nucleus contains protons and neutrons.

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The Modern Atom (cont.)

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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.

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Isotopes (cont.)

• 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

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Isotopes of Carbon

• 12C has 6 protons (atomic number ) and 6 neutrons (atomic mass number = 6 + 6 = 12)

• 13C has 6 protons and 7 neutrons (atomic mass number = 6 + 7 = 13)

• 14C has 6 protons and 8 neutrons (atomic mass number = 6 + 8 = 14)

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Elements

• Arranged in a pattern called the periodic table

• Position on the table allows us to predict properties of the element

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Elements (cont.)

• 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

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The Modern Periodic Table

• Elements with similar chemical and physical properties are in the same column.

• Groups (Families) and Periods

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The Modern Periodic Table (cont.)

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The Modern Periodic Table (cont.)

• Main group = representative elements– “A” columns

• Transition elements– All metals

• Bottom rows = inner transition elements = rare earth elements– Metals– Lanthanides and actinides

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Natural States of Elements

• Gases: monatomic gases such as noble gases and diatomic gases such as O2

• Liquids such as Hg or Br2

• Solids such as metals or allotropes of carbon

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Natural States of Elements (cont.)

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Ions

• Cations: ions that have a positive charge– Form when an atom loses electrons

• Anions: ions that have a negative charge– Form when an atom gains electrons

• Ions with opposite charges attract – Therefore cations and anions attract each other

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Ions (cont.)

• 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.

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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 metalsodium Na Na+ + 1e- sodium ioncalcium Ca Ca+2 + 2e- calcium ion

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Atomic Structures of Ions (cont.)

• 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

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Ionic Charges and Compounds

• Charges on the ions of some elements can be predicted from the periodic table.

• Cations and anions usually form simultaneously to yield an ionic compound.

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Ionic Charges and Compounds (cont.)

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Formulas of Ionic Compounds

• The formula of an ionic compound can be determined by balancing the positive charge of the cation(s) with the negative charge of the anion(s) to yield a net charge of zero.

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Periodic Table

• Periodic Table Tutorial• http://www.wwnorton.com/college/

chemistry/gilbert/overview/ch6.htm#covalent_bond