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Copyright © Houghton Mifflin Company. All rights reserved. 4 | 1

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.


Symbols for the Elements

• Symbolized with one or two letters

• First letter is capitalized, second is smallCarbon: C

Bromine: Br

• Common elements and symbols in Table 4.3


Elements

• Robert Boyle (1627-1691)

• Elements: cannot be broken down into atwo or more simpler substances

• 155 elements: 88 natural



• Atoms of different elements combine to form compounds.



• 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


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


J. J. Thomson’s “plumb pudding” model of the atom (1910)

• Atom contains positive particles that balance the negative charge of the electrons.


Rutherford’s Gold Foil Experiment

• www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf


Rutherford’s Nuclear Model (1911)

• The atom contains a tiny dense center calledthe nucleus.

• The nucleus is essentially the entire massof the atom.

• The nucleus is positively charged.– It is composed of protons (positive charge) and

neutrons (no charge)

– balances the negative charge of the electrons.

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


The Modern Atom (cont.)


Components of an Atom


The Modern Atom

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

• The nucleus contains protons and neutrons.the hydrogen atom (H)

the helium atom (He)


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.


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 = # of protons + # of neutrons

• Isotope symbols:

X = the symbol of the elementA = the mass numberZ = the atomic number (number of protons)

A - Z = # of neutrons

XAZ


Isotope Examples

Symbol Number of

Protons

Number of

Electrons

Number of

Neutrons

1 1 0

1 1 1

1 1 2

H11

H21

H31


Other Examples

Symbol Number of

Protons

Number of

Electrons

Number of

NeutronsS

34

16

Cr54

26

N14

7


The Modern Periodic Table


Properties of Metals

• Good conductors of heat and electricity

• Malleability (thin sheets)

• Ductility (wires)

• Shiny

• All are solids at room temperature except mercury (Hg)


Properties of Nonmetals

• Poor conductors

• Not malleable or ductile

• Exist in various physical states:

solids – carbon, phosphorus

liquids – bromine

gases – hydrogen, oxygen, helium


Metalloids

• Show a mixture of metallic and nonmetallic properties

• Examples:


Problem

Give the symbol and classify as a metal, nonmetal or metalloid:

• Silver

• Sulfur

• Silicon

• Barium


Natural States of Elements

• Most elements are solids at room temperature.• Group 8 Noble gases:

• Diatomic molecules:


Diatomic Molecules

Element Elemental State (25oC) Molecule

hydrogen colorless gas H2

nitrogen colorless gas N2

oxygen pale blue gas O2

fluorine pale yellow gas F2

chlorine pale green gas Cl2

bromine reddish brown liquid Br2

iodine purple solid I2


Allotropes of Carbon


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

• Cations and anions attract each other.


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


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


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.

• When ions are formed the number of protons and neutrons does not change.

• There must be a net charge of zero.


Ionic Charges and Compounds (cont.)


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.

• Ba2+ and Cl-

• K+ and P3-

• Ca2+ and O2-

• Mg2+ and N3-

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