AP Chemistry
Ch. 2 Sections 2.5 – 2.7 Notes
The Modern View of Atomic Structure: An Introduction, Molecules and Ions, and An
Introduction to the Periodic Table
The Atom
• Tiny nucleus: diameter of about 10-13 cm.
• Electrons: move about the nucleus at an average distance of about 10-8 from it.
• If the nucleus were the size of the ball bearing, a typical atom would be the size of the
stadium.
• Nucleus contains protons (positive charge) and neutrons (neutral).
• Electrons (negative charge) surround the nucleus.
• Nucleus
• small size with extremely high density
• accounts for most of atom’s mass
• piece of nuclear material about the size of a pea would have a mass of 250 million tons
• See table 2.1 for mass and charge of the electrons, protons, and neutrons
• Electrons
• account for most of atom’s volume
• their number and arrangement account for an atom’s chemical behavior
• involved when different atoms bond
• Atomic Number
• number of protons in the nucleus of an atom
• the atomic number identifies the element
• has the symbol, Z
• Mass Number
• total number of protons and neutrons in the nucleus of an atom
• has the symbol, A
• mass number = number of protons + number of neutrons
• Isotopes
• atoms with the same number of protons but different number of neutrons
• Isotopes are defined by two numbers, its atomic number and its mass number.
• Isotopes are represented by nuclear symbols or by using hyphen notation.
• Nuclear Symbol:
atomic number
•
•
A
ZX
Hyphen notation:
Element name – mass number
Example: symbol for one particular isotope of sodium
23
11 Na
•
•
where X is element symbol, A is mass number, and Z is
where Na is the element symbol, 23 is the mass number,
and 11 is the atomic number
Two isotopes of sodium.
Both have 11 protons and 11 electrons, but differ in the number of neutrons.
Molecules and Ions
• Chemical bonds: forces that hold atoms together in compounds.
• Covalent bond: results from sharing of electrons between different atoms; the resulting
collection of atoms is called a molecule.
• Simplest method to represent a molecule is a chemical formula.
• Chemical formula: the symbols for the elements are used to indicate the types of atoms
present and subscripts are used to indicate the relative numbers of atoms.
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Chemical formula for methane: CH4
Different representations of molecules.
Structural formula for methane. Individual bonds are shown. May or may not indicate the
actual structure of the compound. Refer to Figure 2.16 on p. 53.
Space-filling model of methane. This type of model shows the relative sizes of atoms as well
as their relative orientation in the molecule. Refer to Figure 2.17 on p. 53
Ball-and-stick model of methane. Refer to Figure 2.18 on p. 53.
A second type of bond results from attractions between ions.
An ion is an atom or groups of atoms that has a net positive or negative charge resulting from
a loss or gain of electrons.
The best known ionic compound is common table salt, or sodium chloride, which forms
when neutral sodium and chlorine react.
When an ion forms, electrons are transferred from one atom to another (the number lost must
equal the number gained).
With one electron transferred, sodium has 11 protons and only 10 electrons so has a net 1+
charge – it has become a positive ion.
A positive ion is called a cation.
Represented in shorthand form: Na → Na+ + eIf an electron is added to chlorine, it has 17 protons and 18 electrons which produce a net 1charge – it has become a negative ion.
A negative ion is called an anion.
Represented in shorthand form: Cl + e- → ClAnions and cations = opposite charges = attraction
Ionic bonding: force of attraction between oppositely charged ions.
Solid composed of oppositely charged ions is called an ionic solid or a salt.
Polyatomic ion: an ion composed of two or more atoms.
Can form ionic bonds with other simple ions or with other polyatomic ions.
Introduction to the Periodic Table
• Periodic Table: a chart showing all the elements arranged in columns with similar chemical
properties.
• Letters in the boxes are the symbols for the elements.
• The number above the symbol is the atomic number (number of protons).
• Metals
• Most of the elements are metals (found to the left of the staircase in the periodic table).
• Characteristic physical properties:
•
efficient conduction of heat and electricity
•
malleability (they can be hammered into thin sheets)
•
ductility (they can be pulled into wires)
•
(often) a lustrous appearance
•
all are solids at room temperature except mercury
• Tend to lose electrons to form positive ions.
• Nonmetals
• Found to the right of the staircase on the periodic table.
• Encountered mostly in the form of compounds or mixtures of compounds.
• Poor conductors of heat and electricity.
• Neither malleable nor ductile.
• Many are gases at room temperature and many are solids (Bromine is the only liquid).
• Usually exist as molecules in their elemental form.
• Tend to gain electrons to form negative ions.
• Metalloids
• Grouped along the staircase.
• Properties lie between those of metals and nonmetals.
• Tend to be semiconductors; they conduct electricity, but not nearly as well as metals.
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Generally, metalloids behave as nonmetals, both chemically and physically.
Periodic Table
Horizontal rows are called periods.
Vertical columns are called groups or families.
Elements in same column have similar chemical properties.
Group 1 elements are called the alkali metals.
very active elements
readily form ions with a 1+ charge
Group 2 elements are called the alkaline earth metals.
reactive and form ions with a 2+ charge
Groups 3 through 12 are called the transition metals.
form colored ions
variable oxidation states
Group 17 are called the halogens.
very reactive nonmetals
form ions with 1- charge
exist as diatomic molecules
Group 18 are called the noble gases.
exist as monatomic (single-atom) gases
little chemical reactivity
The elements in the two long rows below the main body of the periodic table are called the
inner transition elements.
Elements 58-71 are called the lanthanide elements because they follow lanthanum (Z = 57).
Elements 90-103 are called the actinide elements because they follow actinium (Z = 89).