Transcript Chapter 2 cont’

Chapter 2 cont’
Atoms and Elements
Recall:
• Atomic Number
 Number of protons
 Z
• Mass Number
 Protons + Neutrons
 Whole number
 A
• Abundance = relative amount (in %) found in a
sample
Reacting Atoms
when elements undergo chemical reactions, the reacting elements
do not turn into other elements
◦ Dalton’s Atomic Theory
 since the number of protons determines the kind of element, the
number of protons in the atom does not change in a chemical
reaction
 however, many reactions involve transferring electrons from one
atom to another
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Charged Atoms
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when atoms gain or lose electrons, they acquire a charge
charged particles are called ions
when atoms gain electrons, they become negatively charged ions,
called anions (Cl-)
when atoms lose electrons, they become positively charged ions,
called cations (Na+)
ions behave much differently than the neutral atom
◦ e.g., The metal sodium, made of neutral Na atoms, is highly
reactive and quite unstable. However, the sodium cations, Na+,
found in table salt are very nonreactive and stable
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Ionic Charge from Group Numbers
•
The charge of a positive ion is equal to its Group number.
Group 1A(1) = 1+
Group 2A(2) = 2+
Group 3A(3) = 3+
•
The charge of a negative ion is obtained by subtracting 8 or 18
from its Group number.
Group 6A(16) =
6-8
or 16 - 18
= 2-
= 2-
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Atomic Structures of Ions
Nonmetals form anions
 For each negative charge, the ion has 1 more electron than the
neutral atom
◦ F = 9 p+ and 9 e-, F ─ = 9 p+ and 10 e
•Metals form cations
•For each positive charge, the ion has 1 less electron than the
neutral atom
Na atom = 11 p+ and 11 e-, Na+ ion = 11 p+ and 10 e-
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Examples

Predict the charge and identify the number of proton and
electron from that ion
◦ Mg 
◦ Al 
◦ O
◦ P
Collection Terms
A collection term states
a specific number of items.
• 1 dozen donuts
= 12 donuts
• 1 ream of paper
= 500 sheets
• 1 case
= 24 cans
In chemistry, how do chemists know the number of atoms
in an element or compound?
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A Mole of Atoms
A mole is
• a unit of measurement used in chemistry to express amounts
of a chemical substance, the same number of particles as there
are carbon atoms in 12.0 g of carbon.
• a collection term “dozen”
1 mole = NA = 6.022 x 1023 of anything
Avogadro’s Number = 6.0221421 x 1023
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Relationship Between Moles and Mass
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The mass of one mole of
atoms is called the molar
mass
The molar mass of an
element, in grams, is
numerically equal to the
element’s atomic mass, in
amu
E.g
1 H atom = 1.01 amu
1 mol H = 1.01g
6.022 x 1023 atoms of H = ???? g
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Examples
Give the molar mass for each
A. 1 mole of Li atoms
=
________ g
B. 1 mole of Co atoms
=
________g
C. 1 mole of S atoms
=
________g
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Converting between mass, moles
and atoms
gC
mol C
mol C
gC
gC
mol C
atoms
Examples

Assuming all pennies are pure copper and each has a mass of 2.5 g
◦ Without doing calculation, determine the number atoms of
copper present in 1 mole
◦ How many pennies does it take to make a mole?
Examples

In a 3.0 moles of O2 molecules
◦ How many oxygen molecules are there ?
◦ How many oxygen atoms are there?
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Calculate the moles of carbon in 0.0265 g of pencil lead
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Calculate the mass (in grams) of 0.473 moles of titanium