Download Chemistry Chapter 3

Chemistry Chapter 3
Atoms: The Building Block of
Matter
3:1 The Atom: From Philosophical
Idea to Physical Theory
The idea of the atom was first devised by
Democritus in 530 B.C.
In 1808, an English school teacher and
scientist named John Dalton proposed
the modern atomic theory.
Dalton’s Atomic Theory
All matter is made of atoms
All atoms of any element are the same
Atoms of different elements are different (size, mass, and other
properties)
Atoms of different elements can combine to form compounds
In chemical reactions, atoms are not made, destroyed, or changed
(subdivided)
In any compound, the numbers and kinds of atoms remain the
same – atoms combine in whole number ratios
In a chemical reaction atoms are combined separated or
rearranged
Changes
All atoms of any element are not always
the same – different mass
Atoms can be changed (subdivided) –
quarks
Modern Atomic Theory
All matter is composed of atoms
Atoms of any one element differ in
properties from atoms of another element
3.2 Structure of the Atom
Atom is the smallest particle of an element
that retains the chemical properties of that
element
J.J. Thomson’s cathode ray experiment
revealed the existence of the electron
Milikan’s oil drop experiment confirmed that
electron had a negative charge & a very
small mass
Earnest Rutherford’s gold foil experiment
revealed the nucleus and that the nucleus
had a positive charge
In 1932, James Chadwick discovered a
third type of sub-atomic particle which he
named the neutron. Neutrons help
stabilize the protons in the atom's nucleus.
Quiz:
1. Dalton's idea that all matter is composed of atoms was
first introduced in ancient Greece by the philosopher
named?
A. Hippocrates. B. Aristotle. C. Democritus. D. Pythagoras.
2. Magnesium will react with bromine to form a compound
called magnesium bromide, MgBr2. If 10.00 g of
magnesium is mixed with 65.75 g of bromine and allowed
to react, you would expect the mass of the products of the
reaction to be?
A. 75.75 g. B. less than 75.75 g. C. more than 75.75 g. D.
unpredictable.
3. According to Dalton's atomic theory, atoms cannot ?
A. have mass. B. be destroyed. C. combine with other
atoms. D. differ from one element to another.
4. Which of the following statements describes a part of the
atomic theory that has been modified in the 200 years
since Dalton proposed it?
A. All matter is composed of atoms. B. Atoms cannot be
subdivided, created, or destroyed. C. In chemical
reactions, atoms are combined, separated, or rearranged.
D. Nothing has been modified.
5. Since Dalton proposed the atomic theory, scientists have
discovered that atoms of a given element
A. can have different masses. B. all have the same mass.
C. do not combine in whole-number ratios with atoms of
other elements. D. consist of smaller atoms.
3.3 Counting Atoms
– ATOMIC NUMBER (Z) the number of
protons an atom contains in its nucleus.
ex:
Carbon Z=6 Oxygen Z=8 Uranium Z=92
– · Different elements have different numbers
of neutrons in their nuclei but all
atoms of the same element always have
the same number of protons.
– · Atoms of the same element with different
numbers of neutrons are called nuclides or
isotopes.
– · Nuclides differ only in mass. They act alike
chemically.
–·
–·
Carbon-12
Carbon-14
6 p+ and 6 no
6 p+ and 8 no
MASS NUMBER (atomic mass)
symbol = A



the total number of nucleons in
an atom
Therefore: A = p+ + no
To determine the number of
neutrons in an atom,
A = p + + no
- Z = p+
no
– Atomic Mass Unit (u)
–
–
relative mass - mass of an atom
compared to the mass of C-12
The masses recorded on the periodic table
are averages for all the known isotopes of
that element.
Ex: In a sample of 200 carbon
atoms, 199 are C-12 and 1 is C-14.
What is the average atomic mass of
carbon?
Avg. A = (mass x abundance) + (mass x abundance) + ...
# of atoms in the sample
Avg. A = (12 x 199) + (14 x 1)
200
= 12.01
– Mole (mol)
–
–
a group of 6.02 x 1023 atoms
a mole of any type of atom has a mass in
grams equal to the atomic mass on the
periodic table
a mole of C has a mass of 12 g
a mole of O has a mass of 16 g
What is a mole?
SI base unit for amount of a
substance
Avogadro’s Number of particles (NA)
602 000 000 000 000 000 000 000
particles
6.02 x 1023 particles
1 mol
Molar Mass
Mass of 1 mole of a pure substance
is called the molar mass of that
substance:
– Atoms = atomic mass
– Molecules = molecular mass
– Formulas units = formula mass
– Ions = formula mass
1 mole of Carbon = 12.0 g
1 mole of H20 = 18.0 g
Mole Conversions:
Example 1: How many moles are in 48.0 g of C ?
? mol C = 48.0 g C 1 mol C
12.0 g C
= 4.00 mol C
Mole Conversions:
Example 2: How many grams are in 6.00 mol of
HCl ?
? g HCl = 6.00 mol HCl 36.5 g HCl
1.0 mol HCl
= 219 g HCl
Mole Conversions:
Example 3: How many atoms are in
3.00 mol of N?
23 atoms
6.02
x
10
? atoms N = 3.00 mol N
1.0 mol N
= 1.81 x 1024 atoms N
Mole Conversions:
Example 4: How many molecules are in
90.0 g of H20?
? mole. H20 = 90.0 g H20 1 mol H20 6.02 x 1023 mole.
18.0 g H20 1.0 mol H20
= 3.01 x 1024 molecules H20
Problems
P. 87 – 17 – 24, 28