Download Chapter 2 Atoms and Elements

Chapter 2 Atoms and Elements
2.4
The Atom
Dalton’s Atomic Theory
Atoms
♣ Are tiny particles of matter.
♣ Of an element are similar and
different from other elements.
♣ Of two or more different
elements combine to form
compounds.
♣ Are rearranged to form new
combinations in a chemical
reaction.
Subatomic Particles
Atoms contains subatomic particles.
♣ Protons have a positive (+) charge.
♣ Electrons have a negative (-) charge.
♣ Like charges repel and unlike
charges attract.
♣ Neutrons are neutral.
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Rutherford’s Gold-Foil
Experiment
Structure of the Atom
An atom consists
♣ Of a nucleus that
contains protons
and neutrons.
♣ Of electrons in a
large, empty space
around the
nucleus.
Atomic Mass Scale
On the atomic mass scale for subatomic particles
♣ 1 atomic mass unit (amu) is defined as 1/12 of the
mass of the carbon-12 atom.
♣ A proton has a mass of about 1 (1.007) amu.
♣ A neutron has a mass of about 1 (1.008) amu.
♣ An electron has a very small mass, 0.000549 amu.
Particles in the Atom
TABLE 2.6
Learning Check
Identify each statement as describing a
1) proton 2) neutron 3) electron
A. Found outside the nucleus.
B. Has a positive charge.
C. Is neutral.
D. Found in the nucleus.
Atomic Number
The atomic number
♣ Is specific for each element.
♣ Is the same for all atoms of an element.
♣ Is equal to the number of protons in an atom.
♣ Appears above the symbol of an element.
Atomic Number
Symbol
11
Na
Atomic Number and Protons
Examples of atomic number and number of
protons:
♣ Hydrogen has atomic number 1, every H atom
has one proton.
♣ Carbon has atomic number 6, every C atom has
six protons.
♣ Copper has atomic number 29, every Cu atom
has 29 protons.
♣ Gold has atomic number 79, every Au atom has
79 protons.
Learning Check
State the number of protons in each.
A. A nitrogen atom
1) 5 protons
2) 7 protons
3) 14 protons
B. A sulfur atom
1) 32 protons
C. A barium atom
1) 137 protons
protons
2) 16 protons
3) 6 protons
2) 81 protons
3) 56
Number of Electrons in An Atom
An atom
♣ Of an element is electrically neutral; the net
charge of an atom is zero.
♣ Has an equal number of protons and electrons.
number of protons = number of electrons
♣ Of aluminum has 13 protons and 13 electrons.
The net charge is zero.
13 protons (13+) + 13 electrons (13 -) = 0
Mass Number
The mass number
♣ Represents the number of particles in the
nucleus.
♣TABLE
Is equal
to the
2.7
Number of protons + Number of neutrons
Atomic Models
Learning Check
An atom of zinc has a mass number of 65.
A. How many protons are in this zinc atom?
1) 30
2) 35
3) 65
B. How many neutrons are in the zinc atom?
1) 30
2) 35
3) 65
C. What is the mass number of a zinc atom that
has 37 neutrons?
1) 37
2) 65
3) 67
Learning Check
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) 14
2) 16
3) 34
B. Its mass number is
1) 14
2) 16
3) 34
2) Ca
3) Se
C. The element is
1) Si
Isotopes
Isotopes
♣ Are atoms of the same element that have
different mass numbers.
♣ Have the same number of protons, but different
numbers of neutrons.
Nuclear Symbols
For an atom, the nuclear symbol gives the number of
♣ Protons (p+)
♣ Neutrons (n)
♣ Electrons (e-)
16
31
8
15
8 p+
8n
8 e-
15 p+
16 n
15 e-
O
P
65
30
Zn
30 p+
35 n
30 e-
Learning Check
Naturally occurring carbon consists of three
isotopes, 12C, 13C, and 14C. State the number of
protons, neutrons, and electrons in each of the
following.
12C
6
protons
13C
6
14C
6
______
______
______
neutrons ______
______
______
electrons ______
______
______
Learning Check
Write the nuclear symbols for atoms with the
following subatomic particles:
A. 8 p+, 8n, 8e-
___________
B. 17p+, 20n, 17e-
___________
C. 47p+, 60 n, 47 e-
___________
Solution
16O
A. 8p+, 8n, 8eB. 17p+, 20 n, 17e-
8
37Cl
17
C. 47p+, 60n, 47e-
107Ag
47
Learning Check
1. Which of the following pairs are isotopes of the
same element?
2. In which of the following pairs do both atoms
have 8 neutrons?
15X
A. 15X
8
7
B.
12X
6
C.
15X
7
16X
8
14X
6
Solution
B. 12X
6
14X
6
Both nuclear symbols represent isotopes of carbon
with six protons each, but one has 6 neutrons and
the other has 8.
C.
15X
7
16X
8
An atom of nitrogen (7) and an atom of oxygen
(8) each have 8 neutrons.
Isotopes of Magnesium
In naturally occurring
magnesium,
Isotopes
of Mg .
there are three
isotopes
Atomic Mass
23.99 24.98 25.98
% abundance 78.70 10.13 11.17
24Mg
12
25Mg
12
26Mg
12
Isotopes of Sulfur
A sample of naturally
occurring sulfur contains
several isotopes with the
following abundances
Isotope % abundance
32S
95.02
33S
0.75
34S
4.21
36S
0.02
32S, 33S, 34S, 36S
16
16
16
16
Atomic Mass
The atomic mass of an element
♣ Is listed below the symbol of each
element on the periodic table.
♣ Gives the mass of an “average” atom
of each element compared to 12C.
♣ Is not the same as the mass number.
Na
22.99
Some Elements and Their Atomic
Mass
Most elements have two or more isotopes that
contribute to the atomic mass of that element.
TABLE 2.9
32.07 amu
Atomic Mass for Cl
The atomic mass of chlorine
is
♣ Based on all naturally
occurring Cl isotopes.
♣ Not a whole number.
♣ The weighted average
of 35Cl and 37Cl.
Learning Check
Using the periodic table, give the atomic mass of
each element:
A.
calcium
B. aluminum
__________
__________
C. lead
__________
D. barium
__________
E. iron
__________
Solution
Using the periodic table, give the atomic mass of
each element:
A.
calcium
B. aluminum
40.08 amu
26.98 amu
C. lead
207.2 amu
D. barium
137.3 amu
E. iron
55.85 amu
Chapter 2 Atoms and Elements
2.7
Electron Energy Levels
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Electromagnetic Radiation
Electromagnetic radiation
♣ Is energy that travels as waves through
space.
♣ Is described in terms of wavelength and
frequency.
♣ Moves at the speed of light in a vacuum.
speed of light = 3.0 x 108 m/s
Electromagnetic Spectrum
The electromagnetic spectrum
♣ Arranges forms of energy from lower to higher.
♣ Arranges energy from longer to shorter
wavelengths.
♣ Shows visible light with wavelengths from 700400 nm.
Electron Energy Levels
Electrons are arranged in
specific energy levels that
♣ Are labeled n = 1, n = 2, n
= 3, and so on.
♣ Increase in energy as n
increases.
♣ Have the electrons with the
lowest energy in the first
energy level (n=1)closest to
the nucleus.
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Energy Level Changes
♣ An electron absorbs
energy to “jump” to a
higher energy level.
♣ When an electron falls to
a lower energy level,
energy is emitted.
♣ In the visible range, the
emitted energy appears
as a color.
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Energy Emitted
Learning Check
In each of the following energy level changes,
indicate if energy is
1) absorbed
2) emitted 3) not changed
A. An electron moves from the first energy level (n
=1 ) to the third energy level (n = 3).
B. An electron falls from the third energy level to
the second energy level.
C. An electron moves within the third energy level.
Solution
A. An electron in an atom moves from the first
energy level (n =1 ) to the third energy level (n =
3).
1) absorbed
B. An electron falls from the third energy level to
the second energy level.
2) emitted
C. An electron moves within the third energy level.
3) not changed
Energy Diagram for Sublevels