Chapter 4 Atomic Structure
Theories about matter were based on the ideas of Greek philosophers:
Democritus (400 B.C. ) – coins the term “atom” saying matter can be subdivided only as small as an elemental particle.
from Greek “a” means not and “tomos” means cutting
atom means “indivisible”
Aristotle thought that all matter consisted of 4 basic elements:◦earth, air, fire, water
*Their views were not backed by any evidence.
Dalton’s Atomic Theory (1808)
1. All matter is composed of small particles called atoms.
2. Atoms of a given element are identical in size, mass, and other properties.
3. Atoms cannot be subdivided, created, or destroyed.
4. Atoms of different elements combine in simple-whole number ratios to form chemical compounds.
5. In chemical reactions, atoms are combined, separated, or rearranged.
In 1981 we see atoms!
scanning electron microscope
OR(STM – scanning tunneling microscope)
See page. 100 (Ni atoms)
What instrument is used to observe individual atoms?
A: scanning electron microscope (or STM)
B: particle acceleratorC: graduated cylinder
Please make your selection
Democritus, who lived in Greece during the fourth century B.C., suggested thatmatter is made up of tiny particles that cannot be divided. He called these particles
.A) ElectronsB) IonsC) atoms
Please make your selection
Suppose you could grind a sample of the element copper into smaller and smaller
particles. The smallest particle that could no longer be divided, yet still has the chemical properties of copper, is ___________.
a) A copper atomb) An ionc) A proton
Please make your selection
Dalton’s atomic theory included the idea that the atoms of different elements can chemically combine in _______ ratios.
A) Decimal B) FractionC) Partially whole numbersD) Whole numbers
Please make your selection
The modern process of discovery about atoms began with the theories of an Englishschoolteacher named .
A) AristotleB) Albert EinsteinC) John DaltonD) Democritus
Please make your selection
Which of the following is not a part of Dalton’s atomic theory?
a. All elements are composed of atoms.
b. Atoms of the same element are alike.
c. Atoms are always in motion.d. Atoms that combine do so in
simple whole-number ratios.
Please make your selectionDalton theorized that atoms are indivisible
and that all atoms of an element are identical. Scientists now know that
a. Dalton’s theories are completely correct.
b. atoms of an element can have different numbers of protons.
c. atoms are all divisible.d. all atoms of an element are not
identical but they all have the same mass.
Please make your selectionWhich of these statements is included in
Dalton’s atomic theory?a. Chemical reactions occur when atoms
are separated, joined, or rearranged.b. Some but not all elements are
composed of atoms.c. Atoms of the same element can have
different numbers of protons.d. Atoms are divisible.
Please make your selection
The identity of an element can be determined on the basis of which of the following?
a. the number of protons in an atom of the element
b. the number of neutrons in an atom of the element
c. the mass number of the elementd. the atomic mass of the element
What instrument is used to observe individual atoms?
A: scanning electron microscope (or STM)
B: particle acceleratorC: graduated cylinder
Section 2: The Structure of the Atom
consists of: Protons Neutrons Electrons
symbols p+ no e-
location: in nucleus in nucleus outside nucleus
gain/loss: not gained can be can be
gained or lost different number or
lost (isotopes) (forms ions)
consists of: Protons NeutronsElectrons
mass number: 1 amu 1 amu 0 amu
*important # p+ *holds nucleus
*involved in
info: determines together chemical
type of elementreactions
(atomic number)
Please make your selectionWhat is the atomic number and the mass
number of an atom with 11 protons and 12 neutrons?
a. atomic number = 11 and mass number = 12
b. atomic number = 12 and mass number = 11
c. atomic number = 11 and mass number = 23
d. atomic number = 23 and mass number = 12
Nuclear Force:
Very strong, short range force that holds the particles of the nucleus together
Question: What is it about the composition of the nucleus that requires the concept of nuclear forces?
A: Like charges do repel each other, so protons would not be expected to be close to other protons. Nuclear forces prevent the protons from repelling each other.
Question: What are the attraction relationships in the nucleus (application of nuclear forces)?
Answer: proton-proton, neutron-proton, neutron-neutron
Section 4.2Structure of the Nuclear Atom
One change to Dalton’s atomic theory is that atoms are divisible into subatomic particles (p+ n0 e-)
Discovery of the ElectronIn 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively
charged particle: the electron (pg. 106)
Mass of the Electron
1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge
The oil drop apparatus
Mass of the electron is 9.11 x 10-28 g
Conclusions from the Study of the Electron:
a) Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.
b) Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons
c) Electrons have so little mass that atoms must contain other particles that account for most of the mass
Conclusions from the Study of the Electron:
Eugen Goldstein in 1886 observed what is now called the “proton” - particles with a positive charge, and a relative mass of 1 (or 1840 times that of an electron)
1932 – James Chadwick confirmed the existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton
Subatomic Particles
Particle Charge Mass (g) Location
Electron (e-) -1 9.11 x 10-28 Electron
cloud
Proton (p+) +1 1.67 x 10-24 Nucleus
Neutron (no) 0 1.67 x 10-24 Nucleus
Ernest Rutherford’sGold Foil Experiment - 1911
Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil Particles that hit on the detecting screen (film) are recorded
Rutherford’s Findings
a) The nucleus is smallb) The nucleus is densec) The nucleus is positively
charged
Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected
“Like howitzer shells bouncing off of tissue paper!”
Conclusions:
The Rutherford Atomic ModelBased on his experimental evidence:
◦The atom is mostly empty space◦All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus”
◦The nucleus is composed of protons and neutrons (they make the nucleus!)
◦The electrons distributed around the nucleus, and occupy most of the volume
◦His model was called a “nuclear model”
Rutherford’s atom
Section 4.3
IsotopesFor each element there are
different kindsThey are chemically alike & react
the same in chemical reactionsDiffer in mass because there are
different numbers of neutrons in the nucleus.
Isotope hyphen notation:
Element name-dash mass numberExample: carbon-14 where14 is the mass number.(NOTE: mass number = #p+ and
#n0)________ protons________electrons________ neutrons
How many protons, neutrons, and electrons are there in cobalt-60?
_______ protons_______ electrons_______ neutrons
isotope symbols
Contain the symbol of the element, the mass number and the atomic number.
X Massnumber
Atomicnumber
Subscript →
Superscript →
Symbols Find each of these:
a) number of protons
b) number of neutrons
c) number of electrons
d) Atomic number
e) Mass Number
Br80 35
Symbols If an element has 78
electrons and 117 neutrons what is the
a) Atomic number
b) Mass number
c) number of protons
d) complete symbol
Symbols If an element has 91
protons and 140 neutrons what is the
a) Atomic number
b) Mass number
c) number of electrons
d) complete symbol
*Another word for isotope is nuclide.Mass # = p+ + n0
Nuclide p+ n0 e- Mass #
Oxygen - 10
- 33 42
- 31 15
8 8 1818
Arsenic 75 33 75
Phosphorus 15 3116
Hydrogen isotopes have names. All other isotopes are named with the mass number.
Isotope Protons Electrons Neutrons Nucleus
Hydrogen–1 (protium) 1 1 0
Hydrogen-2(deuterium) 1 1 1
Hydrogen-3(tritium)
1 1 2
Measuring Atomic Mass
Instead of grams, the unit we use is the Atomic Mass Unit (amu)
It is defined as one-twelfth the mass of a carbon-12 atom.◦Carbon-12 chosen because of its isotope purity.
Each isotope has its own atomic mass, thus we determine the average from percent abundance.
Atomic Masses pg. 116
Isotope Symbol Composition of the nucleus
% in nature
Carbon-12 12C 6 protons6 neutrons
98.89%
Carbon-13 13C 6 protons7 neutrons
1.11%
Carbon-14 14C 6 protons8 neutrons
<0.01%
Atomic mass is the average of all the naturally occurring isotopes of that element.
Carbon = 12.011
To calculate the average: p. 118
Multiply the atomic mass of each isotope by it’s abundance (expressed as a decimal), then add the results.
If not told otherwise, the mass of the isotope is expressed in atomic mass units (amu)
Example problem:
Element X has two natural isotopes. The isotope with a mass of 10.012 amu (10X) has a relative abundance of 19.91%. The isotope with a mass of 11.009 amu (11X) has a relative abundance of 80.09%. Calculate the atomic mass of this element.
See page 119
Example problem:The relative abundances and atomic
masses are 0.337% (mass = 35.978 amu), 0.063% (mass = 37.963 amu), and 99.600% (mass = 39.962 amu), respectively. Calculate the average atomic mass of argon.