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.