V.Montgomery & R.Smith 1

Dalton’s Atomic Theory (1808)1. All matter is made of tiny indivisible particles

called atoms. 2. Atoms of the same element are identical. The

atoms of any one element are different from those of any other element.

3. Atoms of different elements can combine with one another in simple whole number ratios to form compounds.

4. Chemical reactions occur when atoms are separated, joined, or rearranged;however, atoms of one element are not changed into atoms of another by a chemical reaction.

Summary of the Atom• atoms are the smallest particles that can be

uniquely associated with an element• each element has unique atoms • atoms are composed of e-, p and n• atoms are electrically neutral (# of e- = # of p)• for a single element, isotopes differ only in

number of n (neutrons)• atoms have characteristic masses (atomic

weights)• atoms combine with one another in definite,

whole number proportions to make compounds

~ 10-10 m

nucleusMass 9 x 10Mass 9 x 10-31-31 kg kg Mass > 10Mass > 10-26-26 kg kg

electron

~ 1 – 7 x 10-15 m (1 – 7 fermi)

atoms are dominantly empty space:

If an oxygen atom had a total radius of 100 km, the nucleus

would be a ~1 m diameter sphere in

the middle.

electron orbits

The Spacious Atom

In a simplistic model, electrons float around the nucleus in energy levels called shells.

As the number of

electrons increases,

they start to fill shells

farther out from the

nucleus.

In most cases,

electrons are lost or

gained only from the

outermost shell.

electron orbits

Electrons in Orbit

AtomAtom

NucleusNucleus

The Nuclear Modelof the atom

• Mass of an electron is approximately 1/1840th of a

proton or neutron.

• Mass of a neutron is very close to the mass of a proton.

• 1 atomic mass unit (amu or u)=

1.66054x10-24g

• 1 g= 6.022 x1023amu• amu is defined by assigning a mass of exactly 12

amu to an atom of C-12 isotope.

Subatomic Particles

Particle Symbol Charge RelativeMass

Electron e- 1- 0

Proton p+ 1+ 1

Neutron n 0 1

Atomic Number(nuclear charge)

11

Na

Atomic Number

Symbol

All atoms of an element have the same number of protons

11

Na

11 protons

Sodium

Number of Electrons

If an atom is neutral ;The net charge is zeroNumber of protons = Number of electronsAtomic number = Number of electrons in a

neutral atom

Ions• Have a net electrical charge since the total

number of electrons isn’t equal to the number of protons.

• Can be anions (e->p+ ;formed as a result of gaining electrons; negatively charged), cations (p+>e-;formed as a result of losing electrons; positively charged).

• e- + q(charge of the ion) = proton number

• In chemical rxns, atom never gain or lose protons. It’s the interaction of electrons.

Mass Number(nucleon number)

Counts the number

of

protons and neutrons

in an atom

A= p+ + n0

Atomic Symbols

Show the mass number and atomic number

Give the symbol of the element

23Na

11

(Z)atomic number

(A)mass number

sodium-23

Notation for Atoms

12C13C C

only one isotope of carbon

all isotopes of carbon

only one isotope of carbon

Basic Definitions• “atomic number” = number of protons in the

nucleus;

• “mass number” = sum of protons + neutrons in the nucleus

• “isotopic mass” = mass of a single isotope

More Atomic Symbols

16 31 65

O P Zn

8 15 30

8 p+ 15 p+ 30 p+

8 n 16 n 35 n8 e- 15 e- 30 e-

Isotopes

Atoms with the same number of protons,

but different numbers of neutrons.

Atoms of the same element (same atomic

number) with different mass numbers

Isotopes of chlorine

35Cl 37Cl17 17

chlorine - 35 chlorine - 37

• Since both isotopes have the same number of protons and electrons, they have identical chemical properties.

• Since physical properties depend on the mass of particles as well, isotopes will often have slightly different physical properties such as density, rate of diffusion etc.

Natural abundances of isotopes

Natural chlorine contains :

75 % 35Cl and 25 % 37Cl . 17 17

• These percentages are known as the natural abundances of the isotopes and determined by “mass spectrometry.”

Relative atomic mass

• is weighted mean.

5.35100

)37x25()35x75(

Relative atomic mass has no unit since the atomic mass unit, amu is cancelled out in calculation w/ respect to C-12.

Exercise 1

• The molar mass of iridium is 192.2 g / mol. What are the naturally occurring percentages of the two isotopes of Ir-191 and Ir-193?

solution

40x

2.192100

)x100(193)x(191

Iridium is a mixture of 40% 191Ir and 60 % 193Ir.

Isotopes of Hydrogen element

H

H

H

11

21

31

has the biggest abundance in nature.

Radioactive isotopes

• Are produced by exposing the natural element to a flux(flow) of neutrons in a nuclear reactor. The nucleus of an atom captures an additional neutron and form radioisotope.

Usages of radioisotopes1. The rate of radioactive decay is used to date

objects (C-14). Naturally occurring C has a fixed proportion of

C-14 due to exchange w/ C in the atmosphere. When the plant is dead, the exchange stops & the proportion of C-14 starts to decrease in the plant due to radioactive decay. This decayed amount is used to date the plant. After about 5,700 yrs, the proportion of C-14 falls to about half its initial value.

2. as tracers.Radioactive isotope reacts chemically &

biologically. For example the activity of thyroid gland can be measured w/ monitoring the increase in radioactivity of the gland after taking a drink including iodine radioisotopes (I-125 and I-131) since the thyroid gland absorbs the radioactive iodine when it works.

Usages of radioisotopes

3. Source of gamma rays and therefore, source intense radioactivity.

Cobalt-60 is an example of such a radioactivity source. It’s used in radiation treatment for cancer and industrially as well

Usages of radioisotopes

• A mass spectrometer is an instrument which separates particles according to their masses , records the relative proportions of these, and determine natural abundances of the isotopes of an element. Therefore, it also allows us to calculate the atomic mass of an element.

• The most accurate way for determining atomic and molecular weights is provided by mass spectrometer.

Mass spectrometry

• mass spectrometer, invented by the English physicist Francis William Aston (1877-1945) when he was working in Cambridge with J. J. Thomson. It was in his use of this instrument that the existence of isotopes of elements was discovered.

• Aston eventually discovered many of the naturally occurring isotopes of non-radioactive elements.

• He was awarded the Nobel Prize for Chemistry in 1922.

Mass spectrometry

Mass spectrometry

A B

C

DE

accelerating

F

A: a gaseous sample is very slowly

introduced to the mass spectrometer.

B: atoms/molecules are bombarded by a stream of high energy electrons to produce positive ions, mostly w/ a 1+ charge. These electrons collide w/ electrons in the particle knocking them out and leaving a positive ion.

C: positively charged ions are accelerated high enough to make the particles pass through the slits and magnetic field by high electrical voltage on the negatively charged grid. With the slits, the ions were made a beam of ions.

D: Fast moving ions enter a magnetic field

produced by an electromagnet. Electrons are deflected by a magnetic field into a curved path. The deflection of the ions depends on “charge to mass ratio(q/m). ”The more massive the ion, the less the deflection. The ions w/ equal mass and charge will deflect the same.

E: By changing the strength of the magnetic field or the accelerating voltage on the negatively charged grid, ions of varying masses can be made to enter the detector at the end of the instrument.

• On the detector, ions are collected on a

metal plate and the current flows through the metal plate to neutralise the ions and this current is recorded.

• In this way, the relative abundances of ions of different masses in the sample can be determined and put into a graph called “mass spectrum.”

F: The mass spectrometer must be at a high vacuum for its correct operation and its correct operation depends on particles being able to pass through it w/o colliding with any other particles.

• A: vapourised sample introduced

• B: ionization by electron bombardment

• C: Positive ions accelerated by electrical field

• D: ions deflected by a magnetic field

• E: detector records ions of a particular mass

• F: vacuum prevents molecules colliding

• Mass spectrometer is used to identify the molecular structure of a compound or analyze mixtures of substances.

• When a molecule loses an electron, it falls apart, forming fragments. Mass spectrometer measures the mass of these fragments, producing a chemical “fingerprint” of the molecule and providing clues about how the atoms were connected together in the molecule.

Atomic weight measurements

How was the atomic weight measured?

• By mass spectrometry– This also measures

% natural abundance

for a given isotope

Atomic weight calculation There are three naturally occuring isotopes

of neon (Ne):

20Ne isotopic mass = 19.99244018 amu21Ne isotopic mass = 20.9938467 amu22Ne isotopic mass = 21.9913855 amu

the atomic weight is reported in text as:20.1797 amu

Learning Check 1

Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms.

12C 13C 14C 6 6 6

#P _______ _______ _______

#N _______ _______ _______

#E _______ _______ _______

Solution

12C 13C 14C 6 6 6

#P __6___ _ 6___ ___6___

#N __6___ _ _7___ ___8___

#E __6___ _ 6___ ___6___

Learning Check 2

An atom of zinc has a mass number of 65.

A. Number of protons in the zinc atom

1) 30 2) 35 3) 65

B. Number of neutrons in the zinc atom

1) 30 2) 35 3) 65

C. What is the mass number of a zinc isotope

with 37 neutrons?

1) 37 2) 65 3) 67

Solution

An atom of zinc has a mass number of 65.

A. Number of protons in the zinc atom

1) 30

B. Number of neutrons in the zinc atom

2) 35

C. What is the mass number of a zinc isotope

with 37 neutrons?

3) 67

Learning Check 3

Write the atomic symbols for atoms with the following:

A. 8 p+, 8 n, 8 e- ___________

B. 17p+, 20n, 17e- ___________

C. 47p+, 60 n, 47 e- ___________

Solution

16OA. 8 p+, 8 n, 8 e- 8

B. 17p+, 20n, 17e- 37Cl 17

C. 47p+, 60 n, 47 e- 107Ag 47

Learning Check 4

An atom has 14 protons and 20 neutrons.A. Its atomic number is

1) 14 2) 16 3) 34

B. Its mass number is1) 14 2) 16 3) 34

C. The element is1) Si 2) Ca 3) Se

D. Another isotope of this element is

1) 34X 2) 34X 3) 36X 16 14 14

Solution

An atom has 14 protons and 20 neutrons.A. It has atomic number

1) 14

B. It has a mass number of3) 34

C. The element is1) Si

D. Another isotope of this element would be

3) 36X 14

Masses of Atoms

A scale designed for atoms gives their small

atomic masses in atomic mass units (amu)

An atom of 12C was assigned an exact mass of

12.00 amu

Relative masses of all other atoms was

determined by comparing each to the mass of 12C

An atom twice as heavy has a mass of 24.00

amu. An atom half as heavy is 6.00 amu.

Average atomic mass(atomic weight)

• “atomic weight” = average mass of an atom calculated from the masses and natural abundances of all isotopes

(use atomic weights to calculate the molecular weights of compounds from their constituent elements!)

Atomic Mass

Average atomic mass is based on all the

isotopes and their abundance %

Atomic mass is not a whole number

Na22.99

Calculating Atomic Weight or Mass

Percent(%) abundance of isotopes

Mass of each isotope of that element

Weighted average =

mass isotope1(%) + mass isotope2(%) + …

100 100

• Naturally occurring C is composed of 98.93

% 12C and 1.07 % 13C. The masses of these nuclides are 12 amu (exactly) and 13.00335 amu, respectively.

amu01.12100

)amu00335.13x07.1()amu12x93.98(

Average atomic mass(atomic mass) or atomic weight

Atomic Mass of Magnesium

Isotopes Mass of Isotope Abundance 24Mg = 24.0 amu 78.70%

25Mg = 25.0 amu 10.13%

26Mg = 26.0 amu 11.17%

Atomic mass (average mass) Mg = 24.3 amu

Mg24.3

Atomic mass calculation

How was the atomic mass calculated?

• multiply each isotopic mass by the reported natural abundance for the isotope, then:

• add these individual contributions for each isotope to get the average atomic mass for the element

Atomic mass calculation

There are three naturally occuring isotopes of neon (Ne):20Ne mass # = 19.99244018 amu (90.51%)21Ne mass # = 20.9938467 amu (0.27%)22Ne mass # = 21.9913855 amu (9.22%)

the atomic mass is reported in text as:

20.1797 amu

18.10 + 0.057 + 2.03 = 20.19 amu

Learning Check 5

Gallium is a metallic element found in

small lasers used in compact disc players.

In a sample of gallium, there is 60.2% of

gallium-69 (68.9 amu) atoms and 39.8% of

gallium-71 (70.9 amu) atoms. What is the

atomic mass of gallium?

Solution

Ga-69

68.9 amu x 60.2 = 41.5 amu for 69Ga

100

Ga-71 (%/100)

70.9 amu x 39.8 = 28.2 amu for 71Ga

100

Atomic mass Ga = 69.7 amu

Finding An Isotopic Mass

A sample of boron consists of 10B (mass 10.0 amu) and 11B (mass 11.0 amu). If the average atomic mass of B is 10.8 amu, what is the % abundance of each boron isotope?

Assign X and Y values:X = % 10B Y = % 11B

Determine Y in terms of XX + Y = 100Y = 100 - X

Solve for X:X (10.0) + (100 - X )(11.0) = 10.8

100 100

Multiply through by 10010.0 X + 1100 - 11.0X = 1080

Collect X terms

10.0 X - 11.0 X = 1080 - 1100

- 1.0 X = -20

X = -20 = 20 % 10B

- 1.0

Y = 100 - X

% 11B = 100 - 20% = 80% 11B

Learning Check 6

Copper has two isotopes 63Cu (62.9 amu) and 65Cu (64.9 amu). What is the % abundance of each isotope? (Hint: Check Zumdahl or any other chemistry text for atomic mass)

1) 30% 2) 70% 3) 100%

Solution

2) 70%

Solution

62.9X + 6490 = 64.9X = 6350

-2.0 X = -140

X = 70%

Atomic Masses

13C12C

13.00335 amu (1.11%)

12.0000 amu (98.89%)

atomic weight of C = 12.01115 amu WHY?

Calculating masses of atoms relative to 12C

(mass of 12C atom) * 1.58320 = mass of F atom

= 18.99840

reported atomic weight of F = 18.9984

Charged Atoms: Ions

Left to their own devices, atoms are electrically neutral.

That means that they have an equal number of

protons and electrons.

During the course of most natural events,

protons are not gained or lost, but electrons may be.

Atoms with more or fewer electrons than protons are

electrically charged. They are called ions:

an atom that loses electrons takes on a positive charge

(cation);

an atom that gains electrons takes on a negative charge

(anion).

Complex cations and anions can also occur: (NH4)+1, (SO4)-2

An ISOTOPE is one of a set of nuclides with the same Z and consequently different A. (ie isotopes are the same chemical element but different masses). e.g.

  C126 C136 C146

An ISOBAR is one of a set of nuclides with the same A but different N and Z.

e.g C146 O148N147

An ISOTONE is one of a set of nuclides with the same N and consequently different A. e.g.

21412021

401921

3918 A,K,A

• More on atomic notation, which is based on the nuclear structure:– Isotope: same Z, different A and N– Isobar: same A, different Z and N– Isotone: same N, different Z and A

Example: From the following list of atoms, which are isotopes, isobars, and isotones?

Component

Atom

A Z N

Xe

I

Cs

I

Xe13154 I130

53 Cs13255 I131

53