The Discovery of The Discovery of IsotopesIsotopes

• Francis William Aston and Antoine Becquerel

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An atom consists of aAn atom consists of a

• nucleusnucleus

– (of (of protonsprotons and and neutronsneutrons) )

• electronselectrons in space about the nucleus. in space about the nucleus.

The The AtomAtom

NucleusNucleus

Electron cloudElectron cloud

Copper Copper atoms on atoms on silica silica surface.surface.

• An An __________ is the smallest particle of is the smallest particle of an element that has the chemical an element that has the chemical properties of the element.properties of the element.

Distance across = 1.8 nanometer (1.8 x 10-9 m)

ATOM ATOM COMPOSITIONCOMPOSITION

•protons and neutrons in protons and neutrons in the nucleus.the nucleus.

•the number of electrons is equal to the the number of electrons is equal to the number of protons.number of protons.

•electrons in space around the nucleus.electrons in space around the nucleus.

•extremely small. One teaspoon of water has extremely small. One teaspoon of water has 3 times as many atoms as the Atlantic Ocean 3 times as many atoms as the Atlantic Ocean has teaspoons of water.has teaspoons of water.

The atom is mostlyThe atom is mostlyempty spaceempty space

ATOMIC COMPOSITIONATOMIC COMPOSITION• Protons (pProtons (p++))

– + electrical charge+ electrical charge– mass = 1.672623 x 10mass = 1.672623 x 10-24-24 g g– relative mass = 1.007 atomic relative mass = 1.007 atomic mass mass

units (amu) units (amu) but we can round to 1but we can round to 1

• Electrons (eElectrons (e--))– negative electrical chargenegative electrical charge– relative mass = 0.0005 amu relative mass = 0.0005 amu

but we can round to 0but we can round to 0

• Neutrons (nNeutrons (noo))– no electrical chargeno electrical charge– mass = 1.009 amu mass = 1.009 amu but we can round to 1but we can round to 1

Atomic Number, ZAtomic Number, Z

All atoms of the same element All atoms of the same element have the same number of have the same number of protons in the nucleus, protons in the nucleus, ZZ

1313

AlAl

26.98126.981

Atomic numberAtomic number

Atom symbolAtom symbol

AVERAGE Atomic MassAVERAGE Atomic Mass

Mass Number, AMass Number, A• C atom with 6 protons and 6 neutrons C atom with 6 protons and 6 neutrons

is the mass standard is the mass standard • = 12 atomic mass units= 12 atomic mass units• Mass NumberMass Number (A) (A)

= # protons + # neutrons= # protons + # neutrons• NOT on the periodic table…(it is the NOT on the periodic table…(it is the

AVERAGE atomic mass on the table)AVERAGE atomic mass on the table)• A boron atom can have A boron atom can have

A = 5 p + 5 n = 10 amuA = 5 p + 5 n = 10 amu

A

Z

10

5B

A

Z

10

5B

IsotopesIsotopes

• Atoms of the same element (same Z) Atoms of the same element (same Z) but different mass number (A).but different mass number (A).

• Boron-10 (Boron-10 (1010B) has 5 p and 5 nB) has 5 p and 5 n

• Boron-11 (Boron-11 (1111B) has 5 p and 6 nB) has 5 p and 6 n

10B

11B

IsotopesIsotopes

• The existence of isotopes explains the apparent contradiction between the mass number and atomic mass of the elements. The various isotopes of the elements in nature occur in different proportions. Therefore , the atomic mass on the periodic table is the weighted average of all the natural isotopes of that element,

Two isotopes of sodium

How we classify How we classify IsotopesIsotopes

• Stable isotopes are chemical isotopes that are not radioactive .By this definition, there are 256 known stable isotopes of the 80 elements which have one or more stable isotopes.

• Commonly analyzed stable isotopes include oxygen, carbon, nitrogen, hydrogen and sulfur

Unstable or Unstable or Radioactive Radioactive IsotopesIsotopes

• Naturally occurring fall into three categories:

• primordial • secondary • cosmogenic • Artificially produced radioisotopes can be produced by particle accelerators or by nuclear reactions.

What are Radio What are Radio IsotopesIsotopes

• since they have different numbers of neutrons, isotopes of the same element may have different radioactivity. An isotope that is radioactive is called a radioisotope.

What is Half-life What is Half-life of the Isotopesof the Isotopes

• The half-life he amount of time it takes for half of the atoms in a sample to decay. The half-life for a given isotope is always the same

• Suppose there's an alien species with a half-life of, say, 70 years. You randomly pick out 16 baby aliens and track them to see how long they live. After 70 years, of course, 8 of them will still be alive.

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ExamplesExamples• The most stable isotope of uranium, U-238, has an

atomic number of 92 (protons) and an atomic weight of 238 (92 protons plus 146 neutrons). The isotope of uranium of greatest importance in atomic bombs, U-235, though, has three fewer neutrons. Thus, it also has an atomic number of 92 (since the number of protons has not changed) but an atomic weight of 235 (92 protons plus only 143 neutrons). The chemical behavior of U-235 is identical to all other forms of uranium, but its nucleus is less stable, giving it higher radioactivity and greater susceptibility to the chain reactions that power both atomic bombs and nuclear fission reactors.

ExamplesExamples• Iodine is one of the earliest elements whose

radioisotopes were used in what is now called nuclear medicine. The most common, stable form of iodine has an atomic number of 53 (protons) and an atomic weight of 127 (53 protons plus 74 neutrons). Because its nucleus has the "correct" number of neutrons, it is stable and is not radioactive. A less stable form of iodine also has 53 protons (this is what makes it behave chemically as iodine) but four extra neutrons, for a total atomic weight of 131 (53 protons and 78 neutrons). With "too many" neutrons in its nucleus, it is unstable and radioactive, with a half-life of eight days. Because it behaves chemically as iodine, it travels throughout the body and localizes in the thyroid gland just like the stable form of iodine. But, because it is radioactive, its presence can be detected. Iodine 131 thus became one of the earliest radioactive tracers.

Isotopes Isotopes & Their & Their UsesUses

Bone scans with Bone scans with radioactive radioactive technetium-99. technetium-99.

Separation of Separation of Isotopes Isotopes

• Difference in physical properties enables separation of the isotope to be carried out.

• Evaporation

• Distillation

• Centrifuging

• Laser methods

• Diffusion

Calculating the% Calculating the% abundanceabundance

• Lithium has 2 isotopes. Li-6 and Li-7. If the atomic mass of Lithium is 6.94, calculate the proportion ( % of abundance) of the isotopes.

Atomic Symbols

Show the name of the element, a hyphen, and

the mass number in hyphen notation

sodium-23

Show the mass number and atomic number

in nuclear symbol formmass number 23 Na

atomic number 11

Counting Protons, Neutrons, and Electrons

• Protons: Atomic Number (from periodic table)• Neutrons: Mass Number minus the number of protons

(mass number is protons and neutrons because the mass of electrons is negligible)

• Electrons: – If it’s an atom, the protons and electrons must be

the SAME so that it is has a net charge of zero (equal numbers of + and -)

– If it does NOT have an equal number of electrons, it is not an atom, it is an ION. For each negative charge, add an extra electron. For each positive charge, subtract an electron (Don’t add a proton!!! That changes the element!)

Learning Check – Counting

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+ _______ _______ _______

#no _______ _______ _______

#e- _______ _______ _______

Answers

12C 13C 14C 6 6 6

#p+ 6 6 6

#no 6 7 8

#e- 6 6 6

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 is1) 14 2) 16 3) 34

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

D. Another isotope of this element is1) 34X 2) 34X 3) 36X

16 14 14

Learning Check – Counting

State the number of protons, neutrons, and electrons in each of these ions.

39 K+ 16O -2 41Ca +2

19 8 20

#p+ ______ ______ _______

#no ______ ______ _______

#e- ______ ______ _______

AVERAGE AVERAGE ATOMIC ATOMIC MASSMASS

• Because of the existence of isotopes, the Because of the existence of isotopes, the mass of a collection of atoms has an average mass of a collection of atoms has an average value.value.

• Boron is 20% Boron is 20% 1010B and 80% B and 80% 1111B. That is, B. That is, 1111B is B is 80 percent abundant on earth. 80 percent abundant on earth.

• For boron atomic weightFor boron atomic weight

= 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu= 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu

10B

11B

Isotopes & Average Atomic Isotopes & Average Atomic MassMass

• Because of the existence of isotopes, the mass of a Because of the existence of isotopes, the mass of a collection of atoms has an average value.collection of atoms has an average value.

• 66Li = 7.5% abundant and Li = 7.5% abundant and 77Li = 92.5%Li = 92.5%

– Avg. Atomic mass of Li = ______________Avg. Atomic mass of Li = ______________

• 2828Si = 92.23%, Si = 92.23%, 2929Si = 4.67%, Si = 4.67%, 3030Si = 3.10%Si = 3.10%

– Avg. Atomic mass of Si = ______________Avg. Atomic mass of Si = ______________