Chapter 4

Chapter 4

The Atom – Part 2

Subatomic ParticlesPARTICLE SYMBOL CHARGE MASS

(amu)LOCATION

electron e- -1 0 orbit nucleus

proton p+ +1 1 inside nucleus

neutron n0 0 1 inside nucleus

Alchemy “The Golden Rule”

The number of protons in the nucleus of the atom (atomic number = Z) determines the atoms identity

Gold Atom Lead Atom

Mass Number

• The sum of the protons and neutrons in the nucleus of an atom.

What element is represented below?

Carbon - 12

• This is the symbol for the isotope carbon-12.

• Atomic number is 6.

• Mass number is 12.

Isotopes• Isotopes are different types of atoms of a

chemical element, having the same number of protons in the nucleus, but having different numbers of neutrons in the nucleus.

• Isotopes therefore have the same atomic number but different mass numbers.

• There are 275 isotopes of the 81 stable elements, in addition to over 800 radioactive isotopes, and every element has known isotopic forms.

• Isotopes of a single element possess almost identical properties.

Isotopes

Carbon - 12

• Write the symbols for carbon-13 and carbon-14.

What is the average mass of a carbon atom?

12.01

Atomic Mass

• The atomic mass of carbon is 12.01 amu.

• Atomic mass is the average mass of all the isotopes of an atom. It takes into account the different isotopes of an element and their relative abundance.

What is the average atomic weight for chlorine if it has two isotopes? The percent abundance for chlorine-35 is 75.53%. The percent abundance for chlorine-37 is 24.40%. The mass

for Cl-35 is 35.0 amu and for Cl-37 it is 37.0 amu.

[(0.7553)(35.0 amu)] + [(0.2440)(37.0 amu)]

= [26.4355] + [9.028]

= 35.4635

the atomic weight of Cl = 35.5 amu

What is the average atomic mass for thallium, Tl? The two stable isotopes and their abundances are listed here.Tl-205 has a mass of 205.059 amu with an abundance of 70.48 % and Tl-203 has a

mass of 203.059 amu with an abundance of 29.52 % .

204.5 amu

What is the average atomic mass for thallium, Tl? The two stable isotopes and their abundances are listed here.Tl-205 has a mass of 205.059 amu with an abundance of 70.48 % and Tl-203 has a

mass of 203.059 amu with an abundance of 29.52 % .

[(0.7048)(205.059 amu)] + [(0.2952)(203.059 amu)]

= [144.5256] + [59.943]

= 204.4686

the atomic weight of Tl = 204.5 amu

Common Uses of Isotopes

• Isotope Analysis

• Radiometric Dating

• Nuclear Medicine:– Magnetic Resonance Imaging (MRI)– Positron Emission Tomography (PET

scans)

Importance of Isotopes

Radiometric Dating

Half-Life

• The decay of radioactive elements occurs at a fixed rate. • The half-life of a radioactive isotope is the time required

for one half of the material to degrade into a more stable material.

• Although the half-life pattern is the same for every radioactive isotope, the length of a half-life is different.

• For example, C-14 has a half-life of about 5730 years while Ir-192 has a half-life of about 74 days.

Half-Life

Carbon Dating

Nuclear Medicine refers to imaging techniques that use radioactive isotopes to detect and treat disease. Magnetic resonance imaging (MRI) and positron emission tomography (PET scans) are examples.

MRI PET scan

• An atom of vanadium contains 23 electrons. How many protons does it contain?

• 23 p+

• An atom of silver contains 47 protons. What is its atomic number?

• 47

• An “atom” of sodium contains 11 electrons. What is its atomic number?

• 11

• An atom contains 37 protons. What element is it?

• rubidium

• How many electrons, protons and neutrons are in an atom of actinium with a mass number of 221?

• 89p+

• 89e-

• 132n0

• How many electrons, protons and neutrons are in an atom of rhodium-105?

• 45p+

• 45e-

• 60n0

Homework

Chapter 4 Worksheet 1