atomic number, mass number, atomic mass and isotopes

Atomic Number, Mass Number, Atomic Mass and Isotopes

Atomic Number (Z):

• is the number of protons in the nucleus of the atom. Z=#p+

• The number of protons (atomic number) determine the identity of an element.

What about the electrons? How many?

• Atoms have no overall electrical charge (neutral) so, an atom must have as many electrons as there are protons in its nucleus. • # p+ =# e-

• The atomic number of an element also equals the number of electrons in a neutral atom of that element• Z =#e-

Ex: Sodium

What is the atomic number of Sodium?

How many protons does sodium have?

How many electrons does sodium have?

11

11

11

Mass Number (A):

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

A=#p+ + #n0

Standard atomic Notation

Other ways to write elements:

36

17Cl Cl-36

Atomic Number

Mass Number Mass Number

Nucleons:

protons and neutrons

Isotopes

They have different mass numbers

because they have

different numbers of neutrons,

but they have

the same atomic number because they have the same number of protons.

Atoms of the same element with

• Same number of protons

• Same number of electrons

• Different number of neutrons

Isotopes

Do isotopes have similar chemical properties? Why?

Yes – electrons are the same

Do they have similar physical properties? Why?

NO – mass is different.

Remember:Isotopes have:same # of protonsdifferent # of neutrons

Example:Isotopes of Carbon and Hydrogen

protium deuterium tritium

H H H

Isotopes of Carbon

Isotopes of Hydrogen

Atomic Mass Unit

is a unit used to compare

the masses of atoms

and has the symbol

u

or

amu.

1 amu or u

is approximately equal to

the mass of

a single

proton or neutron.

Carbon-12

Chemists have defined

the carbon-12 atom

as having a mass of

12 atomic mass units.

For carbon: Mass = 12.0199% 12C AND 1% 13C found in nature12.01 = average mass based on abundance NOT just 12

Atomic Mass

is the weighted average

mass

of all the naturally occurring

isotopes

of that element.

How do we deal with the fact that some atoms have several isotopes?

The solution is to measure the mass of each individual isotope in the element and then calculate the weighted average.

The weighted average is known as the atomic mass of the element

MARBLE ACTIVITY

Types of calculations

1. Finding atomic mass, given mass and abundance of each

2. Finding the mass of one of the isotope given atomic mass and abundance.

3. Finding abundance given atomic mass and mass of each

NOTE – if atomic mass is not given for 2 and 3 above use the mass in the periodic table.

Problem 1- finding atomic mass

An element consist of two isotopes. Isotope A has an abundance of 75.00 percent, and its mass is 14.000 atomic mass units. Isotope B has an abundance of 25.00 percent, and its mass is 15.000 atomic mass units. What is the atomic mass of the element?

Atomic mass = (14.00u x 0.7500) + (15.000 u x 0.2500)

= 14.25

Problem 2 – finding mass of one isotope

Bromine has two naturally occuring isotopes. Bromine-79 has a mass of 78.918amu and is 50.69% abundant. Using the atomic mass reported in the periodic table, determine the mass of bromine-81, the other isotope of bromine.

Atomic mass = (MBr79 x Ra) + (MBr81x Ra)

Ra for Br81 = 100 – 50.69 = 49.31

79.90(mass from periodic table) = (78.917 x 0.5069) + (MBr81 x 0.4931)

79.90 = 40.003 + 0.4931Br

79.90 – 40.003 = 0.4931Br

39.897 = 0.4931 Br

80.91 amu = Br

Problem – 3. finding relative abundance

Gallium consists of two naturally occuring isotopes with masses of 68.926 and 70.925 amu. The average atomic mass of Ga is 69.72 amu. Calculate the abundance of each isotope.

Note – if atomic mass is not given use the mass in the periodic table.

Answer should be:69Ga: 60.3%, 71Ga: 39.7%

Average atomic mass = (mass 69Ga)(x) + (mass 71Ga)(y)

Where x and y are the relative abundances of 69Ga and 71Ga respectively. So, 69.72 = 68.926(x) + 70.925(y)Cant have 2 unknowns?????

Let the abundance of 1 isotope = x, then the abundance of the other will be 1-x . In this way the problem will have 1 unknown.(Substitute y for 1-x)So 69.72 = (68.926) (x) + (70.925)(1-x)                                     69.72 =  68.926x + 70.925 - 70.925x                                 - 1.205 = -1.999x                                           x = 0.60280

= 0.603 x 100 = 60.3%