Section 4.2—Atomic Structure

What are atoms?

Atom - smallest piece of matter that has the chemical properties of the element.

What’s in an atom?

An atom is made of three sub-atomic particles

Particle Location

Nucleus

Nucleus

Outside the nucleus

Mass

1 amu = 1.6710-27 kg

1 amu = 1.6710-27 kg

0.00055 amu

9.1010-31 kg

Charge

+1

0

-1

Proton

Neutron

Electron

1 amu (“atomic mass unit”) = 1.66 10-27 kg

What gives an atom its identity?

What makes an atom “carbon” as opposed to “oxygen”?

Every atom has a different number of protons.

The number of protons determines the identity of the atom

The atomic number shows the number of protons.

Atomic number = protons

The Nucleus & Mass

Since the nucleus has protons & neutrons, and the mass of each one is 1 amu…

The mass of the nucleus (in amu’s) is the number of protons + neutrons

Since electrons have relatively no mass (0.054% of one proton or neutron), we don’t need to worry about them when determining mass of an atom

Mass # = protons + neutrons

Charges

Protons have a positive chargeElectrons have a negative chargeNeutrons have no chargeOverall charge = protons + (-1)×electrons

Charge = protons - electrons

How do we show information about an element?

XA C

Z #

Element symbols

Charge# protons - # electrons(assumed to be “0” if blank)

Element Symbol1 or 2 letters, found on

the periodic table

Mass number# protons + # neutrons

Atomic number # of protons Number

How many atoms do you have?

Example: Element symbols

O16 -2

8

Charge-2

Element SymbolO = Oxygen

Mass number16

Atomic number 8 Number

Assumed to be “1” if blank

Let’s Practice

Example:Fill in the missing values

Symbol Name Atomic #

Mass #

Charge Proton Neutron Electron

Magnesium-25 +2

82 126 82

Let’s Practice

Example:Fill in the missing values

Symbol Name Atomic #

Mass #

Charge Proton Neutron Electron

Magnesium-25 +2

82 126 82

Remember: Atomic number is the identityAtomic number = protonsCharge = proton - electronsMass # = protons + neutrons

12 25 12 13 10

Lead-208 208 0 82

22512

Mg

Pb20882

Isotopes

What are isotopes?

Isotopes - n. Atoms of the same element with a different number of neutrons

Some isotopes are radioactive—but not all…many are quite stable!

Isotopes Example

If they have different number of neutrons, and neutrons have a mass of 1 amu…

Then isotopes of the same element will have different masses!

But because their protons are the same, they are the same element!

Hydrogen-2Hydrogen-1

Mass # = 1 amu Mass # = 2 amu

Identifying Isotopes

12C 13C

Carbon-12 Carbon-13

Isotopes can be differentiated by their different mass numbers in the element symbol

Or by the mass number following their name.

Mass Number versus Atomic Mass

Mass Number Average Atomic Mass

# of protons + # of neutrons Average of actual masses

For one specific isotope only

Is not found on the periodic table

Weighted average of all isotopes

Is found on the periodic table

Always a whole number Not a whole number

Calculating Average Atomic Mass

Average atomic mass

= Abundance of isotope

Mass of isotope

( )

Average atomic mass is a weighted average (it takes into account how often each isotope occurs).

“Sum of”

What fraction of the time is that isotope present?

Actual mass (not mass number)

Example of Finding Avg Atomic Mass

966.362422.0969.347578.0 amuMassAvg

Example:Find the atomic mass of

chlorine if Chlorine-35 has a mass of

34.969 amu and Chlorine-37 has a

mass of 36.966 amu and is present 24.22% of the time.

Remember that percents add up to 100.

So they said the second isotope is present 24.22% of the time.

This means that the first isotope is present 100-24.22 = 75.78% of the time

Isotope Mass Percent Decimal

1 34.969 amu 75.78 0.7578

2 36.966 amu 24.22 0.2422

This chart summarizes the information in the problem:

= 35.45 amu (this is what’s on the periodic table for Cl!)