Section 4.2—Atomic Structure

Objectives:•determine the number of protons, neutrons, & electrons in atoms•distinguish between the properties of protons, neutrons, & electrons•define isotopes•differentiate between mass number and average atomic mass•calculate weighted average atomic masses

What are atoms?

An atom is the smallest piece of an element that has all the chemical properties of that element.

What is 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

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

On a periodic table, the atomic number shows the number of protons.

The Nucleus & Mass

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

The mass of the nucleus (called the mass number) 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.

The mass number (protons + electrons) represents the mass of an atom.

Charges in an Atom

Protons have a positive charge (+)Electrons have a negative charge (-)Neutrons have no chargeOverall charge of an atom = 0 because

the number of protons equals the number of electrons in any atom.

Ions

An ion is any charged particle; specifically, if an atom LOSES or GAINS electrons, it will become an ion.

If an atom loses electrons, there will be more protons than electrons. It will become a positively-charged ion.

If an atom gains electrons, there will be more electrons than protons. It will become a negatively-charged ion.

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

Example: Element symbols

Charge-2

Element SymbolO = Oxygen

Mass number16

Atomic number 8

Another element symbolIt is possible for atoms to have the same

number of protons but different numbers of neutrons. Such atoms are called isotopes.isotopes.

To make it easy to identify the various isotopes of an element, scientists add a number after the element’s name.

This number is the mass numbermass number - the sum of the number of protons and neutrons in the nucleus of an atom of the isotope.

Example

168O 17

8O 188O

Let’s Practice

Example:Fill in the missing values

Symbol Name Atomic #

Mass #

Charge Proton Neutron Electron

Magnesium-25 +2

83Li

15 16 18

53 127 -1

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.

Isotopes

Elements are found in nature as a mixture of all of their isotopes.

The percent abundancepercent abundance of the isotopes (percent of each isotope in a sample) is constant no matter where the sample is found.

Isotopes have the same chemical behavior because chemical behavior is determined by the number of ELECTRONS an element has.

Atomic Mass

The atomic massatomic mass of an element is the weighted average mass of the isotopes of that element.

Atomic mass, therefore, takes into account the percent abundance of each isotope as well as the mass of an atom of that isotope.

The isotope in greater abundance will have a greater effect on determining the atomic mass.

Analyzing Atomic Mass

Analyzing an element’s atomic mass can SOMETIMES give you a clue as to what the most abundant isotope for that element is.

The atomic mass of fluorine is 18.998 amu. From this you can conclude that most fluorine occurs in the form of fluorine-19.

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 Atomic Mass

1 Multiply the % abundance of each isotope by its mass. (75%)(35) = 26.25 amu

(25%)(37) = 9.25 amu

2 Add the mass contributions of all the isotopes together.

35.5 amu is the calculated atomic mass of chlorine.

Chlorine-35 Chlorine-37

Practice Problems

Calculate the atomic mass of lithium.

Isotope Percent Abundance

lithium-6 7.6%

lithium -7 92.4%