Atomic Structure

Protons, Neutrons, and Electrons

Section 4.3

Distinguishing Among Atoms

• Just as apples come in different varieties, a chemical element can come in different “varieties” called isotopes.

4.3

Protons

• Protons- determine the identity of an atom– Elements are different because they each have

different #s of protons

• The atomic number of an element is the number of protons in the nucleus of an atom of that element– You can find this number by looking at your

periodic table

• In a neutral atom, the number of protons is equal to the number of electrons

Atomic Number

Practice Finding # of protons and electrons

Finding the number of Neutrons

• The total number of protons and neutrons in an atom is called the mass number.– The number of neutrons in an atom is the

difference between the mass number and atomic number.

– How do I find the mass number? • If given, add the protons and neutrons together• If not, round the mass from the periodic table to the

nearest whole number. This will provide the mass number of the most common isotope.

Finding Protons, Neutrons, Electrons

• You may just be given the name of an element and be expected to look it up on the periodic table.

• You may be given a symbol with mass numbers and atomic numbers beside it.

Writing and Interpreting Chemical Symbols

Mass of an atom• Nucleus- contains most of the mass of

an atom– Protons & Neutrons contain most of an

atom’s mass• Mass of proton = mass of neutron = 1amu

– Electrons- 1/2000th of the mass of a proton

Atomic structure

• Grams are not used to measure mass of atoms because atoms are too small– Amu is used instead – Amu = atomic mass unit

• Amu – 1/12th the mass of a carbon-12 atom, which contains 6 protons and 6 neutrons

Isotopes• Isotopes are atoms that have the same number

of protons but different numbers of neutrons. – Because isotopes of an element have different

numbers of neutrons, they also have different mass numbers.

Isotopes

• Despite these differences, isotopes are chemically alike because they have identical numbers of protons and electrons.

4.3

Isotopes

• To indicate the identity of an isotope, write the element name – mass # – Carbon-12 Carbon-14– Isotopes can also be indicated using the method we

saw earlier

Calculating the Average atomic mass of an element

• Average atomic mass- a weighted average of the masses of the isotopes of an element

Average atomic mass = (% x mass) + (% x mass) + (% x mass)….

Example of average atomic mass for carbon

mass number 13

exact weight percent abundance

12 12.0000 amu 98.90

13 13.003355 amu 1.10

(12.0000amu) (0.9890) + (13.003355amu) (0.0110) = 12.011 amu

Check your answers

• C has an average atomic mass of 12.011 amu.

• This is closer to 12 than to 13, which means most isotopes must be carbon-12.

• This matches our known data, which say that 98.90% of carbon atoms are C-12.

How do we know the mass of isotopes??

• Use a mass spectrometer

Results from mass spectrometer

Throughout this section, we’ve been using the periodic table

• A periodic table is an arrangement of elements in which the elements are separated into groups based on a set of repeating properties. – A periodic table allows you to easily compare

the properties of one element (or a group of elements) to another element (or group of elements).

Periodic Table

• Each horizontal row of the periodic table is called a period.

• Within a given period, the properties of the elements vary as you move across it from element to element.– This occurs because they have a different

number of outer shell (or valence) electrons

The Periodic Table—A Preview

• A Period

4.3

Periodic Table

• Each vertical column of the periodic table is called a group, or family.

• Elements within a group have similar chemical and physical properties.– This occurs because they have the same

number of valence (or outer shell) electrons

The Periodic Table—A Preview

• A Group or Family

4.3