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GHS Honors Chem

Atomic StructureGHS Honors Chem

Section 3.3Distinguishing Between Atoms

� OBJECTIVES:� Explain how the atomic number identifies

an element. � Use the atomic number and mass number of

an element to find the numbers of protons, electrons, and neutrons.

� Explain how isotopes differ, and why the atomic masses of elements are not whole numbers.

� Calculate the average atomic mass of an element from isotope data.

GHS Honors Chem

Atomic Notation• Atomic Notation represents the number of

protons and neutrons in the nucleus of an atom.• The Atomic Number, Z, represents the

number of protons in the nucleus.• The Atomic Number, Z, also represents the

number of electrons in a neutral atom. • The Mass Number, A, represents the total

number of protons and neutrons in the nucleus of an atom.

GHS Honors Chem

Atomic Notation

• The Atomic Number is 11:• # p+ = 11 protons, # e- = 11 electrons

• The Mass Number is 23:• # n0 = 23 – 11 = 12 neutrons

GHS Honors Chem

• Find the • number of protons• number of neutrons• number of electrons• Atomic number• Mass Number

F199

Atomic Notation

GHS Honors Chem

• Find the • number of protons• number of neutrons• number of

electrons• Atomic number• Mass Number

Br8035

Atomic Notation

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GHS Honors Chem

• if an element has an atomic number of 34 and a mass number of 78 what is the • number of protons• number of neutrons• number of electrons• Complete symbol

Atomic Notation

GHS Honors Chem

• if an element has 91 protons and 140 neutrons what is the • Atomic number• Mass number• number of electrons• Complete symbol

Atomic Notation

GHS Honors Chem

• if an element has 78 electrons and 117 neutrons what is the • Atomic number• Mass number• number of protons• Complete symbol

Atomic Notation

GHS Honors Chem

What are Isotopes?

• Dalton was wrong. Atoms of the same element can have different numbers of neutrons.

• These atoms of the same element would have different mass numbers.

• These “cousins” of the same element are called isotopes.

Think of them as cousins …

GHS Honors Chem

Isotopes

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

• All atoms of the same element have the same number of protons.

• Most elements occur naturally with varying number of neutrons.

Atoms of the same element with different number of neutrons in the nucleus are called isotopes.

GHS Honors Chem

Hydrogen IsotopesHydrogen has _____ isotopes

11H

21H

31H

1 proton and 2 neutrons, tritiumradioactive

1 proton and 1 neutron, deuterium

1 proton and 0 neutrons, protium

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GHS Honors Chem

Naming Isotopes

• To name an isotope properly, we put the mass number after the name of the element.

• carbon- 12• carbon -14• uranium-235

GHS Honors Chem

Isotopes

GHS Honors Chem

Cont’d … IsotopesExamples:

1. Write the symbol for Cobalt-60.2. How many protons and neutrons does

an atom of mercury-202 have?3. How many electrons are present in

an atom of Copper-63?

GHS Honors Chem

Cont’d … IsotopesQs. If we look at the Periodic Table, all of the elements have

a Mass Number that is in decimal form, or not a whole number … why??

Ans.The Mass Number reported on the periodic table is a “Weighted Average” of the Isotopes that are naturally occurring in nature.

Let’s look at this a bit further …

GHS Honors Chem

Measuring Atomic Mass• The Mass Number = the Atomic Mass, or

Atomic Weight.• This number not only tells us:

• The mass of one mole of an element (Unit 2)• about the number of neutrons and protons present• It also tells us the mass of one atom of an element

relative to an atom of another element.• Unit is the Atomic Mass Unit (amu).• The standard for the amu scale is carbon:

• Carbon = 6 p+ and 6 n0 = mass standard of 12 amu• All other atoms are designated relative to

Carbon. i.e. hydrogen is 1/12th the mass of carbon, or an Atomic Mass of 1 amu.

GHS Honors Chem

One More Visit to the Properties of Subatomic Particles

Particle Symbol Relative Charge

Mass in Amu’s

Actual Mass(g)

Electron e- 1- 0 9.11 x 10-28

Proton p+ 1+ 1 1.67 x 10-24

Neutron N0 0 1 1.67 x 10-24

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GHS Honors Chem

Mathematics of Isotopes

• A simple average assumes that each object has the same number of occurrences.

• A weighted average means that the numbers of all of the object are not equal, but occur differently.

• A weighted average is calculated by multiplying the percent of the object (as a decimal number) by its mass for each object and adding the numbers together.

GHS Honors Chem

Mathematics of Isotopes

GHS Honors Chem

Mathematics of Isotopes

GHS Honors Chem

Some Natural Isotopes

GHS Honors Chem

Isotope Examples• Magnesium has three isotopes. 78.99%

magnesium 24 with a mass of 23.9850 amu, 10.00% magnesium 25 with a mass of 24.9858 amu, and the rest magnesium 26 with a mass of 25.9826 amu. What is the atomic mass of magnesium?

• Boron is 20% B-10, and 80% B-11. What is the Atomic Mass of Boron?

GHS Honors Chem

What are Ions?Result from the loss or gain of electrons by an

atom …

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GHS Honors Chem

What are Ions?Result from the loss or gain of

electrons by an atom …

Be94 Be9

4+2

+ 2e-

A Positive Ion: A CATIONGHS Honors Chem

What are Ions?Result from the loss or gain of

electrons by an atom …

F199

+ 1 e-

-199 F

A Negative Ion: An ANION

GHS Honors Chem

Determine the number of p+, no, and e- for the following:

Ion p+ no e-

F-

Cl-

Na+

Ca+2

Al+3

O2-

Br-

GHS Honors Chem

Let’s Complete the Table

Element SymbolAtomicNumber

(Z)protons neutrons electrons charge

Mass#

(W)

Oxygen 0 16

Ru 58 2+

26 23 56

17 1- 35

18 2- 34

Mg+2

Nitrogen 8 10

GHS Honors Chem

Let’s Review1. Atoms are composed of ____, ____, and ______.2. The ____ & ______ are located in the nucleus, while the _____ are

outside of the nucleus.3. Atoms are most empty space, or dense clouds of + & - charges?4. The number of protons is referred to as the ____ for the atom.5. All atoms of the same element have the same number of ______.6. Isotopes have the same number of _____, but a different number

of _____.7. The mass number of an element is the total number of ______ &

______.8. Why is the atomic mass of an element given in decimal form in the

periodic table?