Ch 19 Properties of Atomsand the Periodic Table

Section 2Masses of Atoms

The Essential Question

– What does the periodic table tell us about the structures of different atoms?

Ponder the following •A Bohr model of a helium atom and a beryllium atom are given below.

Helium, He Berylium, Be

(cont.)

– List three similarities and three differences.

– How do you think a gold atom is different from a copper atom?

(cont.)

You will be able to:

– Use the periodic table to identify the properties of an elements atom.

– Atomic Mass Unit is one-twelfth the mass of a carbon atom containing six neutrons and six protons

– Atomic number is the number of protons in the nucleus of an atom.

– Mass number is the mass of an individual atom. The sum of the neutrons and protons in the nucleus.

Notes

Unit 1 • Investigation III

Activity

•Purpose: This lesson will formally introduce you to atomic structure.

Beryllium Atom Fluorine Atom Carbon Atom

(cont.)

element chemical symbol

atomic number

# of protons

# of electrons

# of neutrons

mass number

atomic weight

beryllium 5

fluorine 10

6 12

chlorine 18 35.45

lead 126

potassium 19 39

tin 70

tungsten 184 183.85

29 36

gold 118

(cont.)

Making Sense

– If you know the atomic number of an element, what other information can you figure out about the atoms of that element?

– If you know the atomic number of an element, can you figure out how many neutrons an atom of that element has? Can you come up with a close guess? Explain.

– Mass number is the number of

protons plus the number of neutrons.– Atomic mass is the “weight” or mass

of a single atom.– Atomic weight is the decimal number

on the periodic table.

Notes

Check-In

•Use your periodic table to identify the following elements:•a) Atomic number 18•b) Has three electrons•c) Atomic mass of 16.0

Wrap-Up

– Each successive element has one more proton than the element preceding it.

– The atomic number is equal to the number of protons.

(cont.)

– The number of electrons is equal to the number of protons (as long as the atom is neutral).

– The mass number is equal to the number of protons plus the number of neutrons (most of the mass is found in the nucleus).

(cont.)

Isotopes

The Essential Question

– How do isotopes of an atom account for the atomic weight of an element?

Ponder this •A chemist investigating a sample of lithium found that some atoms have a lower mass than other atoms. The chemist determined that the structures of the two types of atoms would be similar to the two drawings below.

(cont.)

– What is different about the two atoms?– What is the atomic number of each atom?– What is the mass number of each atom?– Do you think they are both lithium atoms?

Why or why not?

– Atoms of the same element that have

different numbers of neutrons are called isotopes.

– Atomic mass units (amu) are “invented” measurement units of the atomic mass.

Notes

Unit 1 • Investigation III

Activity•Purpose: In this lesson you will investigate isotopes and how they affect atomic weight.

(cont.)

Unit 1 • Investigation III

boron atom 1 2 3 4 5 6 7 8 9 10

# protons

# neutrons

# electrons

(cont.)

(cont.)

Unit 1 • Investigation III

Element Chemical Symbol

Atomic Number

Atomic Weight

# of protons

# of electrons

# of neutrons

Boron B 5 or 6

Chlorine 17

Lithium 6.94

Vanadium V 23

Nitrogen 7

Magnesium

Argon Ar 39.9 18. 20. or 22

(cont.)

Unit 1 • Investigation III

Making Sense

– Explain why the atomic weights listed in the periodic table are not usually whole numbers.

Unit 1 • Investigation III

– While the element iron is defined as being made up of neutral atoms with 26 protons and 26 electrons, not every iron atom has the same number of neutrons.

– Atoms that have the same number of protons but different numbers of neutrons are called isotopes.

Notes

(cont.)

Unit 1 • Investigation III

– What we call the atomic weight on the periodic table is actually the average atomic mass of that element’s naturally occurring isotopes.

– Isotopes have similar chemical properties in that they combine with other elements to form similar compounds.

Notes (cont.)

(cont.)

Unit 1 • Investigation III

– Atomic Weight is the weighted average of the atomic masses of different isotopes taking into account their abundance.

(cont.)

(cont.)

Unit 1 • Investigation III

(cont.)

Unit 1 • Investigation III

Check-In

– Predict the isotopes of carbon, C. Which isotope is more abundant? How do you know?

Unit 1 • Investigation III

Wrap-Up

– Elements may have anywhere from 2 to 10 naturally occurring isotopes.

– The atomic weight of an element listed on the periodic table is actually the average mass of the naturally occurring isotopes of that element.

– Isotopes have the same number of protons and electrons, but different numbers of neutrons.

(cont.)

Unit 1 • Investigation III

– Isotopes of a single element exhibit similar properties in that they form similar compounds.

– Isotopes are referred to by their mass numbers.

(cont.)