Chapter 18

Properties of Atoms and the Periodic Table

Scientific Shorthand

Scientists use abbreviations to write the name of elements.

Abbreviations are easier to write than the whole element.

For example:Carbon is written asAluminum is written asSilver is written as

C

AlAg

Scientific Shorthand

The first letter in the symbol is ALWAYS written with a capital letter.

If there is a second letter, it is ALWAYS written as a lowercase letter.

Some element symbols are based on their English names, while others are based on their Latin names.Silver is Ag from its Latin name Argentum.

Atoms

Elements are made of atomsAtoms are the smallest piece of matter

that still retains the properties of the element.

Subatomic ParticlesEach atom is

made up of smaller particles.

In the center of the atom is the nucleus.

The nucleus is made of protons and neutrons.

Subatomic ParticlesEach atom is

made up of smaller particles.

Electrons orbit around the nucleus

Subatomic Particles

Properties of ProtonsElectrical charge of +1Mass =1 amu.

Properties of NeutronsElectrical charge of 0.Mass =1 amu.

Properties of Electrons Electrical charge of -1Mass is approximately 1/1800 of a proton.

Quarks

Protons, neutrons and electrons are all made up of even smaller particles called quarks.

Quarks

To study quarks, scientists smash atoms together to break the protons apart.

Quarks

Based on the data collected, they can reconstruct the structure of the atom.

The Atomic Model - History

Democritus (400 B.C.)First proposed that

atoms made up all substances.

AristotleBelieved that matter is

uniform throughout.This idea lasted for

about 2,000 years.

The Atomic Model - History

Dalton’s ModelEach atom was a solid sphere.Allowed scientists to explain how chemical

reactions occurred.Thomson, Rutherford, and Bohr

Each scientist’s experiments helped to identity different parts of the atom and the structure of the atom.

Atomic Model

Electron Cloud Model

The most current model of the atom.

The electron cloud is 100,000 times larger than the diameter of the nucleus, but each electron is smaller than the proton.

Electrons are so small, and moving so fast, that they are difficult to find.

Protons in the nucleus bothrepel and attract each other.The repulsions are due toelectromagnetic force andthe attractions are due to thestrong nuclear force. Thestrong nuclear force onlyacts over very shortdistances, about the size ofan atomic nucleus. Neutronsand protons also attract eachother because of the strongnuclear force.

What holds the Nucleus Together?

Atomic Masses

Most of the mass of an atom is within the nucleus.Protons and neutrons each weigh about

1.6726 x 10-24 kg.That’s 0.000 000 000 000 000 000 000 001 672 6 kg!Electrons are 1/1800 of the mass of a proton

or neutron.

Atomic Masses

To measure atomic mass, we use the “atomic mass unit” or amu.Mass of 1 proton = 1 amuMass of 1 neutron = 1 amu

Atomic Number

The number of protons in an atom identify the element.

Each element has a different number of protons.Atoms with 8 protons are always oxygen.Atoms with 6 protons are always carbon.Atoms with 16 protons are always sulfur.

The number of protons in an atom is called the atomic number.

Atomic Number Practice

Use your periodic table to find the atomic number of the following elements:Aluminum (Al)Titanium (Ti)Tin (Sn)Osmium (Os)Erbium (Er)

How many protons does each atom have?The number of protons are equal to the

atomic number.

Al = 13Ti = 22Sn = 50Os = 76

Er = 68

Mass Number

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

Element Protons Neutrons Mass Number

Boron 5 6

Carbon 6 6

Oxygen 8 8

Sodium 11 12

Copper 29 34

6+6

8+8

5+6

11+12

29+34

11

12

16

23

63

Using the Atomic Number & Mass Number

If you know the atomic number and mass number, you can determine the number of neutrons.

Mass Number

- Atomic Number

Number of Neutrons

Practice

Determine the number of protons and neutrons in the following atoms:Lithium – Atomic Number = 3

Mass Number = 7Argon – Atomic Number = 18

Mass Number = 39Strontium – Atomic Number = 38

Mass Number = 88

Answers

Determine the number of protons and neutrons in the following atoms:Lithium – Protons = 3

Neutrons = 7 – 3 = 4Argon – Protons = 18

Neutrons = 39 – 18 = 21Strontium – Protons = 38

Neutrons = 88 – 38 = 50

Isotopes

Atoms of one type of element must have the same number of protons.

However, they can have different numbers of neutrons.

Atoms with the same number of protons and different numbers of neutrons are called isotopes.

Isotopes

ExamplesCarbon-12 has a mass of 12 amu.

All carbon atoms must have 6 protons.Neutrons = 12 – 6 = 6.C-12 has 6 protons and 6 neutrons.

Carbon-14 has a mass of 14 amu.All carbon atoms must have 6 protons.Neutrons = 14 – 6 = 8.C-14 has 6 protons and 8 neutrons.

Isotopes

ExamplesUranium-235 has a mass of 235 amu.

All uranium atoms must have 92 protons.Neutrons = 235 – 92 = 143.U-235 has 92 protons and 143 neutrons.

Uranium-238 has a mass of 238 amu.All uranium atoms must have 92 protons.Neutrons = 238 – 92 = 146.U-238 has 92 protons and 146 neutrons.

Average Atomic Mass

Because atoms can have different numbers of neutrons, the mass numbers can be different.

To get an overall mass of one element, we take the weighted-average of all the isotopes and determine the average atomic mass for the element.

Neutral Atoms

In the nucleus, which particles have charge?Protons

What charge do they have?Positive

What particle is opposite that charge?Electrons

Neutral Atoms

To balance the charges in the atom, the number of protons must equal the number of electrons.Li has 3 protons, so the nucleus has a charge

of +3.To balance that charge, there must be 3

electrons that add up to -3.Overall, the charge is (+3) + (-3) = 0, so the

atom is neutral.

Practice

Determine how many electrons the following atoms have:Magnesium, Protons = 12Beryllium, Protons = 4Nitrogen, Atomic Number = 7Bromine, Atomic Number = 35

12 Electrons

4 Electrons

7 Electrons

35 Electrons

Practice

Element Symbol Atomic Number

Mass Number

Protons Neutrons Electrons

Iodine I 53 127

Mercury Hg 80 201

Neon 10 20

Mg 12 12 12

Rb 37 48

41 51

75 111

Practice

Element Symbol Atomic Number

Mass Number

Protons Neutrons Electrons

Iodine I 53 127 53 74 53

Mercury Hg 80 201 80 121 80

Neon Ne 10 20 10 10 10Magnesium Mg 12 24 12 12 12Rubidium Rb 37 85 37 48 37

Niobium Nb 41 92 41 51 41

Rhenium Re 75 186 75 111 75

Bohr Models of the Atoms

Each atom had orbitals or shells in which the electrons can fit.1st Shell: up to 2 electrons can fit2nd Shell: up to 8 electrons can fit3rd Shell: up to 18 electrons can fit4th Shell: up to 32 electrons can fit

Must start with the smallest/closest orbital or shell first.

Drawing Bohr Models

HydrogenAN = 1AM = 1e- = 1

BoronAN = 5

AM = 11e- = 5

1 p+, 0 no

5 p+, 6 no

Drawing Bohr Models

ChlorineAN = 17AM = 35e- = 17

17 p+, 18 no

Electron Dot Diagrams

1. Write the symbol of the element

2. Determine the number of valence electrons (electrons in the outermost energy level)

3. Start placing valence electrons around the symbol

4. Pair electrons only after all four sides (top, bottom, left, right) have been used once

Example: Hydrogen

H

Hydrogen has 1 valence electron

Chose one side, and place the electron

H H HOR OR OR

Example: Chlorine

Cl

Chlorine has 7 valence electrons

Place one on each side first

Then begin to pair the remaining electrons

Example: Nitrogen

N

Nitrogen has 5 valence electrons

Place one on each side first

Then begin to pair the remaining electrons

Example: Beryllium

Be

Beryllium has 2 valence electrons

Place each electron on different sides

Example: Helium

He

Helium has 2 valence electrons

Helium is an EXCEPTION

Pair the two electrons because the energy level can only 2 electrons

Periodic Law

Scientists Julius Lothar Meyer and Dmitri Mendeleev individually produced classification schemes for elements in 1869.

Periodic Law: When elements are arranged in order of increasing atomic numbers, elements with similar chemical properties will occur at regular (periodic) intervals.

Periodic Table

The rows on the periodic table are separated into periods.

As chemists worked on determining which substances were elements, they noticed that some elements acted very much like other elements. For example, one atom of some metals always reacts with two atoms of oxygen. Chemists called these similar elements a group of elements.

Periodic Table

Periodic Table

Regions of the Periodic Table

Metals: Elements to the lower left side of the periodic table.

Nonmetals: Elements to the upper right of the periodic table.

Metalloids: Elements in between the metals and nonmetals.

Metalloids

1

2

3 4 5 6 7 8 9 10 11 12

13 14 15 16 17

18

Metals

1

2

3 4 5 6 7 8 9 10 11 12

13 14 15 16 17

18Hydrogen is an exception

Nonmetals

Hydrogen is an exception 1

2

3 4 5 6 7 8 9 10 11 12

13 14 15 16 17

18