Atoms and thePeriodic Table

Dr. ChildsScience Laboratory

Spring, 2004

Cote d'Ivoire

Overview

This lesson has 4 parts:

1. Structure of the atom2. Periodic table3. Valence electrons / chemical reactivity4. Ionic and covalent bonds

1. Structure of the Atom

Nature of the Atom

The periodic table recognizes regular patterns in the atomic structure among the elements.

A review of the structure of the atom is essential to understand the periodic table.

Atoms & Elements

• Atoms are the simplest form of an element.

• An element is a substance that cannot be broken into anther substance (by ordinary chemical means).

Atoms

Atoms are composed of three types of subatomic particles:

•Protons•Neutrons•Electrons

Yes, there are a number of otherSubatomic particles (like quarks), butwe won’t worry about them

Nucleus withprotons and neutrons

Electron cloud

The atom has a nucleus which is composed of positively charged particle (protons) and neutral particles (neutrons). Negatively charged particles (electrons) are in a cloud around the nucleus. Electrons are in energy levels.

Atomic Model

Nucleus

Electroncloud

Atoms are composed of a nucleus with protonsand neutrons surrounded by a cloud of electrons.Electrons are in energy levels.

Subatomic Particles

Particle Location Mass Charge

Proton Nucleus 1 +

Electron Cloud 0 -

Neutron Nucleus 1No charge

Nature of AtomsAtoms have mass:

sum of number of protons plus neutrons.

In a balanced atom (+ charges = - charges):

number of protons = number of electrons

Nature of Atoms

The number of protons determine the element:

all carbon atoms have 6 protons

all oxygen atoms have 8 protons

What element is this?

Nature of Atoms

Finally, the number of electron and their configuration in energy levels (= shells) around the nucleus determines the chemical properties of the atom.

What element is this?

1st energy level

2nd energy level

2. Periodic Table

Elements• Elements are substance that consist of only

a single type of atoms.

• Most elements exist as compounds – combined with different elements – water, carbon dioxide, table salt (sodium chloride).

Elements

• There are about 90 naturally occurring elements on earth.

• There remaining elements on the period table have been man-made.

Periodic Table

Periods

Groups

Columns are “groups”Rows are “periods”

The Periodic Table

Each element in the periodic table contains:atomic number = number of protonssymbol – usually 1 or 2 lettersnamemass – average of isotopes

Metals, Metalloids, & Nonmetals

Boxed elements share properties of both metals and nonmetals – classified as metalloids.

Metals• Metals are generally on the left side of the

periodic table.

• Properties of metals:

• Conduct electricity

• Usually shiny

• Examples: magnesium (Mg), iron (Fe), sodium (Na), copper (Cu), titanium (Ti)

Nonmetals• Nonmetals are generally on the right side of

the periodic table.

• Properties of nonmetals:

• Do not conduct electricity

• Brittle

• Usually dull

• Examples: sulfur (S), oxygen (O), iodine (I), carbon (C), phosphorus (P)

Metalloids

• Metalloids are transitional between metals and nonmetals with properties of each.

• These are often used for transistors

• Examples: germanium (Ge), silicon (Si)

3. Valence Electron & Chemical Reactivity

Valence Electrons• An understanding of the configuration of

electrons around the nucleas is very important for the understanding of:

– How atoms react – How atoms are grouped in the periodic table– How atoms form bonds– How to identify chemical compounds– How to balance chemical equations

Octet Rule

• Electrons in the outer energy level determine the reactivity of the atom. The electrons in the outer energy level are called “valence electrons”.

• Hydrogen and helium need 2 electrons to fill the outer energy level.

• Other elements require 8 electrons to fill their outer energy level (=octet).

Electron Levels

• Electrons are in energy levels.

• The lowest level has two electrons.

• The outer level may have 1 to 8 valence electrons.

• How many electron in the outer level keeps the atom happy? Duh! How about 8?

Chemical Stability• If an element has an outer level that is complete

with 8 valence electrons (2 for helium) the atoms is stable or non-reactive.

• These elements include neon (Ne), Argon (Ar), and Krypton (Kr).

• No! Not “kryptonite”! That’s the green stuff that hurts Superman!!

Valence Electrons

• For the TAKS test it is very important to be able to determine the number of valence electrons from the periodic table and to be able to visualize these.

• Most questions related to chemical reactivity are related to the valence electrons.

Valence• At the top of each column (or group) is a number in

Roman numerals with an “A”. This is the number of valence electrons.

• For the moment ignore the transition metals (Groups 3 thru 12).

• For example: – Group 1 IA 1 valence electron

– Group 15 VA 5 valence electrons

– Group 17 VIIA 7 valence electrons

– Group 18 VIIIA 8 valence electrons

Valence• Elements arranged in columns have the same number

of valence electron and have similar chemical properties.

• Group 2 (IIA) elements (Be, Mg, Ca) all have 2 valence electrons and similar chemical properties. Likewise, group 17 (VIIA) elements (F, Cl, Br) all have 7 valence electrons and similar chemical properties.

Lewis Dot Diagrams

• Lewis dot diagrams show only the electrons in the outer shell – the valence electrons that contribute to the chemical properties of the atom.

Na • Ca ••F ••• •

•• •

Ar ••• •

• •

••

Valence Electrons & Chemical Properties

• Note that oxygen (O) has an 8 protons and 8 electrons, but only 6 valence electrons

• Sulfur (S) has 16 protons and 16 electrons. But, like oxygen (O), only has 6 valence electron.

• Since both O and S have a similar Lewis dot diagrams they have similar chemical properties.

O ••• •

•• S ••

• •

•

•oxygen sulfur

Periodic Table• Elements can be arranged by increasing mass and

by chemical reactivity.

• Columns (or “groups”) are arranged by like chemical reactivity - same Lewis dot diagram.

• Rows (or “periods”) are arranged by increasing atomic number and mass.

Periodic Table

• Elements in “Group IA” and “Group “IIA” have valence electrons that are easily lost.

Please follow along with a periodic table!

Na • Ca ••

Periodic Table

• Groups VIA and VIIA have 6 and 7 valence electron. These readily gain 2 and 1 electron to fill the outer energy levels to 8 electrons.

Please follow along with a periodic table!

O ••• •

•

•Cl ••• •

•

•

•

Periodic Table

• Finally, atoms of elements in Group VIIIA have a complete set of 8 valence electrons. These atoms or “inert” or “nonreactive”. They neither gain nor lose electrons.

Please follow along with a periodic table!

Ne ••• •

•

•

•• Ar ••

• •

•

•

••

4. Ionic & Covalent Bonds

H2O

Bonds

• Atoms are joined together to form compounds or molecules by bonds.

• There are two major classes of bonds:

• ionic

• covalent

Bonds

• Atoms form bonds in order to become more stable.

• According to the Octet Rule, atoms form bonds by gaining, losing, or sharing electrons in order to obtain 8 valence electrons.

Ionic Bonds

• Atoms, typically those in groups IA and IIA lose electron and become positive ions.

• Atom, typically those in groups VIA and VIIA gain electrons and become negative ions.

Ionic Bonds

• Ionic bonds are formed by the attraction of positive and negative ions.

Ionic Bonds

Ionic Bonds

•To make compounds the total positive charges must balance the total negative charges.

Na+ + Cl- NaCl

K+ + Br- KBr

Mg+2 + O-2 MgO

Mg+2 + 2Cl- MgCl2

2H+ + S-2 H2S

Covalent Bonds

Covalent bonds are formed if electrons are shared and each atom will have a complete outer energy level.

Hydrogen atoms have a single valence electron. When they form a a compound, the electrons are shared and the atoms become stable each with two valence electrons/

Covalent Bonds

In a water molecule each hydrogen shares two electrons with the oxygen atom.Essentially, hydrogen has 2 electrons and oxygen has 8 valence electron. So all atoms are stable.

H2O water

Covalent Bonds

Examples of compounds with covalent bonds are water (H2O), oxygen gas (O2), hydrogen gas (H2), and methane (CH4).

Bonds

• But who’s our favorite Bond:

Bond. James Bond!

Bonds

• We also like

Barry Bonds!

For those that get less than an 80% on the test !!