Atoms and the Periodic Table Dr. Childs Science Laboratory Spring, 2004 Cote d'Ivoire

Overview This lesson has 4 parts: 1.Structure of the atom 2.Periodic table 3.Valence electrons / chemical reactivity 4.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 other Subatomic particles (like quarks), but we won’t worry about them Nucleus with protons 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 Electron cloud Atoms are composed of a nucleus with protons and neutrons surrounded by a cloud of electrons. Electrons are in energy levels.

Subatomic Particles ParticleLocationMassCharge ProtonNucleus1+ ElectronCloud0- NeutronNucleus1 No charge

Nature of Atoms Atoms 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? 1 st 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 protons symbol – usually 1 or 2 letters name mass – 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 15VA5 valence electrons –Group 17VIIA7 valence electrons –Group 18VIIIA8 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 oxygensulfur

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 H2OH2O

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

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 Cl -  MgCl 2 2H + + S -2  H 2 S

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. H 2 O water

Covalent Bonds Examples of compounds with covalent bonds are water (H 2 O), oxygen gas (O 2 ), hydrogen gas (H 2 ), and methane (CH 4 ).

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 !!