atoms and the periodic table dr. childs science laboratory spring, 2004 cote d'ivoire
TRANSCRIPT
Overview
This lesson has 4 parts:
1. Structure of the atom2. Periodic table3. Valence electrons / chemical reactivity4. Ionic and covalent bonds
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
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.
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)
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 ••
• •
•
•
••
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
•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).