Atoms, Bonding and the Periodic Table Chapter 5, Section 1

Key Concepts How is the reactivity of elements related to valence electrons in atoms? What does the periodic table tell you about atoms and the properties of elements? Page 12

Key Terms Valence electrons Electron dot diagram Chemical bond Period Group Family Page 12 Say the terms out loud Identify words that they are most familiar with

Electrons and Energy Levels Where are electrons found? Neils Bohr suggested: Electrons have specific amounts of energy leading them to move in specific orbits around the nucleus of the atom Resemble planets orbiting around the sun or like the layers of an onion HowStuffWorks "How Atoms Work"science.howstuffworks.com -

Cloud of Electrons In the 1920’s scientists determined that electrons do not orbit the nucleus like planets but rather in a cloudlike region. This region is where electrons are likely to be found. An electron movement is related to its energy level or specific amount of energy it has. Visualizing an electron cloud: electric or battery operated fan Show students that when the fan is off they can clearly distinguish each blade. Then turn it on and ask, “How would you describe the blades?” Like a blur – possible answer Ask, How are the electrons in a “cloud” of electrons like the blades of the fan?” Electrons in a cloud move in such a way around the nucleus that it is impossible to identify their positions

Valence Electrons Electrons that have the highest energy levels and are held most loosely are known as the atom’s valence electrons. The number of valence electrons in an atom of an element determines many properties of that element, including the ways in which the atom can bond with other atoms. Second bullet – very important

Electron Dot Diagram Each element has a specific number of valence electrons, ranging from 1 to 8. An electron dot diagram includes the symbol for the element surrounded by dots. Each dot stands for one valence electrons. Identifying electrons can be done with an electron dot diagram Turn to page 13 in your book – look at figure 9 Draw a picture of on of the diagrams of figure 9 in your book. Please note that you can place the first two dots on any side, but the rest of the dots should be placed in either a clockwise or counter clockwise manner, with no side receiving two dots until each side gets one . By looking at the electron dot diagram for oxygen we can see that oxygen has two unpaired electrons, so it has two electrons available for standard covalent bonds. Oxygen www.fordhamprep.com/.../sho/lessons/lesson38.htm Chlorine Electron Dot Notation is the shorthand method by which chemists portray the quantity of valence electrons that any given element may contain. www.ibpassociation.org/encyclopedia/Chemistry...

Quick Review: Chemical symbol Atomic number Period Group or Family One or two letters that represents an element. Atomic number Number of protons in the nucleus of an atom. Periodic table is arranged in order of increasing atomic number Period Row of elements across the periodic table Group or Family Elements in the same column

Chemical Bonds and Stability How is the reactivity of elements related to valence electrons in atoms? Most atoms are more stable – less likely to react – when they have eight valence electrons. What are some examples of elements that are stable? Neon, argon, krypton, and xenon Small atoms such as helium are stable with just two valence electrons – which is why it is found in group 18, noble gases Key concept question Examples: neon, argon, krypton, and xenon all have eight valence electrons and are vey unreactive – they do not easily form compounds. Some small atoms such as helium are stable with just two valence electrons – which is why it is found in group 18 – noble gases.

How can an atom react to become more stable? Two ways: Either the valence electrons increases to eight (or two in the case of hydrogen) Atom gives up loosely held valence electrons Atoms that react this way become chemically combined or bonded to other atoms – chemical bond. Chemical bond is the force of attraction that holds two atoms together.

Chemical Bonds and Chemical Reactions Chemical bonds may occur two ways: Electrons may be transferred from one atom to another. Electrons may be shared between the atoms. In both cases a chemical reaction takes place and a new substance is formed. Page 13

How is the periodic table organized? Organized by atomic number If you know the atomic number, what else do you know? The periodic table gives you information about the arrangement of electrons in atoms. How do the atoms change from left to right across a period, or row? As the number of protons – or atomic number increases, the number of electrons also increases. As a result, the properties of the elements change in a regular way across a period. A period ends when the number of valence electrons reaches 8 (except for period 1) Page 16

Reading the Periodic Table What happens when you get to the next period? It will begin with atoms having higher energy. The repeating pattern means that the elements within a group, or column, always have the same number of valence electrons. View the periodic table on page 152 and the electron dot diagrams on page 153. What pattern do you see?

Patterns of Valence Electrons Elements in groups 3-12 follow a slightly different pattern.

What does the periodic table tell you about the properties of elements? The elements within a group have similar properties because they all have the same number of valence electrons in their atoms.

Noble Gases Noble gases Group 18 Have 8 valence electrons Except for Helium which only has 2 They do not react easily with other elements – they are stable Chemists have been able to make noble gases form compounds with a few other elements.

Reactive Nonmetals and Metals Group 17 Name of the group Halogens How many valence electrons do they have? Seven Will they gain electrons or lose electrons? Gain or share electrons As a result, they will react easily with other elements whose atoms can give up or share electrons. Which group would these elements be found in?

What group are alkali metals found? Do alkali metals gain or lose electrons? Alkali metals become chemically more stable by losing their one valence electron This property makes the alkali metals very reactive Which group could these atoms lose their electrons to? Halogens – group 17 Seven valence electrons, just one more electron gives these atoms the stable number of eight electrons, like the noble gases For example, sodium chloride Sodium has 1 valence electron Chlorine has 7 valence electrons Alkaline Earth metals, lose electrons easily and are almost as reactive as the Alkali metals.

Other metals How reactive a metals is, depends on what key factor? Depends on how easily its atoms lose valence electrons. Groups 2-12 are metals, like group 1 Most have 1, 2, or 3 valence electrons They react by losing these electrons, especially when they combine with oxygen or one of the halogens. Elements in group 2 thru 12 are metals Platinum (Pt) and gold (Au) are unreactive Mercury (Hg) is the only metal that is a liquid at room temperature All the other metals are solids, although gallium (Ga) melts just above room temperature

Which of the nonmetals are solid at room temperature? Carbon (C), phosphorus (P), sulfur (S), selenium (Se), and iodine (I) Which nonmetal is the only liquid? Bromine (B) Like the halogens, nonmetals may become stable by gaining or sharing electrons Of the nonmetals, only oxygen and the halogens are highly reactive. Is hydrogen a metal or a nonmetal? It is considered a nonmetal. It is in group 1 only because it has one valence electron. Hydrogen’s properties differ greatly from the alkali metals

Where are the metalloids found on the periodic table? Along the zig zag line between the metals and nonmetals How many valence electrons do metalloids have? Three to six Do the metalloids gain, lose, or share electrons? They can either lose or share electrons when they combine with other elements.