COVALENT BONDING AND STRUCTURE

These oppositely charged ions attract each other and are pulled, or bonded, to one another by strong electrostatic forces. This type of bonding is cal...

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COVALENT BONDING AND STRUCTURE DESCRIPTIF DE L’ACTIVITE DESTINE AU PROFESSEUR Préambule du programme du cycle terminal de la voie S : […] en devant présenter la démarche suivie et les résultats obtenus, l’élève est amené à une activité de communication écrite et orale susceptible de le faire progresser dans la maîtrise des compétences langagières, orales et écrites, dans la langue française, mais Compétences exigibles aussi en langue étrangère, notamment en anglais, langue de communication du B.O. internationale dans le domaine scientifique. […] Programme de première S : Cohésion et transformations de la matière. Cette activité d’approfondissement suit l’activité de soutien portant sur les liaisons ioniques. Elle est prévue pour une durée d’une heure en séance d’accompagnement personnalisé, en classe de première S. Déroulement de l’activité

Déroulement :  Le document est donné aux élèves qui en prennent connaissance.  Selon les difficultés rencontrées, le professeur leur remet, si besoin, une fiche « coup de pouce » permettant de pallier les difficultés de compréhension liées au vocabulaire.  A la fin de la séance, il est préconisé de distribuer l’une des deux fiches pour s’assurer que l’ensemble des élèves s’est approprié l’activité. Compétences linguistiques :  compréhension écrite de la langue anglaise ;  expression écrite.

Compétences évaluées En plus des compétences linguistiques, cette activité permet de travailler les compétences de la démarche scientifique :  s’approprier (APP)  analyser (ANA)  réaliser (REA) Cette activité peut se décliner selon trois niveaux de compétences :

Remarques

Auteurs

Académie d’Orléans-Tours

 niveau 1 : une fiche « coup de pouce » permet de donner le vocabulaire nouveau (ou qui pose problème) directement en français (fiche 1).  niveau 2 : le vocabulaire nouveau est introduit sous forme écrite par le biais de fiche « coup de pouce » proposant les définitions en anglais (fiche 2).  niveau 3 : le vocabulaire nouveau (ou qui pose problème) est explicité oralement par l’enseignant. Séverine Leget – Lycée Marceau – Chartres Delphine Pailler – Lycée Paul-Louis Courier – Tours Florence Trouillet – Lycée Claude de France – Romorantin

De la physique en anglais !

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COVALENT BONDING AND STRUCTURE Compétences travaillées (capacités et attitudes) :  APP : mobiliser ses connaissances, extraire des informations utiles.  ANA : exploiter des informations ; adopter une démarche explicative.  REA : appliquer une consigne donnée.

INTRODUCTION Ionic bonds are usually found in compounds that contain metals combined with non-metals. For example, consider what happens when sodium and chlorine combine to make sodium chloride: sodium + chlorine  sodium chloride The charges on the sodium and chloride ions are equal but opposite. They balance each other and the resulting formula for sodium chloride is NaCl. These oppositely charged ions attract each other and are pulled, or bonded, to one another by strong electrostatic forces. This type of bonding is called “ionic bonding”. Another way in which atoms can bond together is by sharing electron(s). This occurs between non-metal atoms, and the bond formed is called a covalent bond. The simplest example of this type of bonding is the hydrogen molecule H2. Each hydrogen atom has one electron, and the molecule of hydrogen is formed by sharing this pair of electrons between those two hydrogen atoms. Generally, covalent compounds do not conduct electricity when molten or dissolved in water. This is because they do not contain ions.

CARBON, AN EXAMPLE OF ATOM BOUND COVALENTLY Carbon is a non-metallic element present in all organic molecules. In nature, it is a material which exists in more than one solid structural form, these forms are called allotropes. Allotropes of carbon are graphite and diamond. Each of these allotropes has a different structure (Figures A and B) and so the allotropes exhibit different physical properties that lead allotropes being used in different ways. (A) A portion of the graphite structure.

(B) The structure of diamond, and the regent Diamond, on occasion worn by Queen Elisabeth II. Académie d’Orléans-Tours

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Property

Graphite

Diamond

Appearance

A dark grey, shiny solid

A colorless transparent which sparkles in light

Electrical conductivity

Conducts electricity

Does not conduct electricity

Hardness

A soft material with slippery feeling

A very hard substance

Density (g.cm-3)

2.25 3.51 (C) Physical properties of graphite and diamond.

QUESTIONS (a) Complete the table giving examples of the use of carbon graphite and carbon diamond. Graphite

Diamond

(b) How many bonds does carbon have in diamond and graphite? ………………………………………………………………………………………………………………………………………………………………………………… (c) Comparing both structures (Figures A and B), explain why graphite is slippery instead of diamond. ………………………………………………………………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………………………………… (d) Having a look at each arrangement of the lattice and the number of bounds around each carbon (Figures A and B), determine which is the hardest. Justify. ………………………………………………………………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………………………………… (e) - Give the electronic structure for Carbon 6C. ………………………………………………………………………………………………………… How many outer electrons does it have? …………………………………………………………………………………………………………….. - In the case of graphite, how many covalent bonds are there around an atom of carbon? …………………………………… With these two data, explain why graphite can conduct electricity. ……………………………………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………………………………… (f) The density of a material is defined as its mass per unit volume. Thus, what can you deduce about the number of carbon atoms in a small volume of graphite, compared to diamond? ………………………………………………………………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………………………………… Académie d’Orléans-Tours

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ANSWERS (a) Complete the table giving examples of the use of carbon graphite and carbon diamond. (APP) Graphite Pencils Electrodes Lubricant

Diamond Jewels Glass cutters Diamond-studded saws Polishers

(b) How many bonds does carbon have in diamond and graphite? (APP) 3 bonds in graphite and 4 in diamond. (c) Comparing both structures (Figures A and B), explain why graphite is slippery instead of diamond. (ANA) In graphite, carbon atoms are arranged in layers. There are only weak forces between the layers so they can slide over each other quite easily. When we write with a pencil, some layers of carbon atoms slide off the pencil and are left on the paper. (d) Having a look at each arrangement of the lattice and the number of bounds around each carbon (Figures A and B), determine which is the hardest. Justify. (ANA) The atoms in diamond cannot slide like this because of their arrangement in the lattice, moreover, each carbon atoms in diamond is bonded to four others, cons three for graphite, so the structure is reinforced in the diamond case. (e) - Give the electronic structure for Carbon 6C. (REA) (K)2 (L)4 How many outer electrons does it have? Four outer electrons (For example: 1H has one outer electron, 16O has six outer electrons, 9F has seven outer electrons…) - In the case of graphite, how many covalent bonds are there around an atom of carbon? (ANA) There are only three covalent bonds formed between carbon atoms within the layers, consequently, an unbounded electron is present on each carbon atom. With these two data, explain why graphite can conduct electricity. (ANA) An important property of graphite comes from the fact that there are free electrons within its structure, and these free electrons allow graphite to conduct electricity (which diamond simply cannot do). Free electrons found in graphite are called “delocalized electrons”. (f) The density of a material is defined as its mass per unit volume. Thus, what can you deduce about the number of carbon atoms in a small volume of graphite, compared to diamond? (ANA) Diamond density is bigger than graphite density; therefore, the mass of carbon atoms in a small sample of diamond is bigger than the same mass in a sample of graphite with the same volume. This leads us to the conclusion that in a given sample there will be more atoms in diamond structure than in the graphite one.

Académie d’Orléans-Tours

De la physique en anglais !

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USEFUL VOCABULARY - LEVEL 1

bond

liaison chimique

to bond

lier

compound

composé (ici, un solide ionique)

to balance

équilibrer

to pull

tirer

to share

partager

molten

liquéfié

allotrope

allotrope

to exhibit

montrer, faire apparaître une caractéristique

to lead

conduire à, mener à

worn

pp “wear” = porter

hardness

dureté

shiny

brillant

soft

mou

slippery

glissant

colorless

incolore

to sparkle

étinceller

outer electrons

électrons de valence

Académie d’Orléans-Tours

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USEFUL VOCABULARY - LEVEL 2

bond to bond compound to balance to pull to share molten allotrope to exhibit

the way in which atoms are held together in a chemical compound. to join two things firmly together. The atoms bond together to form a molecule. a substance formed by a chemical reaction of two or more elements in fixed amounts relative to each other. Common salt is a compound of sodium and chlorine. to have equal totals of atoms and charges, in the reagents and in the products. to use a force to move something towards yourself. I pulled on the rope to make sure that it was secure. to divide something between two or more people. We shared the pizza out between the four of us. made liquid by very great heat (used about metal or rock). the ability that certain substances have to exist in more than one physical form. to show clearly that you have a particular quality, or a particular feeling. The refugees are exhibiting signs of exhaustion and stress.

to lead

to have something as a result.

worn

pp “wear”.

hardness shiny

to quantify what is not soft to touch, not easy to break or to bend. something causing a bright effect when in the sun or in the light. The shampoo leaves your hair soft and shiny.

soft

not hard or firm. A soft bed.

slippery

when a surface is difficult to walk on because it is smooth or wet. A slippery floor.

colorless

without any color.

to sparkle outer electrons

Académie d’Orléans-Tours

to shine with many small points of light. The river sparkled in the sunlight. electrons which are in the outer shell ; in the highest energy level.

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