Atomic combinations

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Atomic combinations

  1. 1. 1 3. Atomic combinations Chemistry Grade 11Everything Science www.everythingscience.co.za
  2. 2. 2RevisionA chemical bond is the physical process that causes atoms to be attracted together and tobe bound in new compounds.From grade 10 we know that the electrons in an atom are arranged in fixed energy levels(or shells). The electrons in the outermost energy level of an atom are called the valenceelectrons.We note the following:● Electrons always try to occupy the lowest possible energy level.● The noble gases have a full valence electron orbital. The second energy level is theoutermost (valence) shell and is full.● Atoms form bonds to try to achieve the same electron configuration as the noble gases.● Atoms with a full valence electron orbital are less reactive. Everything Science www.everythingscience.co.za
  3. 3. 3Energy and bondingThere are three forces that act between atoms: attractive forces between the positive nucleusof one atom and the negative electrons of another; repulsive forces between like-chargedelectrons, and repulsion between like-charged nuclei.The energy of a system of two atoms is at a minimum when the attractive and repulsive forcesare balanced. Everything Science www.everythingscience.co.za
  4. 4. 4Lewis diagramsLewis diagrams are one way of representing molecular structure. In a Lewis diagram,dots or crosses are used to represent the valence electrons around the central atom. Everything Science www.everythingscience.co.za
  5. 5. 5Covalent bondsA covalent bond is a form of chemical bond where pairs of electrons are shared betweentwo atoms.● A single bond occurs if there is one electron pair that is shared between the same twoatoms.● A double bond occurs if there are two electron pairs that are shared between the sametwo atoms.● A triple bond occurs if there are three electron pairs that are shared between the sametwo atoms.● A dative covalent bond is a description of covalent bonding that occurs between twoatoms in which both electrons shared in the bond come from the same atom. Everything Science www.everythingscience.co.za
  6. 6. 6Valence shell electron pair repulsion (VSEPR) theoryValence shell electron pair repulsion theory is a model in chemistry, which is used to predict theshape of individual molecules. VSEPR is based upon minimising the extent of the electron-pairrepulsion around the central atom being considered.We use the following steps to predict a molecules shape:1. Draw the molecule using a Lewis diagram. Make sure that you draw all the valence electronsaround the molecule’s central atom.2. Count the number of electron pairs around the central atom.3. Determine the basic geometry of the molecule. Everything Science www.everythingscience.co.za
  7. 7. 7ElectronegativityElectronegativity is a chemical property which describes the power of an atom to attractelectrons towards itself in a chemical.Electronegativity can be used to explain the difference between two types of covalentbonds: polar covalent bonds (between non-identical atoms) and non-polar covalentbonds (between identical atoms or atoms with the same electronegativity).A polar molecule is one that has one end with a slightly positive charge, and one endwith a slightly negative charge. Examples include water, ammonia and hydrogenchloride.A non-polar molecule is one where the charge is equally spread across the molecule ora symmetrical molecule with polar bonds. Everything Science www.everythingscience.co.za
  8. 8. 8Energy and bondingBond length is the distance between the nuclei of two atoms when they bond.Bond energy is the amount of energy that must be added to the system to break the bondthat has formed.Bond strength means how strongly one atom attracts and is held to another atom. Bondstrength depends on the length of the bond, the size of the atoms and the number of bondsbetween the two atoms. Everything Science www.everythingscience.co.za
  9. 9. 9Energy and bonding Everything Science www.everythingscience.co.za
  10. 10. 10For more practice see:www.everythingscience.co.zaShortcode: ESBMK Everything Science www.everythingscience.co.za

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