ionic and covalent bond- lesson science.pptx

ppt lesson ionic and covalent bond

1. T YP E S O F C H E MI C A L B O N D S

2. KEY POINTS A chemical bond is any force of attraction that holds two atoms or ions together. In most cases, that force of attraction is between one or more negatively charged electrons held by one of the atoms and the positively charged nucleus of the second atom. Chemical bonds vary widely in their strength, ranging from relatively strong covalent bonds (in which electrons are shared between atoms) to very weak hydrogen bonds. The term chemical bond also refers to the symbolism used to represent the force of attraction between two atoms or ions. For example, in the chemical formula H— O— H, the short dashed lines are known as chemical bonds.

3. HISTORY Theories of chemical bonds go back a long time. One of the first was developed by Roman poet Lucretius (c. 95–c. 55 B.C. ), author of De Rerum Natura (title means "on the nature of things"). In this poem, Lucretius described atoms as tiny spheres with fishhook-like arms. Atoms combined with each other, according to Lucretius, when the hooked arms of two atoms became entangled with each other.

4. IONIC BONDING Ionic bonding Consider now a more difficult situation, one in which two different atoms compete for electrons. One example would be the case involving a sodium atom and a chlorine atom. If these two atoms come close enough to each both nuclei pull on all electrons of both atoms. In this case, a very different result occurs. The chlorine nucleus has a much charge than does the sodium nucleus. It can pull on sodium's electrons much more efficiently than the sodium nucleus can pull the chlorine electrons. In this case, there is a winner in the battle: chlorine is able to pull of sodium's electrons away. It adds that electron to its own of electrons. In a situation in which one atom is able to completely remove an electron from a second atom, the force of attraction between the two particles is known as an ionic bond.

5. IONIC BONDING

6. To determine the chemical formulas of ionic compounds, the following two conditions must be satisfied: Each ion must obey the octet rule for maximum stability. Ions will combine in a way that the overall ionic compound will be neutral. In other words, the charges of the ions must balance out.

7. ionic bond: sodium chloride, or table salt Ionic bonding in sodium chloride. An atom of sodium (Na) donates one of its electrons to an atom of chlorine (Cl) in a chemical reaction, and the resulting positive ion (Na + ) and negative ion (Cl − ) form a stable ionic compound (sodium chloride; common table salt) based on this ionic bond.

8. EXAMPLE 1: CHLORIDE SALTS In this example, the sodium atom is donating its 1 valence electron to the chlorine atom. This creates a sodium cation and a chlorine anion. Notice that the net charge of the resulting compound is 0. In this example, the magnesium atom is donating both of its valence electrons to chlorine atoms. Each chlorine atom can only accept 1 electron before it can achieve its noble gas configuration; therefore, 2 atoms of chlorine are required to accept the 2 electrons donated by the magnesium. Notice that the net charge of the compound is 0.

9. EXAMPLE 2: MAGNESIUM FLUORIDE Mg most commonly forms a 2+ ion. This is because Mg has two valence electrons and it would like to get rid of those two ions to obey the octet rule. Fluorine has seven valence electrons and as such, usually forms the F – ion because it gains one electron to satisfy the octet rule. When Mg 2+ and F – combine to form an ionic compound, their charges must cancel out. Therefore, one Mg 2+ needs two F – ions to balance. The 2+ of the Mg is balanced by having two - 1 charged ions. Therefore, the formula of the compound is MgF 2 . The subscript two indicates that there are two fluorine's that are ionically bonded to magnesium.

10. KEY POINTS Ionic bonds are formed through the exchange of valence electrons between atoms, typically a metal and a nonmetal. The loss or gain of valence electrons allows ions to obey the octet rule and become more stable. Ionic compounds are typically neutral. Therefore, ions combine in ways that neutralize their charges.

11. COVALENT BONDING Covalent bonding is the sharing of electrons between atoms. This type of bonding occurs between two atoms of the same element or of elements close to each other in the periodic table. This bonding occurs primarily between nonmetals; however, it can also be observed between nonmetals and metals.

12. Covalent bonding Today, it is widely accepted that most examples of chemical represent a kind of battle between two atoms for one or more electrons. Imagine an instance, for example, in which two hydrogen atoms are placed next to each other. Each atom has a positively charged nucleus and one electron spinning around its nucleus. If the atoms are close enough to each other, then the electrons of both atoms will be attracted by both nuclei. Which one wins this battle? The answer may be obvious. Both atoms are exactly identical. Their nuclei will pull with equal strength on both electrons. The only possible result, overall, is that the two atoms will share the two electrons with each other equally. A chemical bond in which two electrons are shared between two atoms is known as a covalent bond.

13. EXAMPLE 1: PHOSPHORUS TRICHLORIDE (PHOSPHORUS CHLORIDE, PCL3 ) In this example, a phosphorous atom is sharing its three unpaired electrons with three chlorine atoms. In the end product, all four of these molecules have 8 valence electrons and satisfy the octet rule.

14. KEY POINTS Covalent bonds involve two atoms, typically nonmetals, that share electron density to form strong bonding interactions. Covalent bonds include single, double, and triple bonds and are composed of sigma and pi bonding interactions where 2, 4, or 6 electrons are shared respectively. Covalent compounds typically have lower melting and boiling points than ionic compounds.

15. WHICH TYPE OF BOND IS STRONGER? Ionic bonds take more energy to break than covalent bonds, so ionic bonds are stronger . The amount of energy needed to break a bond is known as bond dissociation energy, which is basically the force it takes to break bonds of any type.