Chemical bond Physics and Chemistry ESO

Transcription

1 CHEMICAL BOND There are three different ways atoms can bond. These three types of chemical bond are ionic bond, covalent bond and metallic bond. Three types of chemical bond: ionic bond, covalent bond and metallic bond Ionic bond Metals tend to lose electrons and become cations (with positive charge). Non metals tend to gain electrons and become anions (with negative charge). Therefore, when metals and non metals are together, metals can give electrons to non metals (metals will become positively charged and non metals will become negatively charged). Positive charges (metals) and negative charges (non metals) are attracted to each other by electrostatic forces. That is the ionic bond. A crystal is formed. For example, in lithium chloride (LiCl), each atom of lithium loses one electron which is gained by one atom of chlorine: Li Li + + 1e - Cl + 1e - Cl - What is a crystal? In a crystal a huge non definite number of atoms are regularly ordered in space. The chemical formula of a crystal represents the relative amounts in which the different elements are combined. For example, in LiCl there are one atom of lithium for each atom of chlorine. In CaCl 2 (calcium chloride) there are one atom of calcium for each two atoms of chlorine. Covalent bond Non metals tend to gain electrons. When two non metals are together, they both can gain electrons by sharing them. That s the covalent bond: non metals sharing pairs of electrons. The sharing of electrons is shown in Lewis dot diagram: 1

2 Covalent compounds can form molecules or crystals: What is a molecule? In a molecule a definite number of atoms are joined. The formula of a molecule represents the total number of atoms in the molecule. For example, in the molecule CH 4, we have exactly 1 atom of carbon and 4 atoms of hydrogen. Most covalent compounds form molecules. Some examples of covalent compounds (or elements) which form crystals are carbon (C), which can be in the form of diamond or graphite; silica (silicon dioxide, SiO 2 ); or carborundum (silicon carbide, SiC). Metallic bond Metals tend to lose electrons. When a lot of atoms of a metal are together, they all lose electrons: there is a crystal formed by cations with the lost electrons moving throughout the crystal, forming a sea of electrons. 2

3 Properties of compounds depending on its chemical bond Properties of ionic compounds They are solid at room temperature. Why? The ionic bond is very strong. Atoms are joined by strong forces. They have high melting points. Why? As the ionic bond is very strong, it requires a lot of energy to separate the atoms, in order for the substance to become liquid. High hardnes (it is difficult to scratch them). Why? As the ionic bond is very strong, it requires a lot of energy to break this bond to scratch the ionic compound. They are fragile. Why? When you hit the crystal, ions are displaced: positive charges face positive charges and negative charges face negative charges. Ions with the same charge repel each other. That s why the crystal breaks when hit by even small forces. They dissolve in water. Why? Electrons in the water molecule are not evenly spread. One end of the molecule is slightly negative compared to the other end. Water molecules interact with the ions, separating them from each other. They can t conduct electricity when they are in their solid state. Why? To conduct electricity it is necessary to have charges which can move freely. That 3

4 doesn t happen when ionic compounds are in their solid state: charges are in fixed positions in the crystal. They can conduct electricity when dissolved in water or melted. Why? Because when they are dissolved or melted, ions can move freely (the crystal no longer exists). Properties of covalent compounds The properties of crystal covalent compounds are: They are solid at room temperature with high melting points. Why? The covalent bond is very strong, so it requires a lot of energy to break the crystal in order to transform the substance into liquid. High hardnes (it is difficult to scratch them). Why? As the covalent bond is very strong, it requires a lot of energy to break this bond to scratch the crystal. They can t dissolve in water. Why? As there are no ions, water doesn t interact with the atoms. They don t conduct electricity. Why? Because there are no ions or free electrons. The properties of molecules are: They can be gas, liquid or solid at room temperature, but always with low boiling and melting points. Why? Covalent bond is very strong, but molecules are not joined by covalent bonds, they are joined by intermolecular forces, which are very weak. Most of them don t dissolve in water. Why? As there are no ions, water doesn t interact with the molecules. They can t conduct electricity. Why? There are no ions or free electrons. Properties of metallic compounds They are solid at room temperature, except mercury (Hg), which is a liquid. They have high melting points. Why? Metallic bond is a strong bond. Atoms are joined to each other by strong forces. It requires a lot of energy to separate these atoms (in order to transform the solid into liquid). High hardnes (it is difficult to scratch them). Why? As the metallic bond is very strong, it requires a lot of energy to break this bond to scratch the crystal. They can conduct electricity. Why? Because they have electrons that can move freely. They can conduct heat. Why? Because when electrons of one area are heated, their kinetic energy (which is related to their speed) increases, and they can transmit that kinetic energy to cations of other areas when they move. They are ductile [dúctiles] (wires can be made) and malleable [maleables] (sheets can be made). Why? The layers of atoms can move easily without changing the structure of the crystal. 4

5 Several metals can be melted together forming alloys (aleaciones). One example is bronze=copper+tin. Why? After melting cations of different elements can mix and easily form a new crystal when it is cooled down. They have high density. Why? Atoms are very close to each other in the crystal. Type of bond Type of elements Structure that is formed Ionic Metals + Non metals Crystals Covalent Non metals Crystals or molecules Metallic Metals Crystals Complete the table: State of matter at room temperature Ionic Covalent Metallic Melting and boiling points Hardness Fragility Solubility in water Conduct electricity in solid state Conduct electricity when dissolved in water or melted 5

6 Why do metals tend to lose electrons and non metals tend to gain electrons? Elements tend to have the electron configuration of noble gases, because this electron configuration is more stable. Noble gases are He, Ne, Ar, Kr, Xe and Rn. Except He, which has 2 electrons in its last layer, the rest of noble gases have 8 electrons in their last layers, so we can say that most elements tend to have 8 electrons in their last layer, which is called the octet rule. Metals are elements that have less than four electrons in their last layer, so it is easier for them to lose electrons than to gain them in order to obtain the electron configuration of a noble gas. For example, the electron configuration of Mg is 1s 2 2s 2 2p 6 3s 2. If it loses 2 electrons it becomes Mg 2+ whose electron configuration is 1s 2 2s 2 2p 6, which is the electron configuration of Ne. Non metals are elements that have more than four electrons in their last layer, so it is easier for them to gain electrons than to lose them to obtain the electron configuration of a noble gas. For example, the electron configuration of chlorine is 1s 2 2s 2 2p 6 3s 2 3p 5. If it gains one electron it becomes Cl -, whose electron configuration is 1s 2 2s 2 2p 6 3s 2 3p 6, which is the same electron configuration as Ar. - 6