Chemical Bonds: Ionic, Covalent And Metallic
Visão Geral
Sodium is a soft metal that bursts into flame on water. Chlorine is a choking green gas that was once loaded into artillery shells. Push the two together and what falls out is the white powder you shook onto your food this morning. Nothing was added and nothing was taken away, yet every property you could measure has flipped. The whole explanation sits in a single layer of electrons on the outside of each atom, and in what that layer does at the instant the two elements meet.
This lesson takes you through the three ways atoms hold on to one another. In the first, electrons are handed over and the atoms end up charged. In the second they are shared between two nuclei and nothing becomes charged at all. In the third they are released into a crowd and belong to no single atom. By the end you will be able to look at the elements in any formula, name which of the three is at work, read the charge on an ion straight off the periodic table, and write the sentence that earns the mark instead of circling round it.
Objetivos
- Compounds are substances in which atoms of two or more elements are chemically combined.
- Chemical bonding involves either transferring or sharing electrons in the highest occupied energy levels (outer shells) of atoms in order to achieve the electron arrangement of a noble gas.
- When atoms form chemical bonds by transferring electrons, they form ions. Atoms that lose electrons become positively charged ions. Atoms that gain electrons become negatively charged ions. Ions have the electron arrangement of a noble gas (Group 0). Compounds formed from metals and non-metals consist of ions. Students should know that metals form positive ions, whereas non-metals form negative ions. Students should be able to represent the electron arrangement of ions in the following form: for sodium ion (Na+). Students should be able to relate the charge on simple ions to the group number of the element in the periodic table.
- The elements in Group 1 of the periodic table, the alkali metals, all react with non-metal elements to form ionic compounds in which the metal ion has a single positive charge. Knowledge of the chemical properties of alkali metals is limited to their reactions with non-metal elements and water.
- The elements in Group 7 of the periodic table, the halogens, all react with metals to form ionic compounds in which the halide ions have a single negative charge. Knowledge of the chemical properties of the halogens is limited to reactions with metals and displacement of less reactive halogens.
- An ionic compound is a giant structure of ions. Ionic compounds are held together by strong electrostatic forces of attraction between oppositely charged ions. These forces act in all directions in the lattice and this is called ionic bonding. Students should be familiar with the structure of sodium chloride but do not need to know the structures of other ionic compounds. Students given appropriate information, should be able to draw or complete diagrams to show how elements form ions and ionic compounds.
- When atoms share pairs of electrons, they form covalent bonds. These bonds between atoms are strong. Some covalently bonded substances, such as H2, Cl2, O2, N2, HCl, H2O, NH3 and CH4, consist of simple molecules. Others, such as diamond and silicon dioxide, have giant covalent structures (macromolecules). Students should be able to represent the covalent bonds in molecules such as water, ammonia, hydrogen, hydrogen chloride, methane and oxygen in the following forms: Students, given appropriate information, should be able to draw or complete diagrams to show how elements form covalent compounds by sharing electrons. Students should be able to recognise other simple molecules and giant structures from diagrams that show their bonding.
- Compounds formed from non-metals consist of molecules. In molecules, the atoms are held together by covalent bonds.
- Metals consist of giant structures of atoms arranged in a regular pattern.
- The electrons in the highest occupied energy levels (outer shell) of metal atoms are delocalised and so free to move through the whole structure. This corresponds to a structure of positive ions with electrons between the ions holding them together by strong electrostatic attractions. The bonding in metals is represented in the following form:
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- What happens when a metal atom and a non-metal atom form an ionic bond? A. Both atoms share a pair of electrons B. The metal atom gains electrons from the non-metal atom C. The metal atom transfers electrons to the non-metal atom D. Both atoms release electrons into a delocalised pool Answer: C
- An atom of an element in Group 2 forms an ion. What is the charge on that ion? A. 2- B. 1- C. 1+ D. 2+ Answer: D
- Which of these substances consists of simple molecules? A. Sodium chloride B. Methane C. Copper D. Silicon dioxide Answer: B
- A nitrogen atom has five electrons in its outer shell. How many covalent bonds does a nitrogen atom form in a molecule of ammonia? A. 1 B. 2 C. 3 D. 5 Answer: C
- A chlorine atom has the electron arrangement 2,8,7. What is the electron arrangement of a chloride ion? A. 2,8 B. 2,8,6 C. 2,8,8 D. 2,8,8,1 Answer: C
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