Ions 1.2 Flashcards
Atoms and Ions
Atoms are electrically neutral (have no charge) because they contain an equal number of positive protons and negative electrons. However, if an electron is removed or added, the tom becomes charge and is now called an ion. When an electron is removed from an atom, that atom becomes a positively charged ion. When an electron is added to an atom the atom becomes negatively charged. Positive ions are known as cations and negative ions are known as anions.
Cations
A cation is formed when an atom loses electrons. An atom will tend to form cations if its outermost electron shell is mostly empty. The atom will usually lose all the electrons in the outermost shell so that only filled shells remain.
Almost all cations come from metal atoms. This is because metal atoms have few electrons in their outermost electron shell and these electrons are only weakly bound to the atom. Cation symbols are similar to atoms symbols, for example, copper atoms (Cu) may lose one or two electrons to produce the copper ions Cu+ and Cu2+. To distinguish between these two ions, scientists add a roman numeral to the ion name that indicates the number of electrons lost. Therefore, the copper ion Cu+ os referred to as the copper(I) ion. The ion Cu2+ os referred to as the copper (II) ion. Iron (Fe) can also form two types of cations: iron(II) Fe2+ or iron(III) Fe3+.
Anions
An anion is produced when an atom gains electrons. This will occur if the outermost electron shell of the atom is almost full. In that case, the atom gains additional electrons until the shell is filled.
All anions come from non-metallic atoms. These atoms gain electrons in their outer electron shell. The symbol used to represent anions is similar to those used for cations. they are made up of the chemical symbol for the atom and the charge of the ion. For example, a chlorine atom gains one electron and so it has a charge of -1. Therefore, the ion is represented by the symbol Cl-. An oxygen atom (O) gains two electrons, so the ion os represented as O2-.
Ionic Compounds
When both anions and cations come together, they form compounds made up of large crystal lattices. These compounds are known as ionic compounds. For example, common table salt is an ionic compound with the chemical name sodium chloride (NaCl). Other ionic compounds are lithium chloride (LiCl), potassium fluoride (KF) and magnesium oxide (MgO).
Chemical Names and Formulas
Naming ionic compounds is very easy. You simply wrote the name of the cation followed by the name of the anion. For example, the ionic compound known as calcium oxide is made up of calcium cations (Ca2+) and oxide anions (O2-). The ionic compound is known as copper (I) chloride is made up of copper (I) cations (Cu+) and chloride anions (Cl-).
Writing the chemical formula is slightly more difficult. When writing the chemical formula for an ionic compound, you must ensure that there is an equal number of positive and negative charges so that the total charge is zero. In the case of sodium chloride, the sodium ion has a charge of +1 and the chloride ion has a charge of -1. Therefore, the chemical formula is just NaCl because you only need one of each to balance the charges fully. The charges are not shown in the chemical formula because the total charge is zero.
However, in the case of magnesium chloride, the magnesium ion has a charge of +2 and the chloride2 ion has a charge of -1. Therefore, two chloride ions are needed to balance the charge of each magnesium ion. This is represented in the chemical formula by writing MgCl2.
Ionic Bonding
Cations and anions are attracted to each other because they have opposite electric charges. When cations come close to anions, they stick together, forming an ionic bond.
The ionic bonds holding crystal lattices together are very strong. Therefore ionic compounds are usually hard, brittle and have melting points.
Ionic compounds are hard because it takes a lot of force to break the ionic bonds. They are brittle because the ionic bonds hold the ions fixed positions and this means the latice shatters rather than bends. They have high melting points because high temperatures are required to break the strong ionic bonds and allow the ions to flow freely as a liquid. Ionic compounds are also often brightly coloured.
Ions in solution
Some ionic compounds are soluble (dissolve) in the water while others are insoluble (don’t dissolve). How easily an ionic compound dissolves is known as its solubility.
When an ionic compound dissolves in water, the water particles surround the cations and anions which breaks the crystal lattices apart and stops the ions from sticking back together. The ions then spread evenly throughout the water which is then called a solution. If the water was removed through boiling or evaporation, the ions can stick together once again in a process called recrystallization.
Ions move more freely through the liquid when they are in a solution. This means they can conduct electricity by creating a flow of electric charge. If positive and negative electrodes were placed in the solution, then the positive cations will love towards the negative electrode and the negative anions will move towards the positive electrode. This will allow the electrical current to flow through the entire circuit. Only liquids which contain ions will allow electrical current to flow. For example, liquids like oil and kerosene do not contain ions, and therefore do not conduct electricity.
