Structures and Bonding - Foundation Level

Lesson 4

All the substances you know can be sorted into groups. One way of grouping substances is into solids, liquids or gases. Can you match up the state of matter with the correct description? Using the mouse, drag the term to the text box beside its description.

Solid, liquid and gas are the 3 states of matter. Each state has different properties. These different properties are due to the way in which the particles are arranged. Can you match up the state of matter with the correct description? Using the mouse, drag the term to the text box beside its description.

Some substances exist naturally in all three states - solid, liquid, and gas. Under normal conditions, whether something appears as a solid, liquid, or gas depends on its temperature. For example, water freezes when its temperature falls to, or below, 0 degrees Celsius. This temperature is the melting point of water. Above 100 degrees Celsius, water exists as a gas, steam. This is its boiling point. Between these two temperatures water is a liquid.

When a solid is heated, the particles gain energy and vibrate faster, and the temperature rises. The more the solid is heated, the more violently the particles vibrate. The temperature continues to rise. At the melting point, the temperature remains the same. This is because the energy being supplied is being used to overcome the forces holding the particles together, freeing them from their solid structure. If you continue to heat the liquid, the particles move around faster and faster, and the temperature rises. Some particles gain enough energy to overcome attractive forces and escape from the liquid to become a gas. This is evaporation. When the temperature is higher, more particles have enough energy to escape so evaporation is faster. When the temperature is high enough, the liquid will boil. The temperature at which a liquid boils is called its boiling point. When the liquid is boiling the temperature stays the same even though heat energy is still being supplied. This energy is being used to separate the liquid particles from each other.

Melting and evaporating are endothermic processes because in both cases energy is required. Changes such as melting and boiling are called physical changes. The particles stay the same, but the way they are arranged changes. That is why physical changes are reversible. If you cool steam back down to 100 degrees Celsius, it will condense to form liquid water again, as the particles lose energy and clump back together. If you continue to cool the water, it will freeze back to solid ice again at 0 degrees Celsius. Condensing and solidifying are exothermic processes because in both cases energy is transferred from the substance to the surroundings.

There are three main types of particle - atoms, molecules and ions. Atoms are the smallest particles that can exist on their own. Everything is made up of atoms. There are 92 kinds of atom which occur naturally on Earth. These 92 atoms combine in millions of different ways to form millions of substances. Atoms of different elements react together to form compounds. The properties of the compound are usually very different from the properties of the elements from which it is made. For example, sodium is a very reactive metal. When it reacts with chlorine, a poisonous gas, the compound sodium chloride or common salt is produced. This chemical reaction can be represented by a word or symbol equation. Sodium and chlorine are the reactants and sodium chloride is the product of the reaction. Reactants and products can be solids, liquids or gases, or can be dissolved in water. We show this by using the state symbols s, l, g, or aq.

Elements react because of the electron arrangement in their atoms. The electrons whizz around the nucleus of an atom in shells or energy levels. The first energy level can only hold two electrons. The second and third energy levels can hold up to eight electrons each. Atoms have no overall charge because there are equal numbers of negative electrons and positive protons. Scientists noticed that the noble gases in Group 8 are different from other elements, because they don’t usually form compounds. Their atoms are described as unreactive or stable. They are stable because they have full outer electron energy levels. When elements react to form compounds they do so in a way that enables them to obtain full electron energy levels, so they form stable compounds. This idea is the basis of chemical bonding. The word bond is used to describe the forces that hold atoms, molecules, and ions together.

Ionic bonds are usually found in the compounds formed when metals, such as sodium, react with non-metals, such as chlorine. Sodium chloride is an ionic compound. A sodium atom, atomic number 11, has just one electron in its outer energy level. The easiest way for it to become stable is to lose this single electron. The next energy level is completely full. The sodium ion now has 11 protons in the nucleus surrounded by 10 electrons, so the overall charge is +1. It is written as Na+, and is a positive ion. A chlorine atom, atomic number 17, has seven electrons in its outer energy level. The easiest way for it to become stable is to gain one electron to fill this energy level. The chloride ion now has 17 protons in the nucleus surrounded by 18 electrons, so the overall charge is -1. It is written as Cl- and is a negative ion. So, when a sodium atom reacts with a chlorine atom, the sodium atom loses its electron to the chlorine atom, and two ions are formed. The two ions have opposite charges, so they attract each other. The electrostatic force of attraction between them is strong. It is called an ionic bond. In this reaction sodium electrons are shown as dots and chlorine electrons as crosses, but remember that all electrons are exactly the same. This type of diagram is called a ‘dot and cross’ diagram.

When sodium reacts with chlorine, billions of sodium and chloride ions form, and are attracted to each other. The ions do not stay in pairs, but cluster together so that each ion is surrounded by six ions of opposite charge. They are held together by strong ionic bonds. The pattern grows until a giant 3D structure of ions is formed. This giant structure is the compound sodium chloride or common salt. Because the sodium chloride is made of ions it is called an ionic compound. Its formula is NaCl. The structure has no charge because it contains equal numbers of sodium and chloride ions. Compounds with giant structures have high melting and boiling points because of the strong electrostatic forces or bonds between the ions.

Other metals also react with non-metals to form ionic compounds. For example, magnesium burns strongly in oxygen to form magnesium oxide. A magnesium atom has 2 electrons in its outer energy level, and an oxygen atom has 6 in its outer energy level. During the reaction, each magnesium atom loses its 2 outer electrons to an oxygen atom. Magnesium ions and oxide ions are formed. The ions attract each other because of their opposite charges, and they group together into a giant ionic structure, called magnesium oxide. Magnesium oxide contains one magnesium ion for each oxide ion, so its formula is MgO. The compound has no overall charge, because there are equal numbers of positive and negative ions. For the formation of magnesium chloride, a magnesium atom has 2 electrons in its outer energy level, and a chlorine atom has 7 in its outer energy level. To obtain full outer shells, a magnesium atom must lose its 2 outer electrons, and a chlorine atom must gain 1 electron. So when magnesium burns in chlorine, each magnesium atom reacts with two chlorine atoms, to form magnesium chloride. The ions attract each other because of their opposite charges, and they group together into a giant ionic structure. Because magnesium chloride contains two chloride ions for each magnesium ion, its formula is MgCl2. The compound has no overall charge.

When two non-metal atoms react together, both of them need to gain electrons to reach full energy levels. They can manage this only by sharing electrons in their outer energy levels. The simplest example of this is seen in the hydrogen molecule. Each hydrogen atom has only 1 electron. To obtain a full outer energy level and gain the structure of the nearest inert gas, helium, each of the hydrogen atoms must have two electrons. To do this, two hydrogen atoms move close to each other, and allow their outer energy levels to overlap. A molecule of hydrogen is therefore formed with two hydrogen atoms sharing a pair of electrons. This shared pair of electrons is a single covalent bond. It can be represented as a single line.

A whole range of non-metal atoms can be held together to form molecules by sharing electrons and forming covalent bonds. These simple molecules only have weak forces between the molecules, so they have very low melting and boiling points. They are usually gases or liquids at room temperature. Examples are chlorine and oxygen. When oxygen atoms bond together to form oxygen molecules, they have to share two pairs of electrons to gain full outer energy levels. The bond between them is called a double covalent bond, or double bond. It can be represented by two lines.

Many non-metal elements exist as molecules, for example hydrogen, oxygen, and chlorine. A huge number of compounds also exist as molecules. In a molecular compound, atoms of different elements share electrons. These compounds are covalent compounds, because of the covalent bonds in them. Hydrogen chloride, water, methane, ammonia and carbon dioxide are all covalent compounds.