In this section we’re going to focus on the changes that occur during chemical reactions and how this impacts an element.
So to start with let’s take a look at word and symbol equations. Word equations can be less descriptive of what is happening in a reaction because there are not numbers to show the amount of reactants and products.
This is demonstrated in the reaction to produce water – the balanced symbol equation provides more chemical information than the word equation.
To illustrate some of the changes in chemical reactions, we’re going to use some examples to help!
If an atom forms an ion with a two positive charge the number of electrons has decreased by two. Atoms in group two of the periodic table form ions with a two positive charge.
They do this when they bond with non-metal atoms, which involves the transfer of electrons. The metal atom will donate two electrons to the non-metal atom so that both atoms have a full outermost energy level (noble gas configuration).
The ions are charged because the number of negatively charged electrons no longer cancels out the number of positively charged protons.
Potassium forms ions with a positive charge. A compound is formed when potassium reacts with chlorine, which forms ions with a negative charge. The formula of this compound would therefore be KCl.
Potassium loses one electron when it reacts with chlorine. This electron is transferred to a chlorine atom to form a chloride ion. In KCl, both ions have full outer energy levels (noble gas configuration).
The ratio of ions in an ionic compound depends on their relative charges. Potassium and chloride ions have the same size charge (one), so there are the same number of potassium ions as chloride ions in the compound.
So far, so good! Only two more examples to go!
A transfer of electrons occurs when fluorine and calcium react to form an ionic compound. This is because calcium is in group two and so forms ions with a two positive charge. Fluorine is in group seven so forms ions with a negative charge.
Calcium ions have twice as much charge as fluoride ions. This means twice as many fluoride ions must be present to make the overall charge of the ionic compound neutral.
And lastly, hydrogen and sulfur are both non-metals. Hydrogen sulfide therefore contains covalent bonds.
This is because when two non-metal atoms react, they do not form ions or ionic compounds. Instead, the outermost energy levels overlap and they share electrons. A pairs of shared electrons makes one covalent bond. The compound formed is known as a molecule.