Unit 3 - Stoichiometry Flashcards
Molecular formula
The molecular formula of a compound is the number and type of different atoms in one molecule.
Writing molecular formula
Subscript numbers represent the number of atoms of that element within the compound.
Empirical formula
The empirical formula of a compound is the simplest whole number ratio of the different atoms or ions. Useful when very large molecules that contain hundreds, if not thousands of atoms.
Ions
An ion is an electrically charged particle formed when an atom (or group of atoms) loses or gains electrons. When an atom becomes an ion it seeks to gain the very stable noble gas electron structure. The charge of the ion shows how many electrons it has gained or lost.
Charge of ionic formula
The overall charge of the positive ions and the negative ions must be neutral.
Valency
The valency of an element, also known as its combining power, is the number of electrons gained, lost or shared by an atom of the element when it forms a compound. The valency of an element is given by the number of electrons in the outer shell (known as valence electrons) and can be used to determine the charge on an ion.
Group 1
All metals in Group 1 have one electron in their outer shell. They can lose this one electron to form ions with a charge of 1+.
Group 2
All metals in Group 2 have two valence electrons, so have a valency of 2. They can lose 2 electrons to form ions with a charge of 2+.
Group 3
The valency of Group 3 is 3 so in this group three electrons can be lost to form a 3+ charge.
Group 4
Group 4 elements have a valency of 4. They do not usually form ions, instead elements tend to achieve a full outer shell by sharing electrons.
Group 5
They have a valency of 3 and five outer electrons. These elements can achieve a full outer shell by gaining three electrons to form ions with a 3- charge.
Group 6
They have six outer shell electrons and a valency of 2. They can gain 2 electrons to form ions with a 2- charge.
Group 7
They have seven outer shell electrons and a valency of 1. They can achieve a full outer shell by gaining one electron.
Group 8
They are the noble gases and already have a outer shell so they don’t need to gain or lose electrons.
Chemical reactions
It is a process when one set of substances, the reactants, are converted into another set of substances, the products. Any chemical reaction can be represented by a chemical equation.
Chemical equation
- A word equation in which the chemical names of all reactants and products are displayed.
- A symbol equation in which the names of the substances are replaced by their elemental symbols.
Polyatomic ions
Some ions are composed of more than one element.
Balancing equations
When creating a symbol equation the number of atoms present on the left and right for each element must be the same. The total mass of the reactants must equal the total mass of the products.
Balancing complex equations
With more complicated equations it is useful to attempt to balance one atom at a time.
State symbols
State symbols add extra information that allow the reader to know which physical state each substance is during the reaction.
Aqueous solution
The is used to represent a substance fully dissolved in water. This is called the aqueous state and is represented by: (aq)
State symbols (representation)
- Solid = (s)
- Liquid = (l)
- Gas = (g)
Molecular ions (polyatomic ions)
Compounds comprised of atoms that have been covalently bonded together but contain unequal numbers of protons and electrons.
If there is more than one molecular ion in a compound, then brackets are required to make sure the reader understands which ions are involved. Also put brackets around it to show how many of them there are.
Ionic compounds
Formed between two or more ionic molecules. The formula of an ionic compound can be involving a molecular ion can be worked out in the same way as of an ionic compound formed of a metal and non metal. Charges must balance each other an result in a overall charge of zero.
