1. Calcium loses 2 electrons. Oxygen gains 2 electrons. The molecule they make is CaO. 2. Sodium loses 1 electron. Oxygen gains 2 electrons. The molecule they make is Na2
O.
3. Phosphorus shares 3 electrons. Hydrogen shares 1 electron. The molecule they make is PH3
.
Chemical equations
explain how mass is conserved in chemical reactions. They show how the atoms and elements in compounds recombine to form new compounds.
Chemical equations Molecular formulae give us exact information about the atoms that form the molecules which are the building blocks of compounds. We use these formulae in chemical equations. Look at the following chemical equations. Count the number of atoms of each element on the left (the reactants) and the number of atoms of each element on the right (the products). What do you notice?
Reactants Reactants H Cl
Na O
H2 Reactants
H S
O Ca Products
HCl + NaOH → NaCl + H2 Number
Products
2 1 1 1
Reactants SO4
H Cl
Na O
Products Number
+ Ca → CaSO4 Products
H S
O Ca
When molecules react, the atoms simply rearrange into new compounds. The atoms are neither created nor destroyed. As a result, mass is always conserved. The following example shows the conservation of mass applied to equations where there is more than one molecule of a reactant or product.
CH4 + 2O2 → CO2 + 2H2 O
You will notice that a number in front of a molecule affects all the elements in that molecule, while a number after an element only applies to the element immediately before it.
