![]() ![]() We’ve learnt that in a balanced equation, the formula masses (Mr) of the reactants will always be equal to the formula masses of the products. This means that the remaining 73% will come from the two atoms of oxygen. This means that 27% of the mass of the whole molecule comes from carbon. We then multiply by 100 to convert it into a percentage. All we need to do is divide the atomic mass of carbon by the formula mass of carbon dioxide. For example, let’s say we need to find out the percentage by mass of carbon in carbon dioxide. You may be asked to calculate the percentage by mass in a compound – this indicates how much weight a particular element is contributing to the whole molecule. Total formula mass of products = 56 + 44 = 100 This is shown below:įormula mass of calcium oxide = 40 + 16 = 56įormula mass of carbon dioxide = 12 + (16 x 2) = 44 Since we know that the Mr of the reactants is equal to the Mr of the products, this means that the formula masses of calcium oxide and carbon dioxide will add to 100. In this reaction, calcium carbonate is the only reactant and we have already calculated its Mr as 100. You can also use the crossing-over method to work out the formula of a molecule if you know the charges of the ions. Remember that when we are writing the formula of a molecule, we multiply atoms by writing numbers after the atom we want to multiply in subscript. For the whole molecule to be unbalanced, we need two iodide ions to balance out the +2 charge on the magnesium ion. Iodide ions have a +1 charge, since iodine needs to gain 1 more electron to get a full outer shell. Magnesium forms an ion with a +2 charge, since it needs to lose 2 outer electrons to get a full outer shell. ![]() Instead, we have to pay attention to the charges on the ions formed by these atoms. When these elements come together to form a molecule, it may not be the case that one magnesium atom bonds with only one iodine atom. This can be done by looking up their symbols on the periodic table: Mg for magnesium and I for iodine. Valency is the maximum number of electrons an atom can gain or lose whereas the oxidation number is the actual number of electrons the atom gained or lost while forming a molecule.To start with, we need to figure out the formula of magnesium iodide. ![]() It is important to note that the valency of an atom may or may not have the same value as that of its oxidation number. Hence, we can conclude that the valency of iodine is $ - 1$. That means it has a tendency to accept an electron i.e., it will acquire a negative charge by accepting an electron. Therefore, the iodine atom needs one electron to complete its octet and to reach its nearest noble gas configuration. It is not completely soluble in water but breaks down in some organic solvents like carbon tetrachloride ($CC$Īs the number of electrons present in its valence shell $ = 7$. In brine pools and oceans, it is present as a water-soluble iodide on earth. Due to its larger size, it is the least reactive and most electropositive halogen. It is a dark greyish or purple blackish element which is present in the halogen family of group 17 in the periodic table. Iodine is a chemical element with atomic number $53$. Hint: The property of an element which determines the combining power of an atom of that element, it determines the number of other atoms with which an atom of the element can combine to form a chemical compound. ![]()
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