Explain, using the periodic table, how and whyelements combine in specific ratios to formcompounds.

Elements combine in specific ratios to form compounds due to the concept of valency and the octet rule, which are derived from the arrangement of electrons in the different energy levels or shells of an atom.

1. Understanding the Periodic Table: The periodic table is a tabular arrangement of chemical elements, organized based on their atomic number, electron configuration, and recurring chemical properties. Elements are listed in order of increasing atomic number, which corresponds to the number of protons in an element's atomic nucleus. Elements in the same group (vertical columns) have the same number of valence electrons, which are the electrons in the outermost shell of an atom that are involved in forming chemical bonds with other elements.

2. Valency and the Octet Rule: The valency of an element is the number of electrons an atom of that element either gains, loses, or shares during a chemical reaction. The octet rule states that atoms are most stable when their outermost shell is full, typically with eight electrons. This is why elements combine in such a way to satisfy this rule.

3. Combining in Specific Ratios: When elements combine to form compounds, they do so in a way that allows both elements to achieve a full outer shell. For example, in the formation of water (H2O), two hydrogen atoms (each with 1 electron) can share their electrons with one oxygen atom (with 6 valence electrons) to form two covalent bonds, thereby filling the outer shell of oxygen with 8 electrons. This results in a ratio of 2:1 of hydrogen to oxygen atoms.

4. Predicting Ratios with the Periodic Table: By looking at the periodic table, we can predict the ratios in which elements will combine. For example, elements in Group 1 (like hydrogen or sodium) have one valence electron and are likely to form a compound with elements in Group 17 (like fluorine or chlorine) that have seven valence electrons, resulting in a 1:1 ratio.

In conclusion, the periodic table allows us to understand the electron configuration of elements and predict how and in what ratios they will combine to form compounds, based on their valency and the octet rule.