How can I learn valencies of the first 20 elements?

Learning the valencies of elements is an important aspect of understanding chemical bonding and reactions. Here's a guide to help you learn the valencies of the first 20 elements:

Hydrogen (H): Hydrogen typically forms one bond and has a valency of 1.

Helium (He): Helium is a noble gas and is generally unreactive. It does not readily form bonds and thus has a valency of 0.

Lithium (Li): Lithium has a valency of 1 and readily forms one bond.

Beryllium (Be): Beryllium usually has a valency of 2 and forms two bonds.

Boron (B): Boron typically has a valency of 3 and forms three bonds.

Carbon (C): Carbon can have a valency of 4, forming four bonds. However, it can also have a valency of 2 in some cases.

Nitrogen (N): Nitrogen usually has a valency of 3, forming three bonds. However, it can also have a valency of 5 in some cases.

Oxygen (O): Oxygen typically has a valency of 2 and forms two bonds.

Fluorine (F): Fluorine has a valency of 1 and forms one bond.

Neon (Ne): Neon is a noble gas and is generally unreactive. It does not readily form bonds and thus has a valency of 0.

Sodium (Na): Sodium has a valency of 1 and forms one bond.

Magnesium (Mg): Magnesium typically has a valency of 2 and forms two bonds.

Aluminum (Al): Aluminum usually has a valency of 3 and forms three bonds.

Silicon (Si): Silicon typically has a valency of 4 and forms four bonds.

Phosphorus (P): Phosphorus usually has a valency of 3 and forms three bonds. However, it can also have a valency of 5 in some cases.

Sulfur (S): Sulfur typically has a valency of 2 and forms two bonds. However, it can also have a valency of 4 or 6 in some cases.

Chlorine (Cl): Chlorine has a valency of 1 and forms one bond.

Argon (Ar): Argon is a noble gas and is generally unreactive. It does not readily form bonds and thus has a valency of 0.

Potassium (K): Potassium has a valency of 1 and forms one bond.

Calcium (Ca): Calcium typically has a valency of 2 and forms two bonds.

To learn the valencies of the first 20 elements, it's important to review and understand the periodic table. You can find many online resources, textbooks, or educational videos that explain the concept of valency and provide more detailed information about each element's valency. Additionally, practicing writing chemical formulas and balancing equations will reinforce your understanding of valencies.