Chemical Formulas (Class 6-9)

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1. Basics of Chemical Formulas A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule. It uses chemical element symbols, numbers, and sometimes other symbols such as parentheses, dashes, brackets, commas, and plus and minus signs. Subscripts indicate the number of atoms of each element in the compound. If there's no subscript, it means one atom. 2. Common Elements and Their Symbols Element Symbol Element Symbol Hydrogen H Oxygen O Carbon C Nitrogen N Sodium Na Chlorine Cl Iron Fe Copper Cu Calcium Ca Sulfur S Potassium K Magnesium Mg Aluminium Al Zinc Zn 3. Valency and Ion Charges Valency: The combining capacity of an element. It's the number of electrons an atom needs to gain, lose, or share to achieve a stable outer shell. Ions: Atoms that have gained or lost electrons, resulting in a net electrical charge. Cation: Positively charged ion (lost electrons), e.g., $Na^+$ Anion: Negatively charged ion (gained electrons), e.g., $Cl^-$ Common Valencies/Ion Charges Ion/Element Charge/Valency Ion/Element Charge/Valency Hydrogen (H) +1 Chloride ($Cl^-$) -1 Sodium ($Na^+$) +1 Hydroxide ($OH^-$) -1 Potassium ($K^+$) +1 Nitrate ($NO_3^-$) -1 Calcium ($Ca^{2+}$) +2 Sulphate ($SO_4^{2-}$) -2 Magnesium ($Mg^{2+}$) +2 Carbonate ($CO_3^{2-}$) -2 Aluminium ($Al^{3+}$) +3 Phosphate ($PO_4^{3-}$) -3 Oxygen (O) -2 Sulphide ($S^{2-}$) -2 4. Writing Chemical Formulas (Criss-Cross Method) Steps to write a chemical formula for a binary compound: Write the symbols of the elements/ions. Write their valencies/charges below their symbols. Criss-cross the valencies/charges (ignore the signs for charges, just use the numerical value). Write the resulting numbers as subscripts. If the subscript is 1, omit it. If polyatomic ions are involved, use parentheses if there's more than one of them. Simplify subscripts by dividing by the greatest common divisor if possible. Examples: Sodium Chloride: Symbols: Na, Cl Valencies: $Na^1$, $Cl^1$ Criss-cross: $Na_1Cl_1$ Formula: $NaCl$ Magnesium Oxide: Symbols: Mg, O Valencies: $Mg^2$, $O^2$ Criss-cross: $Mg_2O_2$ Formula: $MgO$ (simplified from $Mg_2O_2$) Calcium Hydroxide: Symbols: Ca, OH Valencies: $Ca^2$, $OH^1$ Criss-cross: $Ca_1(OH)_2$ Formula: $Ca(OH)_2$ Aluminium Sulphate: Symbols: Al, $SO_4$ Valencies: $Al^3$, $SO_4^2$ Criss-cross: $Al_2(SO_4)_3$ Formula: $Al_2(SO_4)_3$ 5. Types of Chemical Formulas Molecular Formula: Shows the actual number of atoms of each element in a molecule. Water: $H_2O$ Glucose: $C_6H_{12}O_6$ Empirical Formula: Shows the simplest whole-number ratio of atoms in a compound. For Glucose ($C_6H_{12}O_6$), the empirical formula is $CH_2O$ (divide subscripts by 6). For Hydrogen Peroxide ($H_2O_2$), the empirical formula is $HO$. 6. Common Chemical Formulas Compound Name Formula Water $H_2O$ Carbon Dioxide $CO_2$ Sodium Chloride (Table Salt) $NaCl$ Sulphuric Acid $H_2SO_4$ Hydrochloric Acid $HCl$ Nitric Acid $HNO_3$ Ammonia $NH_3$ Methane $CH_4$ Glucose $C_6H_{12}O_6$ Calcium Carbonate (Limestone) $CaCO_3$ Sodium Hydroxide (Caustic Soda) $NaOH$ Calcium Oxide (Quicklime) $CaO$ 7. Balancing Chemical Equations (Basic) A chemical equation is balanced when the number of atoms of each element is the same on both the reactant and product sides. Use coefficients (numbers placed in front of chemical formulas) to balance equations. Do NOT change the subscripts in the chemical formulas. Example: Formation of Water Unbalanced: $H_2 + O_2 \rightarrow H_2O$ Balanced: $2H_2 + O_2 \rightarrow 2H_2O$ Here, on both sides, there are 4 Hydrogen atoms and 2 Oxygen atoms.