Atoms and Molecules

SO₄²⁻

Step 1: A polyatomic ion is a group of atoms that carries a charge. Step 2: SO₄²⁻ (sulphate ion) contains one sulfur and four oxygen atoms with a 2- charge. Step 3: Na⁺, Cl⁻, and Ca²⁺ are monoatomic ions as they contain only single atoms. Step 4: Other examples of polyatomic ions include NO₃⁻ (nitrate) and OH⁻ (hydroxide). Step 5: These ions act as single units in chemical reactions.

1/12th mass of one carbon-12 atom

Step 1: The atomic mass unit was standardized in 1961 using carbon-12 isotope. Step 2: One atomic mass unit equals exactly 1/12th the mass of one carbon-12 atom. Step 3: This standard was chosen because carbon reacts with many elements and gives convenient whole number masses. Step 4: Previously, oxygen-16 was used as the standard (1/16th of oxygen mass). Step 5: All relative atomic masses are now measured against this carbon-12 standard.

2

Step 1: In CaCl₂, calcium combines with 2 chlorine atoms. Step 2: Each chlorine atom has valency 1, so total negative charge is 2. Step 3: For the compound to be neutral, calcium must have valency +2. Step 4: This means calcium can lose 2 electrons to form Ca²⁺ ion. Step 5: The formula CaCl₂ shows the 1:2 ratio, confirming calcium's valency is 2.

Elements in a compound are present in definite proportions by mass

Step 1: The Law of Definite Proportions was stated by Joseph Proust. Step 2: It says that in any pure compound, elements are always present in the same fixed ratio by mass. Step 3: For example, in water (H₂O), hydrogen and oxygen are always in 1:8 ratio by mass. Step 4: This ratio remains constant regardless of the source or method of preparation. Step 5: This law helped establish that compounds have fixed compositions, leading to atomic theory.