Chapter 3: Atoms and Molecules
1. Laws of Chemical Combination
Before atoms were fully understood, scientists established two critical laws:
● L aw of Conservation of Mass: Mass can neither be created nor destroyed in a
chemical reaction. The total mass of reactants always equals the total mass of
products.
● Law of Constant Proportions: In a chemical substance, the elements are always
present in definite proportions by mass. (e.g., In Water (H_2O), the ratio of the mass
of Hydrogen to Oxygen is always 1:8, no matter where the water comes from).
2. Dalton’s Atomic Theory
John Dalton provided the basic theory about the nature of matter:
● ll matter is made of very tiny particles called atoms.
A
● Atoms are indivisible particles (they cannot be created or destroyed).
● Atoms of a given element are identical in mass and chemical properties.
● Atoms of different elements have different masses and properties.
● Atoms combine in the ratio of small whole numbers to form compounds.
3. What is an Atom?
An atom is the smallest unit of matter that maintains the properties of an element.
● A tomic Radius: Measured in nanometers (1\text{ nm} = 10^{-9}\text{ m}).
● Symbols: Modern symbols are based on the IUPAC system. Usually, it's the first one
or two letters of the element's name (e.g., Al for Aluminum). Some come from Latin
names (e.g., Fe for Ferrum/Iron, Na for Natrium/Sodium).
4. Atomic Mass
Since atoms are so small, we use Atomic Mass Unit (u).
● O ne atomic mass unit is a mass unit equal to exactly one-twelfth (1/12\text{th}) the
mass of one atom of Carbon-12.
● Molecule: A group of two or more atoms that are chemically bonded together.
○ Molecules of Elements: Contain atoms of the same type (e.g., O_2, P_4).
○ Molecules of Compounds: Atoms of different elements join together (e.g.,
H_2O, CO_2).
5. Ions
Compounds composed of metals and non-metals contain charged species known as ions.
1. Laws of Chemical Combination
Before atoms were fully understood, scientists established two critical laws:
● L aw of Conservation of Mass: Mass can neither be created nor destroyed in a
chemical reaction. The total mass of reactants always equals the total mass of
products.
● Law of Constant Proportions: In a chemical substance, the elements are always
present in definite proportions by mass. (e.g., In Water (H_2O), the ratio of the mass
of Hydrogen to Oxygen is always 1:8, no matter where the water comes from).
2. Dalton’s Atomic Theory
John Dalton provided the basic theory about the nature of matter:
● ll matter is made of very tiny particles called atoms.
A
● Atoms are indivisible particles (they cannot be created or destroyed).
● Atoms of a given element are identical in mass and chemical properties.
● Atoms of different elements have different masses and properties.
● Atoms combine in the ratio of small whole numbers to form compounds.
3. What is an Atom?
An atom is the smallest unit of matter that maintains the properties of an element.
● A tomic Radius: Measured in nanometers (1\text{ nm} = 10^{-9}\text{ m}).
● Symbols: Modern symbols are based on the IUPAC system. Usually, it's the first one
or two letters of the element's name (e.g., Al for Aluminum). Some come from Latin
names (e.g., Fe for Ferrum/Iron, Na for Natrium/Sodium).
4. Atomic Mass
Since atoms are so small, we use Atomic Mass Unit (u).
● O ne atomic mass unit is a mass unit equal to exactly one-twelfth (1/12\text{th}) the
mass of one atom of Carbon-12.
● Molecule: A group of two or more atoms that are chemically bonded together.
○ Molecules of Elements: Contain atoms of the same type (e.g., O_2, P_4).
○ Molecules of Compounds: Atoms of different elements join together (e.g.,
H_2O, CO_2).
5. Ions
Compounds composed of metals and non-metals contain charged species known as ions.
