CHEMICAL INTERACTION – PHYSICAL SCIENCE

Structures of Matter

Atomic Particles; All atoms are made up of three basic subatomic particles: protons, neutrons, and electrons.

• Protons carry a positive electric charge.
• Electrons carry a negative electric charge.
• Neutrons have no charge (they are neutral).

Another important difference is mass. Protons and neutrons are each about 2000 times heavier than an electron.

“Model of a Helium Atom” Shows the nucleus with protons and neutrons. Electrons orbiting in surrounding space. Highlights that atoms are mostly empty space.

The center of the atom is called the nucleus, which contains all the protons and neutrons. Because the protons are positive, the nucleus as a whole has a positive charge. Electrons move rapidly around the nucleus in regions called electron clouds or orbitals.

Atomic Symbols and Numbers

Scientists use letters and numbers to describe atoms: The element symbol (e.g., He for helium) identifies the atom. The atomic number tells the number of protons in the nucleus. The mass number is the total number of protons plus neutrons. For example, a helium atom has: 2 protons, 2 neutrons, Mass number = 4, Atomic number = 2.  Because atoms have equal numbers of protons and electrons, they are electrically neutral overall.

Ions and Isotopes

Atoms do not always stay the same. Changes in their particles lead to ions and isotopes. Ion: An atom that has gained or lost electrons.

Gaining electrons = negative ion. Losing electrons = positive ion.

Isotope: Atoms of the same element with different numbers of neutrons. Changing the number of protons turns the atom into a different element entirely.

Molecules, Elements, and Compounds

Atoms often bond together to form larger structures:

Molecule: Two or more atoms bonded together.

 Example: O₂ (oxygen gas), H₂O (water). If all atoms in a molecule are the same type, it is an element.

Example: O₂ (pure oxygen gas). If a molecule is made of different types of atoms, it is a compound.

Example: H₂O (water).

The Periodic Table

Scientists organize the known elements in a chart called the periodic table. Each element has a unique number of protons, neutrons, and electrons, giving it distinct chemical properties.

> 📌 Diagram Placeholder: “Sample Square from the Periodic Table”

Elements in the periodic table are grouped according to their physical and chemical properties: Metals are found on the left side. They are typically shiny, malleable, and good conductors. Nonmetals are grouped on the right side. They are often gases or brittle solids and are poor conductors.

Chemical and Physical Properties

Chemical properties describe how a substance behaves when it interacts with other substances. These properties are observed during chemical reactions. Examples include:

• Flammability: Ability to burn.
• Combustibility: Ability to catch fire and burn easily.
• Reactivity: Ability to undergo chemical reactions with other substances.

In contrast, physical properties can be observed without changing the substance into something new. Examples include: Color, Melting point, Density

Density is an especially important physical property. It describes the relationship between mass and volume:

Density = Mass/Volume

Objects with the same volume but different masses will have different densities. The density of a substance depends on the mass, size, and arrangement of its atoms.

States of Matter

States of matter are considered physical properties because they describe the physical form a substance takes.

Solids

Particles are tightly packed in a regular pattern. High density, little space between particles. Most metals (like gold and silver) are solids at room temperature. When heated to the melting point, solids become liquids.

Liquids

Particles are close together but not in fixed positions. Less dense than solids. Liquids take the shape of their container. Examples: Mercury (a metal) and bromine (a nonmetal) are liquid at room temperature. Heating to the boiling point causes liquids to become gases (evaporation). Cooling turns liquids back into solids (solidification).

Gases

Very low density because of large spaces between particles. Easily compressed and have no fixed shape. Fill any container completely. Examples: Helium, oxygen, and hydrogen are gases at room temperature. Condensation is when gas cools and becomes liquid.

Chemical Formulas and Equations

Chemical formulas use element symbols to represent compounds. A chemical formula shows which elements are present and their ratios. Example: CO₂ (carbon dioxide) has 1 carbon atom and 2 oxygen atoms.

Example: H₂O (water) has 2 hydrogen atoms and 1 oxygen atom.

> Note: A subscript of 1 is understood and not written.

Chemical equations show what happens in chemical reactions:

Reactants: Substances you start with.

Products: Substances you end up with.

Example: Fe +  S.                  FeS}

Reactants: Iron (Fe) and sulfur (S).

Product: Iron sulfide (FeS).

Conservation of Mass

In a chemical reaction, matter is neither created nor destroyed. The total mass of reactants equals the total mass of products. The number of atoms for each element is the same before and after the reaction. Example:

Reactant side: 1 Fe atom and 1 S atom.

Product side: 1 Fe atom and 1 S atom.

Principle: Conservation of Mass ensures balanced equations.

Balancing Chemical Equations

Equations must be balanced so the number of atoms of each element is the same on both sides.

Example 1: Water Formation

Unbalanced: H₂+O₂               H₂O

Left: 2 H atoms, 2 O atoms; Right: 2 H atoms, 1 O atom.

Balanced: 2H₂ + O₂                2H₂O

Left: 4 H atoms, 2 O atoms. Right: 4 H atoms, 2 O atoms.

 Coefficient: A number placed in front of a formula to balance atoms.

Limiting Reactants: In real reactions, one reactant may run out first.

Limiting reactant: Used up first, limits the amount of product.

Excess reactant: Left over after the reaction.

Example: Start with 4 H₂ molecules and 1 O₂ molecule. Can you make 4 H₂O?

4H₂ + O₂               4H₂O (Impossible)

Limiting reactant: O₂ (only 1 molecule available).

Types of Chemical Reactions

Synthesis Reaction: Two or more substances combine to form a more complex substance.

Example: 2Mg + O₂                2MgO

Decomposition Reaction: A compound breaks down into simpler substances.

Example: 2H₂ O₂                          2H₂O+ O₂

Replacement Reaction: One element replaces another in a compound, or two elements in different compounds switch places.

Example: 2CuO +  C                     2Cu  +  CO₂