UNIT TWO: COMPOSITION OF MATTER

Learning Outcomes

At the end of this unit, you will be able to:

• Narrate the historical development of the atomic nature of substances.
• Appreciate that atoms are the building blocks which make up all substances.
• Demonstrate understanding of the idea that the identity of a substance is determined by its atomic structure.
• Differentiate molecules of elements from molecules of compounds.
• Demonstrate scientific inquiry skills along this unit: communicating, asking questions, drawing conclusions, applying concepts.

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2.1 Early Thinking about the Composition of Matter

Have you ever wondered what everything is made of? For thousands of years, brilliant thinkers have pondered this very question!

The Big Debate: Continuous vs. Discrete Matter

Ancient Greek philosophers had two main ideas about matter:

• Continuous Theory: Some believed matter could be divided endlessly into smaller and smaller pieces, with no limit. Imagine always being able to cut a piece of cheese into smaller and smaller slices forever!
• Discrete (Discontinuous) Theory: Others thought there was a smallest, indivisible particle that matter was made of. Like tiny, uncuttable LEGO bricks!

Key Thinkers

Democritus (460 - 370 B.C.) was a big believer in the discrete theory. He proposed that all matter consists of very small, indivisible particles which he named atomos (meaning "uncuttable" or "indivisible"). He thought these atoms moved around, differed in shape and size, and could join together to form different substances.

Aristotle (384 – 322 B.C.), on the other hand, argued for the continuous theory. He believed matter could be divided infinitely and did not think microscopic building particles existed. Aristotle famously believed that matter consisted of combinations of four basic elements: fire, earth, air, and water.

Comparing the Theories

Discreteness Theory	Continuous Theory
Proposed by Democritus	Proposed by Aristotle
There is a limit to which matter can be broken down.	Matter is infinitely divisible.
Believed in the existence of atoms.	Rejected the idea of atoms.

2.2 Inside of an Atom

Now, let's fast forward to modern understanding! We know that atoms aren't just tiny, indivisible spheres. They have their own fascinating internal structure!

Parts of an Atom

An atom consists of two main parts:

1. The Nucleus: This is a tiny, dense core at the center of the atom. It contains positively charged particles called protons and neutral (no charge) particles called neutrons. Because of the protons, the nucleus is positively charged.
2. Electron Shells (or Energy Levels): These are regions surrounding the nucleus where tiny, negatively charged particles called electrons move very rapidly.

Most of an atom's mass is concentrated in its nucleus, as protons and neutrons are much heavier than electrons. However, most of an atom's volume is actually empty space, occupied by the electrons whizzing around!

The Subatomic Particles

Let's look closer at the three main subatomic particles:

Particle Name	Location	Relative Mass (amu)	Relative Charge
Proton	Nucleus	~1	+1
Neutron	Nucleus	~1	0 (Neutral)
Electron	Outside nucleus (shell)	~0 (negligible)	-1

Since a neutral atom has an equal number of protons (positive) and electrons (negative), its overall charge is zero.

Atomic Number and Mass Number

These numbers help us identify and describe different atoms:

• Atomic Number (Z): This is the most important identifier! It equals the number of protons in an atom's nucleus. For a neutral atom, it also tells you the number of electrons.
• Mass Number (A): This is the total number of protons AND neutrons in an atom's nucleus. Electrons contribute very little to the mass, so they are ignored.

We often write these numbers like this:

^A_ZX

Where X is the symbol of the element, A is the Mass Number, and Z is the Atomic Number.

Example: For ¹²₆C (Carbon):

• Mass Number (A) = 12
• Atomic Number (Z) = 6
• Symbol = C (Carbon)

Determining Protons, Neutrons, and Electrons

Using the atomic number (Z) and mass number (A), you can figure out the number of each particle:

• Number of Protons = Atomic Number (Z)
• Number of Electrons = Atomic Number (Z) (for a neutral atom)
• Number of Neutrons = Mass Number (A) - Number of Protons (Z)

2.3 Molecules

Atoms are rarely found alone in nature. They usually join together to form larger units called molecules!

A molecule is two or more atoms held together by chemical bonds.

Molecules of Elements

These molecules are made up of only one type of atom.

• Monoatomic molecules: Contain one atom. These are typically the noble gases, which are very stable and don't usually react with other atoms.
  Examples: Helium (He), Neon (Ne), Argon (Ar)
• Diatomic molecules: Contain two atoms of the same element chemically bonded together.
  Examples: Oxygen gas (O₂), Hydrogen gas (H₂), Nitrogen gas (N₂)
• Polyatomic molecules: Contain three or more atoms of the same element chemically bonded together.
  Examples: Ozone (O₃), Sulfur (S₈), Phosphorus (P₄)

Molecules of Compounds

These molecules are made up of two or more different types of atoms chemically bonded together.

• Examples: Water (H₂O), Ammonia (NH₃), Carbon Dioxide (CO₂), Hydrochloric Acid (HCl)