The Building Blocks of Matter: Atoms and Molecules
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"narration": "Understanding them helps us see how everything is connected, revealing the fundamental composition of all matter.",
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"narration": "For centuries, thinkers pondered what matter was truly made of. Was it infinitely divisible, or were there ultimate, uncuttable particles?",
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"narration": "Around four hundred years before Christ, Democritus proposed that all matter consists of tiny, indivisible particles he called 'atomos', meaning uncuttable.",
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"narration": "Aristotle, however, disagreed! He believed matter could be divided endlessly, like water flowing, consisting of combinations of fire, earth, air, and water.",
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"narration": "This set up a fundamental debate: Democritus's discrete theory of atoms versus Aristotle's continuous theory of matter.",
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"narration": "Centuries later, the idea of atoms resurfaced and was refined. Today, we understand the atom as the basic unit of matter.",
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"narration": "Think of an atom like a super tiny solar system. At its center is the dense nucleus, surrounded by electrons in specific shells.",
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"narration": "The nucleus itself is composed of two types of subatomic particles: positively charged protons and neutral neutrons.",
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"narration": "So, we have protons, with a positive one charge; neutrons, with no charge; both residing in the nucleus. And electrons, with a negative one charge, orbiting outside.",
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"narration": "The identity of an element is defined by its atomic number, or 'Z', which is simply the number of protons in its nucleus.",
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"narration": "The mass number, or 'A', tells us about the atom's overall mass. It's the sum of protons and neutrons in the nucleus.",
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"narration": "Now that we understand atoms, let's explore how they combine to form molecules. A molecule is a group of two or more atoms held together by chemical bonds.",
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"narration": "Molecules of elements consist of only one type of atom. Monoatomic molecules, like Helium, are single, unbonded atoms, often noble gases.",
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"narration": "Diatomic molecules, such as Oxygen, involve two atoms of the same element bonded together.",
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"narration": "And polyatomic molecules contain more than two atoms of the same element, like Ozone, which has three oxygen atoms.",
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"narration": "Finally, molecules of compounds are formed when two or more atoms of *different* elements chemically combine, like water, H₂O.",
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UNIT TWO: COMPOSITION OF MATTER
💡 The world around us, from the air we breathe to the chair you're sitting on, is made of incredibly tiny building blocks called atoms and molecules! Understanding them helps us see how everything is connected.
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.
Main Contents
2.1 Early thinking about the composition of matter
2.2 Inside of an atom
Parts of an atom (nucleus and electron Shells)
The Subatomic Particles of the atom
Relative mass, the charge and location of sub-atomic particles
Atomic number and mass number
Determination of the electrons, protons and neutrons
2.3 Molecules
Molecules of elements
Molecules of Compounds
2.1 Early Thinking about the Composition of Matter
At the end of this section, you will be able to:
Give a short history of the concept of the atom;
Compare and contrast the continuity and discreteness (discontinuity) theory of matter;
Compare earlier conceptions of the structure of matter with their conceptions.
Activity 2.1: Discussing Matter
Form groups and discuss the following questions. Share your opinion with the class:
What is matter?
What do you think matter is made up of?
The earliest recorded discussions about what makes up matter come from ancient Greek philosophers, who were like the scientists of their day. Some thought matter could be divided forever into smaller pieces (continuous theory), while others believed there was a smallest, indivisible particle (discrete theory).
Democritus vs. Aristotle: Two Big Ideas!
Democritus (460 - 370 B.C.) believed that all matter is made of very tiny, indivisible particles, which he called atomos (meaning "uncuttable" or "indivisible"). He imagined atoms as moving particles with different shapes and sizes that could join together. According to Democritus, matter is discrete.
Aristotle (384 – 322 B.C.) disagreed! He argued that matter could be divided into smaller and smaller parts endlessly, without any limit. He didn't believe in tiny building blocks like atoms. For Aristotle, matter was continuous, and he thought it consisted of combinations of fire, earth, air, and water.
Key Concept: Continuous Theory
The idea, proposed by Aristotle, that matter can be divided into smaller and smaller pieces without end, meaning there are no ultimate, indivisible particles.
Key Concept: Discreteness Theory
The idea, proposed by Democritus, that matter is made up of very small, individual particles (atoms) that cannot be divided further, meaning there is a limit to how small matter can be broken down.
Table 2.1: Comparison between the Discrete and Continuous Theory of Matter
Discreteness Theory
Continuous Theory
Proposed by Democritus
Proposed by Aristotle
There is a limit to which matter is broken
Matter is infinitely divisible
Believed in the existence of atoms
Rejected the idea of atoms
Quick Check-in: Ancient Ideas!
Which ancient Greek philosopher believed that matter was made of tiny, indivisible particles called "atomos"?
Activity 2.2: Debate!
Form two groups and debate one of the following ideas. Present your reasons to the class.
If matter is divided and subdivided again and again, what would ultimately be obtained?
Group 1: According to Aristotle's belief
Group 2: According to Democritus's belief
Exercise 2.1: Compare and Contrast
1. Compare and contrast the continuity and discreteness theory of matter.
2.2 Inside of an Atom
At the end of this section, you will be able to:
Describe the structure of an atom as a nucleus containing protons and neutrons, surrounded by electrons in shells (energy levels);
State the relative charge and approximate relative mass of a proton, a neutron and an electron;
Draw hydrogen atoms, including the location of the protons and electrons, with respect to the nucleus;
Differentiate between mass number and atomic number;
Determine the number of protons, neutrons, and electrons in an atom.
Simple Analogy: The Tiny Solar System!
Think of an atom like a super tiny solar system. At its center is the nucleus, like the Sun, which is very dense. Around the nucleus, tiny electrons whiz around, like planets orbiting the Sun!
What are the two main parts of an atom?
An atom consists of a tiny, dense core called the nucleus, surrounded by even smaller particles called electrons. The nucleus is where most of the atom's mass is, and it contains positively charged particles called protons and neutral particles called neutrons. The electrons, which are negatively charged, orbit the nucleus in specific paths called electron shells or energy levels.
Atoms are made up of even smaller particles, which are called subatomic particles. These are the building blocks for atoms.
Figure 2.1: Diagrammatic Representation of the Atom (Click to learn!)
Click on parts of the atom to learn more!
Activity 2.3: Draw a Hydrogen Atom!
On your exercise book, draw a simple sketch of a hydrogen atom model using a colored pen, following these instructions:
Draw a small circle labeled "nucleus".
Add an even smaller circle labeled "proton" inside the nucleus.
Add another circle around the nucleus (this is the electron shell) and place a small dot (for the electron) on it.
The Subatomic Particles: Proton, Neutron, and Electron
The relative charge of a proton is +1. An electron is assigned a charge of −1. A neutron has zero charge. Since a neutral atom has an equal number of protons and electrons, it has no overall electrical charge.
Protons and neutrons have almost the same mass, and they are much heavier than electrons (about 1800-2000 times more massive!). This means that almost all the mass of an atom is concentrated in its tiny nucleus.
Table 2.2: Nature and Location of Sub-Atomic Particles
Particle Name
Location
Relative Mass (amu)
Relative Charge
Proton
Nucleus
1
+1
Electron
Outside nucleus (shells)
~0 (negligible)
-1
Neutron
Nucleus
1
0
Project Work: Build a Hydrogen Model!
In groups, prepare a hydrogen atom model using locally available materials (e.g., clay, beads, wire, cardboard) and present your model to the class.
Atomic Number and Mass Number
Key Concept: Atomic Number (Z)
The number of protons in an atom's nucleus. This number uniquely identifies an element! For a neutral atom, it's also the number of electrons.
Key Concept: Mass Number (A)
The total count of protons and neutrons in an atom's nucleus. It tells us about the atom's overall mass.
All atoms can be identified by the number of protons they contain. The atomic number (Z) of an atom equals the number of protons in its nucleus. For a neutral atom, the atomic number is also the number of electrons.
Atomic number (Z) = Number of protons = Number of electrons
The mass number (A) is the sum of the number of protons and the number of neutrons in the nucleus of an atom.
Mass number (A) = Number of protons + Number of neutrons
Mass number (A) = Atomic number (Z) + Number of neutrons
We often write the atomic and mass numbers for an element like this:
A
Z X
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
C is the symbol for Carbon.
Determination of Electrons, Protons, and Neutrons
Knowing the atomic number and mass number allows us to figure out the number of subatomic particles:
Number of protons: Equal to the atomic number (Z).
Number of electrons: In a neutral atom, it's equal to the number of protons (and thus the atomic number Z).
Number of neutrons: Equal to the Mass number (A) minus the number of protons (or atomic number Z).
Number of neutrons = Mass number (A) - Number of protons (Z)
Activity 2.5: Particle Count Challenge!
Form groups and discuss the following. Share your opinion with your group members.
1. Use a periodic table to tell the atomic number, mass number, proton numbers, neutron numbers, and electron numbers of the first 10 elements.
Quick Check-in: Atom Math!
An atom has an atomic number (Z) of 8 and a mass number (A) of 16. How many neutrons does this atom have?
Exercise 2.2: Fill in the Blanks & Calculate!
1. Complete the following table (use "Approx. 0" for negligible mass):
Particle
Location
Relative Mass (amu)
Relative Charge
Proton
Nucleus
1
+1
Electron
Outside nucleus (shells)
Approx. 0
-1
Neutron
Nucleus
1
0
2. A nucleus consists of 9 protons and 10 neutrons. Determine:
The element by referring to a periodic table
Mass number
3. How many neutrons, protons, and electrons are there in an atom of the element 147N?
2.3 Molecules
At the end of this section, you will be able to:
Define molecules;
Give examples of monatomic, diatomic, and polyatomic molecules;
Use models or particle model diagrams to represent molecules of elements and compounds.
Activity 2.6: What's a Molecule?
Form groups and discuss the following. Share your opinion with your group members and present your findings to the class.
What is a molecule?
Mention some examples of monoatomic, diatomic, and polyatomic molecules.
Key Concept: Molecule
A group of two or more atoms held together by chemical bonds. It's the smallest part of a substance that can exist independently and still have the properties of that substance.
Key Concept: Element Molecule
A molecule made up of only one type of atom (e.g., O₂, H₂).
Key Concept: Compound Molecule
A molecule made up of two or more different types of atoms chemically combined (e.g., H₂O, CO₂).
Molecules of Elements
A molecule of an element consists of only one type of atom joined together. These can be classified by how many atoms are in each molecule:
Monoatomic molecules: Contain only one atom of the element. These are usually noble gases (they don't like to bond with other atoms!). Examples: He (Helium), Ne (Neon), Ar (Argon), Kr (Krypton), Xe (Xenon), Rn (Radon).
Diatomic molecules: Contain two atoms of the same element bonded together. Examples: O₂ (Oxygen), H₂ (Hydrogen), F₂ (Fluorine), Cl₂ (Chlorine), I₂ (Iodine).
Polyatomic molecules: Contain more than two atoms of the same element bonded together. Examples: O₃ (Ozone - three oxygen atoms), P₄ (Phosphorus - four phosphorus atoms), S₈ (Sulfur - eight sulfur atoms).
Figure 2.2: Diagrammatical representations of Ne (monoatomic) and H₂ (diatomic)
Molecules of Compounds
A molecule of a compound always contains two or more atoms of different elements combined chemically.