Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define atom, element, proton, neutron, and electron.
• Draw a simple diagram of an atom.
• Find the number of protons, neutrons, and electrons using the periodic table.
• Explain atomic number and mass number.

Lesson summary

Welcome, Grade 8 learners, to an exciting journey into the fundamental building blocks of everything around us: atoms! This week, we'll be diving into the world of atomic structure and how it relates to the periodic table. Understanding atoms is crucial because everything is made of them, from the air we breathe to the food we eat, the soil beneath our feet, and even ourselves. In South Africa, a strong understanding of matter at this basic level can help us to appreciate the resources we have, from minerals used in manufacturing to the chemical processes involved in agriculture and the development of new technologies.

Lesson notes

Let's begin with the basics! What is Matter? Everything around you that has mass and takes up space is called matter. The chair you are sitting on, the air you breathe, and the water you drink are all examples of matter. Matter is made up of tiny particles called atoms. What is an Atom? An atom is the smallest particle of an element that can exist and still retain the chemical properties of that element. Imagine you have a gold ring. If you could keep cutting it into smaller and smaller pieces, eventually, you would reach a point where the smallest piece is a single gold atom. Any further division would result in particles that are no longer gold.

Subatomic Particles: The Parts of an Atom Atoms themselves are made up of even smaller particles called subatomic particles.

These include: Protons: Protons are found in the nucleus (the center) of the atom. They have a positive (+) electrical charge. The number of protons determines what element the atom is. For example, all atoms with 1 proton are hydrogen atoms.

Neutrons: Neutrons are also found in the nucleus of the atom. They have no electrical charge; they are neutral. Neutrons contribute to the mass of the atom but don't affect its chemical properties directly.

Electrons: Electrons are found orbiting the nucleus in specific energy levels or shells. They have a negative (-) electrical charge. Electrons are much smaller and lighter than protons and neutrons. The number of electrons often (but not always - we'll learn about ions later!) equals the number of protons in a neutral atom.

Analogy: Think of an atom like a peach. The pit (stone) in the center is like the nucleus, containing the protons and neutrons. The fleshy part of the peach surrounding the pit is like the space where the electrons orbit. Atomic Number and Mass Number Atomic Number: This is the number of protons in the nucleus of an atom. It is unique for each element and identifies the element. The atomic number is usually written as a subscript to the left of the element symbol (e.g., ₁H for hydrogen).

Mass Number: This is the total number of protons and neutrons in the nucleus of an atom. It is written as a superscript to the left of the element symbol (e.g., ¹H for a hydrogen atom with 1 proton and 0 neutrons).

Therefore, the number of neutrons can be calculated as: Number of Neutrons = Mass Number - Atomic Number Example 1: Carbon (C) Looking at the periodic table, carbon (C) has: Atomic Number =

6. This means a carbon atom has 6 protons. A common isotope of carbon has a Mass Number =

1

2. Therefore: Number of Protons = 6 Number of Electrons = 6 (in a neutral carbon atom) Number of Neutrons = 12 - 6 = 6 Example 2: Sodium (Na) Sodium (Na), used in table salt (NaCl), has: Atomic Number =

1

1. This means a sodium atom has 11 protons. A common isotope of sodium has a Mass Number =

2

3. Therefore: Number of Protons = 11 Number of Electrons = 11 (in a neutral sodium atom) Number of Neutrons = 23 - 11 = 12 The Periodic Table The periodic table is an organized arrangement of elements based on their atomic number and chemical properties. It's a vital tool for scientists! We will go deeper into understanding the table in the following weeks, but for now, focus on finding an element on the table and reading off its atomic number. Important

Note: Atoms are electrically neutral when the number of protons (positive charges) equals the number of electrons (negative charges). Guided Practice (With Solutions)

Question 1: An atom has 8 protons and 8 neutrons in its nucleus. How many electrons does it have if it's a neutral atom? What is the element?

Solution: Since the atom is neutral, the number of electrons equals the number of protons.

Therefore, it has 8 electrons. The atomic number is the number of protons, which is

8. Looking at the periodic table, the element with atomic number 8 is Oxygen (O).

Question 2: A certain atom has a mass number of 35 and contains 17 protons. How many neutrons does it have? What is the element?

Solution: Number of Neutrons = Mass Number - Atomic Number Number of Neutrons = 35 - 17 = 18 neutrons. The atomic number is 17, so the element is Chlorine (Cl).

Question 3: Draw a simple diagram of a Helium (He) atom, showing the protons, neutrons, and electrons. (Helium has atomic number 2 and a common isotope has mass number 4).

Solution: Helium has 2 protons (atomic number = 2) Helium has 2 neutrons (mass number - atomic number = 4 - 2 = 2) Helium has 2 electrons (same as the number of protons in a neutral atom).

Diagram: Draw a circle representing the nucleus. Inside the nucleus, label two protons (2p+) and two neutrons (2n). Then, draw a circle around the nucleus to represent the electron shell. Place two electrons (2e-) on this shell.

Question 4: An atom of Potassium (K) has a mass number of

3

9. How many neutrons does it have?

Solution: First, find the atomic number of Potassium (K) on the periodic table. It is 19.