Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define an atom and its basic components.
• Describe the structure of an atom.
• State the electrical charges of subatomic particles.
• Use atomic and mass numbers to find protons, neutrons, and electrons.
• Describe electron arrangement in the first three shells.

Lesson summary

This week, we embark on an exciting journey into the microscopic world of atoms! Understanding atomic structure is fundamental to understanding everything around us – from the air we breathe to the food we eat and the materials we use every day. In South Africa, this knowledge is vital for understanding the properties of our natural resources like gold, platinum, and coal, and for developing new technologies and sustainable practices related to these resources. For example, understanding the properties of platinum is essential for the development of catalytic converters, which help to reduce pollution from car exhausts – a significant issue in our urban areas.

Lesson notes

What is an Atom? An atom is the smallest particle of an element that can exist. Everything around us is made up of atoms. Imagine building with LEGO bricks – atoms are like the individual LEGO bricks that make up larger structures. The term "atom" comes from the Greek word "atomos," meaning "indivisible," but we now know that atoms can be divided, although it requires a nuclear reaction.

The Structure of an Atom: Atoms are made up of three main subatomic particles: Protons: Positively charged particles found in the nucleus (the center) of the atom. The number of protons determines what element the atom is.

Think of it like this: having 1 proton makes the atom Hydrogen, having 6 protons makes it Carbon, and so on.

Neutrons: Neutral (no charge) particles also found in the nucleus. Neutrons add mass to the atom and contribute to its stability.

Electrons: Negatively charged particles that orbit the nucleus in specific regions called electron shells or energy levels. Electrons are much lighter than protons and neutrons. The nucleus, containing the protons and neutrons, is tiny but contains almost all the mass of the atom. The electrons, whizzing around the nucleus, occupy most of the volume of the atom.

Electrical Charges: Proton: +1 (positive)

Neutron: 0 (neutral)

Electron: -1 (negative) Atoms are normally electrically neutral because they have the same number of protons (positive charges) and electrons (negative charges).

Atomic Number and Mass Number: Atomic Number (Z): The number of protons in the nucleus of an atom. The atomic number defines what element the atom is. For example, all carbon atoms have 6 protons, so carbon's atomic number is

6. You can find the atomic number on the periodic table.

Mass Number (A): The total number of protons and neutrons in the nucleus of an atom.

Calculating the Number of Neutrons: You can calculate the number of neutrons using the following formula: Number of Neutrons = Mass Number (A) - Atomic Number (Z)

Example 1: Carbon (C) Atomic Number (Z) = 6 (This means carbon has 6 protons) Mass Number (A) = 12 (This is the most common isotope of Carbon) Number of Neutrons = 12 - 6 = 6 Neutrons So, a carbon atom with a mass number of 12 has 6 protons, 6 neutrons, and (since it's neutral) 6 electrons.

Example 2: Sodium (Na) Atomic Number (Z) = 11 (This means sodium has 11 protons) Mass Number (A) = 23 Number of Neutrons = 23 - 11 = 12 Neutrons So, a sodium atom with a mass number of 23 has 11 protons, 12 neutrons, and 11 electrons. Electron Arrangement (Electron Shells/Energy Levels): Electrons don't just orbit the nucleus randomly. They occupy specific energy levels or shells. These shells are numbered, starting with the shell closest to the nucleus: 1, 2, 3, and so on. Each shell can hold a maximum number of electrons: Shell 1: Maximum 2 electrons Shell 2: Maximum 8 electrons Shell 3: Maximum 8 electrons (for Grade 8, we primarily focus on the first 20 elements) Electrons fill the shells starting with the innermost shell (shell 1) first.

Example 3: Oxygen (O) Atomic Number = 8 (8 protons and 8 electrons)

Electron Configuration: Shell 1: 2 electrons Shell 2: 6 electrons (Total: 2 + 6 = 8 electrons)

Example 4: Magnesium (Mg) Atomic Number = 12 (12 protons and 12 electrons)

Electron Configuration: Shell 1: 2 electrons Shell 2: 8 electrons Shell 3: 2 electrons (Total: 2 + 8 + 2 = 12 electrons)

Reactivity (Brief Introduction): The number of electrons in the outermost shell (also called the valence shell) determines how reactive an element is. Atoms "want" to have a full outermost shell. This desire to have a full outermost shell drives chemical reactions. Elements with full outermost shells are very stable and unreactive (like noble gases). Guided Practice (With Solutions)

Question 1: An atom has an atomic number of 17 and a mass number of 35. a) How many protons does it have? b) How many neutrons does it have? c) How many electrons does it have?

Solution: a)

Protons: The atomic number is 17, so it has 17 protons. b)

Neutrons: Mass number – Atomic number = 35 – 17 = 18 neutrons. c)

Electrons: In a neutral atom, the number of electrons equals the number of protons. So it has 17 electrons.

Question 2: Draw the electron arrangement for an atom of Nitrogen (N). The atomic number of Nitrogen is

7. Solution: Nitrogen has 7 electrons.

Shell 1: 2 electrons Shell 2: 5 electrons We can represent this as: N (2, 5)

Question 3: An atom has 13 protons and 14 neutrons. a) What is its atomic number? b) What is its mass number? c) What element is it? (Hint: Use the periodic table) d) How many electrons does it have? e)

Draw the electron arrangement Solution: a)

Atomic number: 13 (the number of protons) b)

Mass number: 13 (protons) + 14 (neutrons) = 27 c)

Element: Aluminum (Al) – found by looking up atomic number 13 on a periodic table.