Grade 9Integrated Science

Structure of the Atom

Protons, electrons, neutrons; atomic & mass number; electron arrangement; first 20 elements.

📖 4 min read · 3 worked examples · 4 practice questions

📚 Practise Structure of the Atom with the AI tutor

Free email sign-in · AI tutor in English, Kiswahili or Sheng

The lesson

Today we begin our journey into the Structure of the Atom, a core part of Grade 9 Integrated Science. By the end of this lesson you will be able to identify the three sub‑atomic particles—protons, neutrons and electrons—explain what atomic and mass numbers mean, describe how electrons are arranged in shells, and even name the first twenty elements. Think about the minerals we find in Kenya, like mica or copper ore—those are made of atoms, and understanding their inner parts helps us see why some minerals are magnetic while others are not. We'll start by looking at the nucleus, then move outward to the electron shells, and I'll pause often to check your understanding, so feel free to raise your hand with questions.

Let's explore the basics of atomic number and mass number, the two key numbers that tell us what an atom is made of. First, the atomic number, written as Z, is simply the count of protons in the nucleus. It defines the element itself—carbon always has Z equals six. Next, the mass number, A, adds the neutrons to those protons. A equals protons plus neutrons. At this bar chart showing three carbon isotopes. Notice they all share the same Z = 6, but their mass numbers differ: carbon‑12, carbon‑13, and carbon‑14. These differences illustrate isotopes—atoms of the same element with different numbers of neutrons. Any questions before we move on?

Everyone, let's dive into how electrons are arranged around the nucleus using the shell model. First, the shell capacity rule tells us that each energy level can hold a certain maximum number of electrons: 2 in the first shell, 8 in the second, 8 in the third, and then 18 in the fourth, and so on. Why does this matter? Because the outermost, or valence, electrons are the ones that participate in chemical reactions. For example, sodium atoms in Kenyan table‑salt have an electron configuration ending in 3s¹, meaning they have one valence electron that they readily give away, forming Na⁺. Remembering the shell capacities helps us predict how atoms will bond and why substances like table‑salt behave the way they do.

Let's dive into our first look at the periodic table: the first twenty elements. Here we see a quick list from hydrogen (atomic number 1) up through calcium (atomic number 20). Notice how the symbols are short—just one or two letters. Can anyone tell me why hydrogen is so important for us here in Kenya? This table expands the list with two extra columns: the atomic number and a short note on how each element is used in Kenya. For example, iron (Fe) is a key component of the soils that support our tea and coffee farms, while calcium (Ca) is abundant in milk and helps strengthen our bones. Take a moment to look at the rows for hydrogen, iron, and calcium. Any questions about how these elements show up in everyday life? Great, we've covered the basic list, their symbols, and why they matter locally. Keep these examples in mind—they'll help you remember the elements more easily.

Worked examples

Identify the Atom

All right, let's dive into our worked example: identifying an atom from its sub‑atomic particle counts. We're given 11 protons, 12 neutrons, and 11 electrons. First, notice the number of protons—that tells us the atomic number. Since the atomic number is 11, the element is sodium, symbol Na. Next, add protons and neutrons to get the mass number: 11 + 12 = 23. We are looking at the isotope ^23Na. Finally, arrange the 11 electrons around the nucleus. Following the 2‑8‑1 rule, the electron configuration is 2, 8, 1. To recap: 11 protons means sodium, mass number 23 gives ^23Na, and the electron arrangement is 2‑8‑1. Any questions before we move on?

Isotope Comparison

Everyone, let's dive into our worked example on isotope comparison. We'll look at two forms of carbon and see why they behave the same chemically. First, notice the composition: Carbon‑12 has 6 protons, 6 neutrons, and 6 electrons, while Carbon‑14 has 6 protons, 8 neutrons, and 6 electrons. Can anyone tell me what stays the same between these two isotopes? Yes, the number of protons—and therefore the atomic number—remains six. Because the atomic number is the same, they occupy the same place in the periodic table and share identical chemical properties. The extra neutrons only affect the mass, not the way the atoms bond. However, the extra neutrons make Carbon‑14 heavier and less stable, which is why it's radioactive and decays over time, while Carbon‑12 is stable. To recap: same protons = same chemistry, different neutrons = different mass and stability. Any quick questions before we move on?

Electron Shell Filling

Let's work through Example 3: filling the electron shells for chlorine, which has an atomic number of 17. First, we distribute the 17 electrons as 2‑8‑7: two in the first shell, eight in the second, and seven in the third. Here you see the two electrons represented in the innermost shell. The next eight electrons fill the second shell completely. Finally, seven electrons occupy the outer shell, leaving it one short of a full ten-electron capacity, which makes chlorine highly reactive. Because that outer shell is incomplete, chlorine readily seeks an extra electron to achieve stability. In Kenya, we harness this reactivity: chlorine is added to drinking water to kill harmful microbes, keeping communities safe.

Practice questions

First, remember that the atomic number tells us the number of protons in an atom's nucleus. It's the same number you'll see on the periodic table for each element.
When we write electron configurations, we fill the shells in order of increasing energy: 1s, 2s, 2p, 3s, 3p, and so on. For a neutral magnesium atom (atomic number 12), the correct configuration ends with 3s².
For isotopes, the mass number is simply the total of protons plus neutrons. An atom with 9 protons and 10 neutrons has a mass number of 19, and we write it as ¹⁹F for fluorine.
Finally, based on soil surveys in Kenya, the element that dominates the mineral composition of most soils is aluminum, largely because of the prevalent lateritic soils.

Ask the tutor

Explain Welcome & Learning Objectives in simple words.
Give me 3 worked examples on Welcome & Learning Objectives.
Quiz me with 5 questions on Welcome & Learning Objectives.
What's the most common mistake students make on Welcome & Learning Objectives?

Free email sign-up — the tutor answers in English, Kiswahili or Sheng and walks you through structure of the atom step by step.

Structure of the Atom

📖 The lesson

✏️ Worked examples