IGCSE Chemistry: Atomic Structure, Bonding and the Periodic Table

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IGCSE Chemistry: Atomic Structure, Bonding and the Periodic Table

TL;DR

[58 words]
Atoms have a nucleus (protons + neutrons) surrounded by electrons in shells; the number of protons defines the element. Chemical bonds form when atoms share electrons (covalent), transfer electrons (ionic), or have weak attractions (metallic/van der Waals). The periodic table organises elements by atomic number and groups elements with similar bonding behaviour.

1. The Mental Model

[68 words]
Think of an atom as a solar system: the nucleus is the sun, electrons orbit in shells like planets. The number of protons never changes—that's what makes an element that element. Electrons want to fill their outermost shell, and they'll do it by stealing, sharing, or borrowing from other atoms. How they do that depends entirely on how many outer electrons they've got. The periodic table is just a cheat sheet showing which elements will behave the same way.

2. The Core Material

[~900 words]

Atomic Structure: Protons, Neutrons and Electrons

Every atom has three types of particle. Protons are in the nucleus and carry a positive charge. Neutrons are also in the nucleus, have no charge, and add mass. Electrons orbit the nucleus in shells (or energy levels) and carry a negative charge.

Here's the critical bit: the number of protons defines what element you have. Carbon always has 6 protons. Oxygen always has 8. If you change the number of protons, you've got a different element. The number of neutrons can vary—those are called isotopes, and they're the same element but different mass.

Electrons fill shells in order: the first shell holds max 2, the second holds max 8, the third holds max 8 (though it can hold more). The outermost shell is called the valence shell, and what's in it determines how the atom will bond.

Atomic number = number of protons (this is the small number at the bottom left of an element symbol). Mass number = protons + neutrons (the big number at the top left). So Carbon-12 has atomic number 6 (six protons) and mass number 12 (six protons + six neutrons).

Bonding: Why Atoms Stick Together

Atoms bond because electrons in the outermost shell want to reach a stable state. For most elements, a full outer shell has 8 electrons (or 2 for hydrogen/helium). Atoms will achieve this by bonding.

Ionic bonding happens when one atom steals electrons from another. A metal (like sodium) has just a few outer electrons and doesn't want to protect them. A non-metal (like chlorine) is hungry for electrons and will grab them. When sodium loses its one outer electron to chlorine, sodium becomes a positive ion (Na⁺) and chlorine becomes a negative ion (Cl⁻). They stick together because opposite charges attract. Ionic bonds form between metals and non-metals.

Covalent bonding happens when two atoms share electrons. Two non-metals (like hydrogen and chlorine) both want more electrons to fill their outer shell, so instead of one stealing from the other, they agree to share. Hydrogen has 1 outer electron; chlorine has 7. They share one electron each—that shared pair "counts" for both atoms. Hydrogen now has 2 in its outer shell (full), chlorine has 8 (full). Covalent bonds form between non-metals.

Metallic bonding is a weird middle ground. Metal atoms sit in a "sea" of loose electrons that move freely. The electrons don't belong to one atom—they float around and let all the metal atoms share them. This is why metals conduct electricity and are bendy (the electrons let layers slide).

Van der Waals forces are the weakest attractions—they happen between molecules (not atoms). Picture two molecules drifting close: briefly, electrons bunch up on one side, making that side slightly negative and the other side slightly positive. These tiny temporary dipoles attract. They're weak, but they pile up when you have millions of molecules, which is why ice doesn't melt at room temperature.

flowchart TD
    A["Atom needs to fill outer shell"] --> B{Which atoms are bonding?}
    B -->|Metal + Non-metal| C["Electrons transfer<br/>IONIC BOND<br/>Opposite charges attract<br/>Forms ionic compounds"]
    B -->|Non-metal + Non-metal| D["Electrons shared<br/>COVALENT BOND<br/>Shared pair fills both shells<br/>Forms molecules"]
    B -->|Metal + Metal| E["Electrons delocalized<br/>METALLIC BOND<br/>Electron sea<br/>Forms metals"]
    B -->|Between molecules| F["Temporary dipoles<br/>VAN DER WAALS FORCES<br/>Very weak<br/>Holds molecules together"]
    C --> G["Conduct when melted/dissolved<br/>Brittle solids<br/>High melting points"]
    D --> H["Don't conduct<br/>Soft/hard depending on structure<br/>Variable melting points"]
    E --> I["Conduct electricity<br/>Malleable and ductile<br/>High melting points"]

The Periodic Table: Why It's Arranged That Way

The periodic table arranges all elements by atomic number (left to right, top to bottom). But the clever bit is the groups (vertical columns). Elements in the same group have the same number of electrons in their outer shell, so they bond in similar ways.

Group 1 (the alkali metals: lithium, sodium, potassium) all have 1 outer electron. They're desperate to lose it, so they're reactive and form 1+ ions.

Group 7 (the halogens: fluorine, chlorine, bromine) all have 7 outer electrons. They're desperate to gain 1, so they're reactive and form 1− ions.

Group 0 (the noble gases: helium, neon, argon) all have full outer shells (2 or 8 electrons). They don't need to bond with anyone—they're stable and inert.

This is why chlorine and iodine behave similarly even though they're different elements: they're in the same group, so they have the same valence configuration.

Periods (horizontal rows) are less about bonding and more about size and shell count. Across a period, atomic number increases, so atoms get smaller and the outer shell fills up. Down a group, atoms get larger (more shells), but the outer shell configuration stays the same.

3. Worked Example

[~200 words]

Let's build sodium chloride (table salt) from scratch.

Sodium atom: atomic number 11, so it has 11 protons. Electrons fill: shell 1 gets 2, shell 2 gets 8, shell 3 gets 1. It has 1 outer electron—very lonely.

Chlorine atom: atomic number 17, so it has 17 protons. Electrons: shell 1 gets 2, shell 2 gets 8, shell 3 gets 7. It has 7 outer electrons—almost full.

When they meet, sodium loses its 1 outer electron to chlorine. Now sodium has 2 + 8 = 10 electrons but still 11 protons, so it's a 1+ ion (Na⁺). Chlorine now has 2 + 8 + 8 = 18 electrons but only 17 protons, so it's a 1− ion (Cl⁻).

Both now have full outer shells (Na⁺ has 8, Cl⁻ has 8). They attract because they're oppositely charged and stick together as ionic compound NaCl.

At room temperature, NaCl is a white crystalline solid. It doesn't conduct electricity because the ions are locked in place. But if you melt it (around 801°C), the ions can move freely and it conducts. This is how you know it's ionic.

4. Examiner's Breakdown

4.1 What Examiners Actually Reward

1. "The atomic number equals the number of protons [and the number of electrons in an atom]." Marks are lost if you confuse this with mass number.
2. "Electrons are arranged in shells/energy levels; the first shell holds a maximum of 2 electrons, the second shell holds a maximum of 8." State the actual numbers, not "it varies."
3. "Ionic bonding involves the transfer of electrons from a metal to a non-metal; the metal forms a positive ion and the non-metal forms a negative ion." The word "transfer" is crucial—they're checking you didn't say "sharing."
4. "Covalent bonding is the sharing of a pair (or pairs) of electrons between non-metal atoms." The shared pair must be explicitly mentioned.
5. "Elements in the same group of the periodic table have the same number of electrons in their outer shell, so they have similar chemical properties." Not just "they're similar"—link it to outer shell electrons.
6. "Ionic compounds conduct electricity when melted or dissolved because the ions are free to move." Temperature or state matters—solid ionic compounds do NOT conduct.

4.2 Trapdoor Mistakes

4.3 Score-Boosting Comparisons

5. Now Try It

[~80 words]

Task: Draw the electron structure of magnesium (Mg, atomic number 12) and oxygen (O, atomic number 8) as separate atoms. Then predict: will they form an ionic or covalent bond? If ionic, what ions form and what's the formula? If covalent, how many electrons are shared?

Write out your answer step-by-step: (1) electron configuration for each atom, (2) which is metal/non-metal, (3) type of bond and why, (4) final formula.

Success looks like: You've correctly placed 12 electrons around Mg (2, 8, 2) and 8 around O (2, 6), identified Mg as metal and O as non-metal, concluded ionic bonding, and written the formula as MgO (because Mg loses 2 electrons and O gains 2).

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