Sciences · Topic 2.2
Atomic Structure and the Periodic Table
All matter is made of atoms. The periodic table organises elements by their atomic number and properties. Understanding atomic structure explains why elements behave the way they do and how they combine to form compounds.
What You'll Learn
- Describe the structure of an atom (protons, neutrons, electrons)
- Calculate the number of protons, neutrons, and electrons from atomic and mass numbers
- Explain how the periodic table is organised by periods and groups
- Predict the number of electrons in each shell using filling rules
- Distinguish between isotopes of the same element
- Describe properties of metals, non-metals, and metalloids
IB Assessment Focus
Criterion A: Recall subatomic particles, their charges, and locations; use atomic and mass numbers to calculate particle counts.
Criterion B: Identify patterns in the periodic table (e.g., trends in reactivity, metallic character).
Criterion C: Use correct chemical notation (e.g., ²³₁₁Na) and atomic symbols.
Criterion D: Apply knowledge to explain properties of everyday materials and chemical phenomena.
Sciences · Topic 2.2
Atomic Structure
Atoms are the smallest units of an element that retain its chemical properties. Every atom consists of a central nucleus surrounded by electrons.
Subatomic Particles
| Particle | Location | Charge | Relative Mass |
|---|---|---|---|
| Proton | Nucleus | +1 | 1 |
| Neutron | Nucleus | 0 (neutral) | 1 |
| Electron | Shells around nucleus | −1 | ~1/1836 (negligible) |
Atomic Number and Mass Number
Key Definitions
Atomic number (Z) = number of protonsMass number (A) = protons + neutrons
Neutrons = Mass number − Atomic number
Example: Sodium (Na) — Atomic number = 11, Mass number = 23
- Protons = 11
- Electrons = 11 (equal to protons in a neutral atom)
- Neutrons = 23 − 11 = 12
Critical Rule: The atomic number (number of protons) identifies the element. Change the number of protons and you change the element entirely. In a neutral atom, the number of electrons always equals the number of protons.
Sciences · Topic 2.2
The Periodic Table
The periodic table arranges all known elements in order of increasing atomic number. Its structure reveals patterns in properties.
Organisation of the Periodic Table
| Feature | Definition | Example |
|---|---|---|
| Period | Horizontal row — elements in the same period have the same number of electron shells | Period 3: Na, Mg, Al, Si, P, S, Cl, Ar |
| Group | Vertical column — elements in the same group have similar chemical properties | Group 1: Li, Na, K (alkali metals) |
| Atomic number | Elements are arranged in order of increasing atomic number (left to right, top to bottom) | H=1, He=2, Li=3... |
Metals, Non-metals, and Metalloids
| Category | Location | Properties | Examples |
|---|---|---|---|
| Metals | Left and centre | Good conductors of heat/electricity; shiny; malleable; ductile; solid at room temperature (except Hg) | Iron, copper, gold, sodium |
| Non-metals | Top right | Poor conductors; often gases or brittle solids at room temperature; lower melting points | Oxygen, carbon, sulfur, chlorine |
| Metalloids | Staircase boundary | Properties intermediate between metals and non-metals; semiconductors | Silicon, germanium, boron |
Key Groups
- Group 1 (Alkali metals): Li, Na, K — very reactive; react vigorously with water; have 1 electron in outer shell
- Group 7 (Halogens): F, Cl, Br — highly reactive non-metals; have 7 electrons in outer shell
- Group 0/18 (Noble gases): He, Ne, Ar — unreactive; full outer electron shells
Sciences · Topic 2.2
Electron Shells
Electrons are arranged in energy levels (shells) around the nucleus. The arrangement of electrons determines how an element reacts.
Shell Filling Rules
| Shell | Maximum Electrons |
|---|---|
| 1st shell (innermost) | 2 |
| 2nd shell | 8 |
| 3rd shell | 8 (at Grade 7 level) |
Examples of electron configurations:
- Hydrogen (Z=1): 1 — 1 electron in 1st shell
- Carbon (Z=6): 2, 4 — 2 in 1st shell, 4 in 2nd shell
- Oxygen (Z=8): 2, 6 — 2 in 1st shell, 6 in 2nd shell
- Sodium (Z=11): 2, 8, 1 — 2 in 1st, 8 in 2nd, 1 in 3rd shell
- Chlorine (Z=17): 2, 8, 7 — 2 in 1st, 8 in 2nd, 7 in 3rd shell
- Argon (Z=18): 2, 8, 8 — full shells = unreactive noble gas
Why Outer Shell Electrons Matter
- The number of electrons in the outermost shell determines chemical reactivity
- Elements with full outer shells (noble gases) are unreactive
- Elements in the same group have the same number of outer shell electrons and thus similar properties
- Atoms react to achieve a full outer shell (usually 8 electrons — the octet rule)
Sciences · Topic 2.2
Isotopes and Ions
Isotopes are atoms of the same element with different numbers of neutrons. Ions are atoms that have gained or lost electrons and carry a charge.
Isotopes
Isotopes
Atoms of the same element (same atomic number) with different numbers of neutrons (different mass numbers)
Example: Carbon isotopes
All three are carbon atoms — same atomic number (6), different mass numbers. Carbon-14 is radioactive and used in carbon dating.
| Isotope | Protons | Neutrons | Electrons |
|---|---|---|---|
| Carbon-12 (¹²C) | 6 | 6 | 6 |
| Carbon-13 (¹³C) | 6 | 7 | 6 |
| Carbon-14 (¹⁴C) | 6 | 8 | 6 |
Ions
- Positive ion (cation): Atom loses electrons. Fewer electrons than protons. E.g., Na¹♠ (sodium loses 1 electron)
- Negative ion (anion): Atom gains electrons. More electrons than protons. E.g., Cl− (chlorine gains 1 electron)
- Metals tend to form positive ions; non-metals tend to form negative ions
Critical Rule: Changing the number of neutrons gives a different isotope of the same element. Changing the number of protons gives a completely different element. Changing the number of electrons gives an ion (charged atom) of the same element.
Sciences · Topic 2.2
Worked Examples
These examples demonstrate the level of reasoning and precision required at Grade 7.
EXAMPLE 1An element has atomic number 11 and mass number 23. Identify the number of protons, neutrons, and electrons.
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Full Solution
Protons = atomic number = 11
Electrons = 11 (equal to protons in a neutral atom)
Neutrons = mass number − atomic number = 23 − 11 = 12
This element is Sodium (Na).
Electrons = 11 (equal to protons in a neutral atom)
Neutrons = mass number − atomic number = 23 − 11 = 12
This element is Sodium (Na).
EXAMPLE 2Oxygen has atomic number 8 and mass number 16. Draw its electron shell diagram.
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Full Solution
Protons = 8, Electrons = 8, Neutrons = 16 − 8 = 8
Electron configuration: 2, 6
• 1st shell: 2 electrons (full)
• 2nd shell: 6 electrons (2 short of full)
This explains why oxygen readily gains 2 electrons to form O²− ions, achieving a full outer shell.
Electron configuration: 2, 6
• 1st shell: 2 electrons (full)
• 2nd shell: 6 electrons (2 short of full)
This explains why oxygen readily gains 2 electrons to form O²− ions, achieving a full outer shell.
EXAMPLE 3Explain why elements in Group 1 all have similar chemical properties.
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Full Solution
Elements in Group 1 (Li, Na, K, Rb, Cs) all have 1 electron in their outermost shell. Because chemical reactions involve the outer shell electrons, elements with the same number of outer shell electrons undergo similar types of reactions. All Group 1 elements react with water to form alkaline solutions and hydrogen gas, because all react by losing that single outer electron.
EXAMPLE 4Two atoms: Atom X has 6 protons, 6 neutrons. Atom Y has 6 protons, 8 neutrons. Are these the same element? Are they isotopes?
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Full Solution
Both atoms have 6 protons, so both are carbon (atomic number 6). They are the same element.
Atom X has mass number = 6 + 6 = 12 (Carbon-12)
Atom Y has mass number = 6 + 8 = 14 (Carbon-14)
Since they have the same number of protons but different numbers of neutrons, they are isotopes of carbon.
Atom X has mass number = 6 + 6 = 12 (Carbon-12)
Atom Y has mass number = 6 + 8 = 14 (Carbon-14)
Since they have the same number of protons but different numbers of neutrons, they are isotopes of carbon.
EXAMPLE 5An element is in Period 3, Group 7 of the periodic table. State its number of electron shells and outer electrons.
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Full Solution
Period 3 tells us the atom has 3 electron shells.
Group 7 tells us the atom has 7 electrons in the outer shell.
This element is Chlorine (Cl), with electron configuration 2, 8, 7. Chlorine is a halogen — it is highly reactive because it needs to gain only 1 more electron to complete its outer shell.
Group 7 tells us the atom has 7 electrons in the outer shell.
This element is Chlorine (Cl), with electron configuration 2, 8, 7. Chlorine is a halogen — it is highly reactive because it needs to gain only 1 more electron to complete its outer shell.
EXAMPLE 6Why are noble gases (Group 0/18) unreactive?
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Full Solution
Noble gases (helium, neon, argon, etc.) are unreactive because they have full outer electron shells. Helium has 2 electrons in its only shell (full); neon has 2, 8 (full); argon has 2, 8, 8 (full). Since chemical reactivity involves gaining, losing, or sharing electrons to achieve a full outer shell, atoms that already have full outer shells have no tendency to react.
EXAMPLE 7A student claims: "A sodium ion (Na⁺) is a different element from sodium (Na) because it has a different number of particles." Is the student correct?
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Full Solution
The student is incorrect. Na and Na¹♠ are both sodium because they both have 11 protons — the atomic number is unchanged. The element is determined solely by the number of protons. Na has 11 electrons; Na¹♠ has lost one electron and has only 10 electrons. The difference in electrons creates the positive charge, but the identity of the element (sodium) is unchanged.
Sciences · Topic 2.2
Practice Q&A
Attempt each question before revealing the model answer.
IDENTIFYAn element has atomic number 17 and mass number 35. State the number of protons, neutrons, and electrons.
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Model Answer
Protons = 17 (atomic number). Electrons = 17 (neutral atom). Neutrons = 35 − 17 = 18. This is Chlorine (Cl).
EXPLAINWhat is the difference between atomic number and mass number?
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Model Answer
Atomic number = the number of protons in the nucleus. It identifies the element. Mass number = protons + neutrons. It gives the total mass of the nucleus. For a neutral atom, the number of electrons equals the atomic number.
DESCRIBEDescribe the location and charge of each subatomic particle.
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Model Answer
Protons: in the nucleus, charge +1. Neutrons: in the nucleus, charge 0. Electrons: in shells around the nucleus, charge −1. The nucleus is positively charged because it contains protons; electrons are attracted to it by electrostatic force.
EXPLAINWhy are the elements in the same group of the periodic table chemically similar?
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Model Answer
Elements in the same group have the same number of electrons in their outer shell. Because chemical reactions involve the outer shell electrons, elements with the same outer shell configuration react in similar ways.
CALCULATEWrite the electron configuration for magnesium (atomic number 12).
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Model Answer
12 electrons total. Fill shells: 1st shell = 2 electrons, 2nd shell = 8 electrons, 3rd shell = 2 electrons. Configuration: 2, 8, 2. Magnesium is in Group 2 because it has 2 outer electrons.
DESCRIBEDescribe two physical properties that distinguish metals from non-metals.
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Model Answer
1. Electrical conductivity: Metals are good conductors; non-metals are poor conductors (insulators). 2. Physical state at room temperature: Most metals are solid at room temperature (mercury is a rare exception); many non-metals are gases (oxygen, nitrogen, chlorine) or brittle solids (sulfur, iodine).
IDENTIFYCarbon-12 and Carbon-14 have the same atomic number but different mass numbers. What does this tell us?
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Model Answer
Both are carbon atoms (same element, same atomic number = 6). They are isotopes — same number of protons and electrons, but different numbers of neutrons. C-12 has 6 neutrons; C-14 has 8 neutrons. Same chemical properties, different physical properties (C-14 is radioactive).
APPLYSilicon is described as a metalloid. Explain what this means and why it is useful.
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Model Answer
A metalloid has properties intermediate between metals and non-metals. Silicon can conduct electricity under certain conditions (a semiconductor) but not as well as metals. This property makes it extremely useful in electronics — the ability to control its conductivity allows it to be used in transistors, computer chips, and solar cells.
Sciences · Review
Flashcard Review
Tap each card to reveal the answer. Try to answer from memory first.
What is an atom?
The smallest unit of an element that retains its chemical properties. Made of protons and neutrons in a nucleus, with electrons in shells around it.
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What is the atomic number?
The number of protons in an atom's nucleus. It uniquely identifies the element. In a neutral atom, it also equals the number of electrons.
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What is the mass number?
Total number of protons + neutrons in the nucleus. Mass number - atomic number = number of neutrons.
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Charges of subatomic particles?
Proton: +1. Neutron: 0. Electron: -1. In a neutral atom, protons and electrons are equal in number.
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What is the maximum number of electrons in the first three shells?
1st shell: 2 electrons. 2nd shell: 8 electrons. 3rd shell: 8 electrons (at introductory level).
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What is a period in the periodic table?
A horizontal row. Elements in the same period have the same number of electron shells. Period 1 has 1 shell; Period 2 has 2 shells, etc.
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What is a group in the periodic table?
A vertical column. Elements in the same group have the same number of outer shell electrons and similar chemical properties.
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What are isotopes?
Atoms of the same element with the same number of protons but different numbers of neutrons (and therefore different mass numbers).
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Why are noble gases unreactive?
They have full outer electron shells and therefore no tendency to gain, lose, or share electrons in chemical reactions.
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What is a metal? Give 3 properties.
A metal is on the left/centre of the periodic table. Properties: good electrical conductor, shiny, malleable (can be shaped), high melting point (generally).
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How many protons, neutrons, and electrons does oxygen (Z=8, A=16) have?
Protons = 8. Electrons = 8. Neutrons = 16 - 8 = 8.
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What is a positive ion?
An atom that has lost one or more electrons, leaving it with more protons than electrons and therefore a positive charge. Metals typically form positive ions.
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What is the electron configuration of sodium (Z=11)?
2, 8, 1 — 2 electrons in the 1st shell, 8 in the 2nd, 1 in the 3rd (outer) shell.
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What identifies an element?
The number of protons (atomic number). Changing the number of protons changes the element entirely.
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How many outer electrons does a Group 7 element have?
7 outer electrons. Group 7 elements (halogens) need to gain 1 electron to complete their outer shell, making them highly reactive.
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Sciences · Practice Test
Practice Test — 20 Questions
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