1st Term Week 8 Lesson Note: The Periodic Table For SSS 1
Subject: Chemistry
Topic: The Periodic Table
Sub-Topics: Groups, Periods, and Periodicity
Duration: 40 Minutes
1. Introduction to the Periodic Table
The Periodic Table is a systematic, tabular arrangement of chemical elements organized by their increasing atomic numbers (the number of protons in the nucleus).
The modern periodic table is based on the Periodic Law, which states that:
"The properties of elements are a periodic function of their atomic numbers."
This organization allows chemists to identify trends and predict the chemical and physical behaviors of elements without having to study each one individually.
2. Rows and Columns: Periods and Groups
The grid layout of the periodic table splits elements into horizontal rows and vertical columns.
A. Periods (Horizontal Rows)
There are 7 horizontal rows in the modern periodic table, known as Periods.
What it represents: The period number tells us the number of electron shells (energy levels) an atom of that element possesses.
Trends across a period: As you move from left to right across any period:
The atomic number increases by 1 from one element to the next.
The number of valence electrons (outermost electrons) increases progressively.
Elements transition from highly metallic solids on the left to non-metals and eventually to unreactive gases on the extreme right.
Example: Sodium ($Na$, Atomic Number 11) is in Period 3. This means a neutral sodium atom distributes its 11 electrons across exactly 3 electronic shells (2, 8, 1).
B. Groups (Vertical Columns)
There are 8 main vertical columns in the periodic table, known as Groups (traditionally numbered using Roman numerals I to VIII or 0, or modern IUPAC numbering from 1 to 18).
What it represents: The group number matches the number of valence electrons in the outermost shell of the atom.
Family similarities: Elements in the same group share very similar chemical properties because they have identical outer electron configurations.
Names of Common Element Families:
Group I (Alkali Metals): Highly reactive metals (e.g., Lithium, Sodium, Potassium). They have 1 valence electron.
Group II (Alkaline Earth Metals): Reactive metals with 2 valence electrons (e.g., Magnesium, Calcium).
Group VII (Halogens): Highly reactive non-metals with 7 valence electrons (e.g., Fluorine, Chlorine, Bromine).
Group VIII / 0 (Noble / Inert Gases): Completely unreactive, stable non-metals with a full outer shell of electrons (e.g., Helium, Neon, Argon).
3. Periodicity
Periodicity refers to the regular, recurring variations or trends in the physical and chemical properties of elements as you move across a period or down a group.
Three fundamental periodic trends include:
Atomic Radius (Size of the atom)
Across a Period: Atomic radius decreases. As more protons are added to the nucleus, the positive nuclear charge increases, pulling the electron shells closer to the center.
Down a Group: Atomic radius increases. Each step down adds an entirely new electron shell, making the atom larger.
Ionization Energy
(The energy required to remove the most loosely bound electron from a neutral gaseous atom).
Across a Period: Ionization energy increases because the smaller atomic size and stronger nuclear charge hold outer electrons tighter.
Down a Group: Ionization energy decreases because the outer electrons are further from the nucleus and easier to remove.
Electronegativity
(The relative ability of an atom to attract shared electrons to itself in a chemical bond).
Across a Period: Electronegativity increases as non-metals seek to gain electrons to achieve stability.
Down a Group: Electronegativity decreases because increased atomic size shields the nucleus's pulling power.
Summary of Trends
----------------- IONIZATION ENERGY INCREASES --------------->
----------------- ELECTRONEGATIVITY INCREASES --------------->
<----------------- ATOMIC RADIUS INCREASES ------------------
+----------+ +----------+
| Group | | Group |
| I | | VIII |
+----------+ +----------+
| | | | | |
A | | | A | | |
T | | | T | | |
O | V | O | V |
M | | M | |
I | ATOMIC | I | IONIZATION|
C | RADIUS | C | ENERGY |
| INCREASES| | DECREASES|
R | | | E | | |
A | | | L | | |
D | V | E | V |
+----------+ +----------+
Evaluation Exercises (For Classwork/Assignment)
State the modern Periodic Law.
An element has an atomic number of 12.
Write its electronic configuration (using K, L, M shell notation).
Identify its Period and Group on the periodic table.
Why do elements belonging to the same vertical group exhibit similar chemical properties?
Explain clearly why the atomic size (radius) decreases as you move from left to right across a period.
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