Atoms, Elements and the Periodic Table
Explore the building blocks of all matter — from subatomic particles to the organisation of elements in the periodic table.
Atomic Structure
All matter is composed of atoms — incredibly small particles that are the fundamental building blocks of every substance in the universe. An atom consists of three types of subatomic particles:
Protons
Positive charge (+1)
Found in the nucleus
Mass ≈ 1 amu
Neutrons
No charge (neutral)
Found in the nucleus
Mass ≈ 1 amu
Electrons
Negative charge (−1)
Orbit in electron shells
Mass ≈ 1/1836 amu
Bohr Model of an Atom (Carbon-12)
Carbon has 6 protons, 6 neutrons, and 6 electrons arranged in two electron shells (2, 4).
Atomic Number and Mass Number
Every element is uniquely identified by two key numbers:
Atomic Number (Z)
The number of protons in the nucleus of an atom.
- Defines which element it is
- In a neutral atom, Z also equals the number of electrons
- Elements are ordered by atomic number in the periodic table
Mass Number (A)
The total number of protons + neutrons in the nucleus.
- A = Z + N (where N is the number of neutrons)
- Neutrons = Mass Number − Atomic Number
- Isotopes have the same Z but different A
Element Notation
Elements, Compounds and Molecules
Element
A pure substance made up of only one type of atom. There are 118 known elements (e.g., hydrogen, oxygen, gold). Each element has a unique atomic number.
Compound
A substance formed when two or more different elements are chemically bonded together in fixed proportions. For example, water (H₂O) contains hydrogen and oxygen in a 2:1 ratio.
Molecule
A group of two or more atoms bonded together. A molecule can be an element (e.g., O₂) or a compound (e.g., CO₂). All compounds are molecules, but not all molecules are compounds.
The Periodic Table
The periodic table organises all known elements by increasing atomic number. It was first developed by Dmitri Mendeleev in 1869 and has been refined since. Elements with similar chemical properties are arranged in groups (vertical columns).
Groups (Columns)
- 18 groups numbered 1–18
- Elements in the same group have the same number of valence electrons
- Similar chemical properties within a group
- Group 1: Alkali metals; Group 17: Halogens; Group 18: Noble gases
Periods (Rows)
- 7 periods numbered 1–7
- The period number tells you the number of electron shells
- Properties change gradually across a period
- Metallic character decreases left to right
Metals
Found on the left and centre of the periodic table. Good conductors, malleable, ductile, shiny. Most elements are metals.
Non-metals
Found on the right side. Poor conductors, brittle (if solid), dull appearance. Includes gases like O₂ and N₂.
Metalloids
Found along the staircase line. Properties of both metals and non-metals. Used as semiconductors (e.g., silicon).
Key Vocabulary
| Term | Definition |
|---|---|
| Atom | The smallest unit of an element that retains the chemical properties of that element. |
| Nucleus | The dense, central region of an atom containing protons and neutrons. |
| Electron shell | An energy level around the nucleus where electrons orbit. |
| Isotope | Atoms of the same element with different numbers of neutrons. |
| Valence electrons | Electrons in the outermost shell that participate in chemical bonding. |
| Ion | An atom that has gained or lost electrons, giving it a net electrical charge. |
Worked Examples
Finding the number of subatomic particles in sodium (Na).
Given: Sodium has atomic number 11 and mass number 23.
Step 1: Number of protons = atomic number = 11
Step 2: Number of electrons = number of protons (neutral atom) = 11
Step 3: Number of neutrons = mass number − atomic number = 23 − 11 = 12
Answer: Sodium has 11 protons, 12 neutrons, and 11 electrons. Its electron configuration is 2, 8, 1.
Identifying an element from its electron configuration.
Given: An element has the electron configuration 2, 8, 7.
Step 1: Total electrons = 2 + 8 + 7 = 17
Step 2: In a neutral atom, protons = electrons, so atomic number = 17
Step 3: Look up atomic number 17 in the periodic table.
Answer: The element is chlorine (Cl). It is in Group 17 (7 valence electrons) and Period 3 (3 electron shells).
Distinguishing between an element, compound and molecule.
Question: Classify each as element, compound, or both a molecule and a compound: (a) Fe, (b) O₂, (c) NaCl, (d) H₂O.
(a) Fe (iron): A single type of atom → Element. It exists as individual atoms in a metallic lattice.
(b) O₂ (oxygen gas): Two atoms of the same element bonded together → Element and molecule (a diatomic molecule).
(c) NaCl (sodium chloride): Two different elements chemically bonded → Compound (ionic, not a discrete molecule).
(d) H₂O (water): Two different elements bonded together as discrete units → Compound and molecule.
Knowledge Check
Select the correct answer for each question. Click “Check Answer” to see feedback.
Question 1
An atom has 8 protons, 8 neutrons, and 8 electrons. What element is it?
Question 2
An element has mass number 40 and atomic number 20. How many neutrons does it have?
Question 3
Which subatomic particle determines the identity of an element?
Question 4
Elements in the same group of the periodic table have the same number of:
Question 5
Which of the following is a compound?
Key Concepts Summary
- •Atoms consist of protons (+), neutrons (neutral) in the nucleus, and electrons (−) in shells.
- •Atomic number = number of protons; mass number = protons + neutrons.
- •Elements contain one type of atom; compounds contain two or more elements chemically bonded.
- •The periodic table is organised into groups (same valence electrons) and periods (same number of shells).
- •Metals are on the left, non-metals on the right, and metalloids along the staircase boundary.