⚛️ Atomic Structure & the Periodic Table: Building Blocks of Everything
Explore how atoms are built from protons, neutrons, and electrons, and discover how the periodic table organizes every element around you.
Learning Goals
By the end of this lesson, you will be able to:
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Identify the three subatomic particles and their charges, masses, and locations in an atom
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Calculate atomic number, mass number, and the number of each particle in an atom
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Explain how elements are arranged in the periodic table by atomic number
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Relate an element's position in the periodic table to its atomic structure
Before You Begin
Before this lesson, you should know:
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Understand that all matter is made of tiny particles
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Know the difference between an element and a compound
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Be comfortable with simple addition and subtraction of whole numbers
Notation & Symbols
Key notation and symbols used in this topic:
Z
Atomic number — the number of protons in an atom's nucleus
A
Mass number — the total number of protons plus neutrons
ZAX
Isotope notation — element symbol X with mass number A on top and atomic number Z below
p+
Proton — a positively charged particle in the nucleus
n0
Neutron — a neutral (no charge) particle in the nucleus
e−
Electron — a negatively charged particle orbiting the nucleus
Inside the Atom
An atom is the smallest unit of an element, made of a central nucleus containing protons and neutrons, surrounded by electrons. The number of protons defines which element the atom is.
Protons: Found in the nucleus with a charge of +1 and a relative mass of 1. The number of protons (the atomic number Z) uniquely identifies the element — every carbon atom has 6 protons.
Neutrons: Also found in the nucleus, with no charge and a relative mass of 1. They add mass and help hold the nucleus together.
Electrons: Tiny negatively charged particles (−1) that move in shells around the nucleus. Their mass is almost negligible compared to protons and neutrons.
Neutral atoms: In a neutral atom the number of electrons equals the number of protons, so the positive and negative charges balance to zero.
Mass number: Calculated as A=protons+neutrons, so the number of neutrons is A−Z.
Example
A sodium atom is written 1123Na. Here Z=11 means 11 protons (and 11 electrons in a neutral atom), and A=23, so the neutrons are 23−11=12.
100%
A lithium atom showing 3 protons and 4 neutrons in the nucleus, with 3 electrons arranged in two shells.
Worked Example
Basic
A carbon atom has an atomic number of 6 and a mass number of 12. Find the number of protons, electrons, and neutrons.
Worked Example
Intermediate
A sodium atom has atomic number 11 and mass number 23. Write its electron arrangement (electron configuration by shells) and state its period and group in the periodic table.
Worked Example
Advanced
Chlorine exists as two isotopes: chlorine-35 (abundance 75%) and chlorine-37 (abundance 25%). Calculate the relative atomic mass of chlorine and explain why the value is not a whole number.
Why It Works
Understanding the reasoning behind the rule:
The relative atomic mass is a weighted average, not a simple average. Here is why the weighting matters.
Your Turn
Try this one yourself — use the hint if you get stuck:
An atom of magnesium has atomic number 12 and mass number 24. State its number of protons, neutrons, and electrons, write its electron arrangement, and give its group number.
Watch Out For...
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Confusing mass number with atomic number when identifying an element.
The atomic number (number of protons) alone identifies the element. For carbon it is always 6. The mass number A=p+n can vary between isotopes, so use the proton count, not the mass, to name the element.
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Thinking the number of electrons always equals the number of neutrons.
In a neutral atom the number of electrons equals the number of protons (the atomic number), not the neutrons. Neutrons are counted separately as n=A−Z.
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Assuming electrons fill shells randomly.
Electrons fill the lowest energy shells first: up to 2 in the first shell, then up to 8 in the second, then 8 in the third (for early elements). Always fill inner shells before starting a new one.
In the Real World
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Metals like iron rust because iron atoms lose outer electrons to oxygen and water — the reactivity of a metal is predicted by its position in the periodic table.
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Neon signs glow because electrons in neon atoms absorb energy, jump to higher shells, then release light as they fall back — a direct demonstration of electron shells.
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Nutrients like iron, calcium, and potassium that your body needs are all elements from the periodic table, each with a specific atomic structure that determines its role.
Extended Practice
Test yourself with a mix of multiple choice and short-answer questions:
Q1.
An atom has atomic number Z=11 and mass number A=23. How many neutrons does it have?
MCQ
Q2.
Which particle has a negative charge?
MCQ
Q3.
A neutral atom has 17 protons. How many electrons does it have, and in which group of the periodic table is it likely found? Explain.
It has 17 electrons (equal to protons in a neutral atom). With electron arrangement 2, 8, 7 it has 7 outer electrons, placing it in Group 7 (the halogens).
Q4.
Explain why elements in the same group have similar chemical properties.
Elements in the same group have the same number of electrons in their outer shell. Because chemical reactions depend on outer-shell electrons, these elements behave similarly.
Extension
Advanced
A harder challenge that connects to the next skill:
Chlorine has two common isotopes: chlorine-35 and chlorine-37. In a sample, about 75% of atoms are chlorine-35 and 25% are chlorine-37. Calculate the relative atomic mass of chlorine.
Key Takeaways
1.An atom is made of protons (positive), neutrons (neutral) in the nucleus, and electrons (negative) orbiting in shells.
2.The atomic number Z equals the number of protons and identifies the element; the mass number A=p+n.
3.Electrons fill shells from the inside out (2, then 8, then 8), and the number of outer-shell electrons determines chemical behaviour.
4.The periodic table organizes elements by atomic number; groups (columns) share outer-electron counts and react similarly, while periods (rows) show shell filling.
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Ready to Practice?
Now that you've learned Atomic Structure & the Periodic Table, put it to the test with a quick quiz!