Everything is built from atoms, and the periodic table organises all known elements. Learn what is inside an atom and why its structure decides its chemistry.
Every substance around you, from the air to a diamond to your own body, is built from atoms. Atoms are so small that millions of them would fit across the thickness of a page. Yet their tiny structure explains why gold shines, why sodium explodes in water and why helium never reacts. The periodic table is the map that organises this knowledge.
The idea that matter is made of tiny particles is old, but modern atomic theory grew from experiments:
Today’s model describes electrons by their probability of being found in regions called orbitals, but the simple shell model is still the best way to understand school chemistry.
An atom has a tiny central nucleus made of protons and neutrons, with electrons moving around it in shells. Nearly all of the mass is in the nucleus, but nearly all of the volume is the space the electrons occupy.
| Particle | Relative mass | Relative charge | Where |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | Very small (about 1/1840) | −1 | Shells around the nucleus |
An atom is electrically neutral because it has equal numbers of protons and electrons, so the positive and negative charges cancel.
Question: An atom of potassium has atomic number 19 and mass number 39. How many protons, neutrons and electrons does it have?
Answer: 19 protons, 20 neutrons, 19 electrons.
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same chemical properties because they have the same number of electrons, but slightly different masses. Chlorine has two common isotopes. Because its relative atomic mass is an average, it is not a whole number.
Question: A sample of an element contains 90% of an isotope of mass 20 and 10% of an isotope of mass 22. Calculate its relative atomic mass.
Answer: 20.2. (This matches neon, which has isotopes of masses 20, 21 and 22.)
Some isotopes are radioactive: their nuclei are unstable and give out radiation. Carbon-14 is used in radiocarbon dating of ancient wood and bones, and cobalt-60 and iodine-131 have uses in medicine, but radioactive materials need careful, regulated handling.
Electrons occupy shells (energy levels) around the nucleus. The first shell is closest to the nucleus and holds up to 2 electrons. The second holds up to 8, and the third up to 8 for the first 20 elements. Electrons fill the lowest available shell first.
| Element | Atomic number | Electron arrangement |
|---|---|---|
| Hydrogen | 1 | 1 |
| Carbon | 6 | 2,4 |
| Oxygen | 8 | 2,6 |
| Neon | 10 | 2,8 |
| Sodium | 11 | 2,8,1 |
| Chlorine | 17 | 2,8,7 |
| Argon | 18 | 2,8,8 |
| Calcium | 20 | 2,8,8,2 |
The electrons in the outer shell (valence electrons) decide how an element reacts. Atoms react to achieve a full outer shell, like the unreactive noble gases: they lose, gain or share electrons. Sodium (2,8,1) loses one electron to become Na⁺ (2,8), and chlorine (2,8,7) gains one to become Cl⁻ (2,8,8).
Question: Write the electron arrangement of phosphorus (atomic number 15).
Answer: 2,8,5. Phosphorus is in Group 15 (5 outer electrons) and Period 3 (3 shells).
The periodic table arranges the elements in order of increasing atomic number. The rows are periods and the columns are groups.
Dmitri Mendeleev published an early periodic table in 1869. He arranged the elements by atomic mass and left gaps for elements not yet discovered, predicting their properties. Later, elements such as gallium and germanium were found with properties close to his predictions, which strongly supported his table. The modern table is arranged by atomic number, a refinement that followed the work of Henry Moseley.
Lithium, sodium and potassium are soft metals with low density and low melting points. They have one outer electron, which they lose easily to form +1 ions. They react with water to give hydrogen and an alkaline hydroxide, for example 2Na + 2H₂O → 2NaOH + H₂. Going down the group, the outer electron is further from the nucleus and is lost more easily, so reactivity increases while melting points decrease.
Fluorine, chlorine, bromine and iodine have seven outer electrons and gain one to form −1 ions. Going down the group, the colour darkens, melting points rise and reactivity decreases, since gaining an electron becomes harder as the outer shell gets further from the nucleus.
Helium, neon, argon and others have full outer shells, so they are very unreactive and exist as single atoms. They are used in balloons, lighting and welding.
In the central block (for example iron, copper and zinc), the elements are typically hard, strong, dense metals with high melting points. They often form coloured compounds, have more than one ion charge (such as Fe²⁺ and Fe³⁺) and are often used as catalysts.
Alkali metals, halogens and radioactive materials are hazardous and are handled by teachers or trained specialists only. Never touch or collect samples of these elements.
An atom that gains or loses electrons becomes an ion, a charged particle. Atoms of Groups 1, 2 and 3 (aluminium) lose electrons to become positive ions (cations) with a full outer shell, and atoms of Groups 15, 16 and 17 gain electrons to become negative ions (anions). The charge matches the number of electrons transferred.
| Atom | Change | Ion | Electron arrangement of ion |
|---|---|---|---|
| Sodium (2,8,1) | Loses 1 electron | Na⁺ | 2,8 |
| Magnesium (2,8,2) | Loses 2 electrons | Mg²⁺ | 2,8 |
| Aluminium (2,8,3) | Loses 3 electrons | Al³⁺ | 2,8 |
| Oxygen (2,6) | Gains 2 electrons | O²⁻ | 2,8 |
| Chlorine (2,8,7) | Gains 1 electron | Cl⁻ | 2,8,8 |
Notice that the number of protons does not change, so a sodium ion still has 11 protons but only 10 electrons, giving an overall charge of +1. Ions have the same electron arrangement as a noble gas, which makes them stable. This is the basis of ionic bonding, covered in the Chemical Bonding article.
Question: How many protons, neutrons and electrons are in an ion of ²⁷Al³⁺ (atomic number 13)?
Answer: 13 protons, 14 neutrons, 10 electrons.
Moving from left to right across a period, such as Period 3 (sodium to argon), the number of protons increases while electrons are added to the same shell. The stronger pull of the nucleus makes atoms smaller. The elements change from metals on the left (sodium, magnesium, aluminium), through the metalloid silicon, to non-metals (phosphorus, sulfur, chlorine), and finally the noble gas argon.
The oxides change in the same pattern: sodium oxide is strongly basic, aluminium oxide is amphoteric, and sulfur dioxide is acidic. This shows how the position in the periodic table helps you predict properties without memorising each element separately.
Try each question first, then tap it to reveal the answer.
Answer: Charge 0; relative mass 1.
Answer: Atomic number 17; mass number 35.
Answer: 2,8,2.
Answer: They have the same number of electrons, especially in the outer shell.
Answer: The outer electron is further from the nucleus and is held less strongly, so it is lost more easily.
Answer: Three.
Answer: Their outer shells are full, so they do not need to gain, lose or share electrons.
Suggested use: have students build atoms with coloured beads for protons, neutrons and electrons, then draw the electron arrangement for the first 20 elements. A blank periodic table of the first 20 elements makes a good group-and-period colouring task. Parents can quiz with flash cards of symbols and electron arrangements. Please follow your own syllabus for how groups are numbered and for the required content on atomic models.
These reputable sources were consulted to verify the facts in this article:
Copyright note: This article is original EDUSAMBAM educational writing. The diagrams are original inline SVG schematics created for this page and are not copied from any source. Links open external websites; the sources were used to check facts and are suggested for further study.
Last updated: 21 October 2026. Written for students up to age 18 and checked against standard school-level chemistry references. This is general education, not a substitute for your teacher, textbook or syllabus.
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