ELEMENTS & THE PERIODIC TABLE
Every atom in the universe belongs to one of 118 known elements — and the periodic table is the map that organises them all.
At this point we have mentioned the concept of an element a few times — but what exactly is one? Think of elements like scents of flowers or flavours of ice cream: each one is distinct, with its own unique characteristics and chemically different behaviour. No two elements are the same, and you cannot reduce one into another by ordinary means.
As we touched on in the last lesson, an element is defined entirely by the number of protons in an atom's nucleus — a value known as its atomic number. Change the proton count and you change the element. Every single atom in the universe with one proton is hydrogen. Every atom with two protons is helium. It is that simple, and that absolute.
The first few elements follow in order:
This numbered sequence — one element per proton count, no gaps, no duplicates — is the backbone of the periodic table. There are 118 known elements in total, ranging from hydrogen, the lightest and most abundant in the universe, all the way to oganesson, a synthetic element so unstable it exists for only milliseconds before decaying.
// INTERACTIVE — PERIODIC TABLE OF ELEMENTS
TO VIEW ITS ATOM
HOW TO READ A CELL
Each cell in the periodic table packs three key pieces of information. The number in the top corner is the atomic number — the count of protons in that element's nucleus, and the value that uniquely identifies it. The large symbol in the centre is the element's chemical abbreviation, used universally in equations, labels, and lab notation. Finally, the number at the bottom is the atomic mass — the average mass of one atom of that element measured in atomic mass units (u), accounting for the natural mix of its isotopes.
// INTERACTIVE — ANATOMY OF A PERIODIC TABLE CELL
That atomic mass is not arbitrary — it is the weighted average of all naturally occurring isotopes of that element, weighted by how abundant each isotope is in nature. Because different samples of an element always contain isotopes in the same natural ratio, the weighted average is consistent everywhere on Earth — and is the number printed on every periodic table.
Lithium is a clean example. It has two stable isotopes: lithium-6 (3 neutrons) and lithium-7 (4 neutrons). In any natural sample, about 7.6% of lithium atoms are Li-6 and 92.4% are Li-7. To find the weighted average you multiply each isotope's exact mass by its fractional abundance, then add the results:
The result matches what is printed on the periodic table. Notice that 6.941 is close to 7 rather than 6 because the heavier Li-7 isotope makes up the vast majority of natural lithium. This is why atomic masses are almost never whole numbers — they are averages, not counts.
A word of warning: the abbreviations do not always match the English name of the element. Many symbols come from older Latin or Greek names that were established before modern naming conventions existed. A few notable examples:
These mismatches are a quirk of scientific history — names were often standardised in Latin across Europe before English became dominant in science. Once you know the common ones, they become second nature. For now, hover over any cell above to explore the full details of each element.
HOW THE TABLE IS ORGANISED
At first glance the layout of the periodic table can look arbitrary — rows of varying length, a detached block floating beneath the main grid, columns that seem to start and stop at random. In reality every position is deeply intentional.
Each vertical column is called a group, and the elements within a group share remarkably similar chemical behaviour. They react with other substances in the same kinds of ways, form the same types of bonds, and often even look alike as pure materials. This is no coincidence — it is a direct consequence of how many electrons sit in each element's outermost orbital shell. Elements in the same group have the same number of outer electrons, and it is those outer electrons that determine almost everything about how an atom behaves chemically. The horizontal rows, called periods, represent the filling of successive electron shells as the atomic number increases.
A handful of groups are important enough to have their own names that you will encounter constantly in chemistry. Here are the key ones:
Notice that noble gases sit at the far right of every period — they are the endpoint of each row, with completely filled outer shells and no tendency to react. This pattern of shell-filling is what gives the periodic table its shape, and understanding it is the key to predicting how any element will behave. We will explore groups, periods, and electron configuration in much greater depth in future lessons.
READY TO TEST YOUR KNOWLEDGE?
Take the Chemistry I quiz — 15 questions covering everything in lessons 1.1 through 1.3.