A zigzag staircase line on the periodic table marks the boundary between metals and nonmetals, running diagonally from boron down to astatine. But that line is just a visual shorthand for deeper differences in how these elements behave: how they bond, how they conduct energy, what happens when they react, and what their atoms do with electrons. About 80% of known elements are metals, clustered on the left and center of the table, while nonmetals occupy a small wedge in the upper right.
The Staircase Line on the Periodic Table
If you look at a standard periodic table, you’ll see a stepped diagonal line starting between boron and aluminum on one side and running down between silicon and germanium, arsenic and antimony, tellurium and polonium, and astatine and tennessine. Everything to the left of this line (plus all the elements in the middle block) is a metal. Everything to the upper right is a nonmetal.
The line itself is somewhat arbitrary. Elements sitting right along it, like silicon, germanium, arsenic, antimony, and tellurium, don’t fit neatly into either camp. These are the metalloids, and they share traits with both groups. The staircase is useful as a quick reference, but the real separation comes down to measurable physical and chemical properties.
Physical Properties That Set Them Apart
The differences you can see and feel between metals and nonmetals are striking. Metals are shiny (lustrous), and they stay shiny when you polish them. Gold, silver, and copper all have that characteristic gleam. Nonmetals like sulfur and coal are dull. Diamond is a notable exception: it’s made entirely of carbon, a nonmetal, but appears brilliant because of how it bends light.
Metals are malleable, meaning you can hammer or press them into thin sheets without them cracking. Silver is malleable enough to be beaten into foil thin enough to decorate food. Metals are also ductile, so you can stretch them into wires. Copper wiring in your walls exists because of this property. Nonmetals are brittle. Hit a chunk of sulfur with a hammer and it shatters.
Thermal and electrical conductivity draw another sharp line. Metals conduct heat efficiently, which is why aluminum and stainless steel are used in cookware. They also conduct electricity, making them essential for wiring and electronics. Nonmetals are poor conductors of both heat and electricity. Wood, rubber, and plastic (all built from nonmetal elements like carbon, hydrogen, and oxygen) are insulators for exactly this reason. Metals also tend to ring when struck, a property called sonority. Nonmetals don’t.
What Happens at the Atomic Level
The physical differences trace back to how each type of element handles its outermost electrons. Metal atoms hold their outer electrons loosely. In a chunk of metal, those electrons aren’t locked to individual atoms. Instead, they form a shared “sea” of electrons that flows freely through the material. This electron sea is what makes metals conduct electricity and heat so well, and it’s what allows metal atoms to slide past each other without breaking apart, giving metals their malleability and ductility.
Nonmetal atoms grip their outer electrons tightly. Rather than sharing electrons in a communal pool, nonmetals tend to either grab electrons from other atoms or share them in fixed pairs with specific neighbors. This tight grip on electrons means there’s no free-flowing charge to carry electricity or heat, and the rigid, directional bonds between atoms make the material brittle rather than flexible.
Electronegativity, a measure of how strongly an atom attracts electrons, captures this difference numerically. The Pauling scale runs from 0 to 4. Metals cluster at the low end (sodium is 0.9, iron is 1.8), while nonmetals sit at the high end (oxygen is 3.4, fluorine tops the scale at 4.0). The higher the number, the more aggressively an atom pulls electrons toward itself. This single property does a remarkably good job of predicting which side of the staircase an element falls on.
How They Bond With Other Elements
The electron behavior described above directly determines how metals and nonmetals form compounds. When a metal reacts with a nonmetal, the metal’s loosely held electrons transfer to the nonmetal. This creates a positively charged metal ion and a negatively charged nonmetal ion. The attraction between these opposite charges is an ionic bond. Table salt is the classic example: sodium (a metal) gives an electron to chlorine (a nonmetal), and the resulting sodium and chloride ions lock together in a crystal lattice.
When two nonmetals react with each other, neither atom is willing to give up electrons. Instead, they share electron pairs in covalent bonds. Water is a covalent compound: oxygen shares electrons with two hydrogen atoms. Covalent bonds hold atoms in fixed orientations, giving molecules definite shapes. This is why water is always bent at a specific angle and carbon dioxide is always linear.
Metals bonding with other metals form metallic bonds, that communal electron sea. This is the bonding in pure metal objects and alloys like steel or bronze. The lack of rigid directional bonds is why metals can be reshaped without fracturing.
Oxides Tell the Story Too
One of the clearest chemical tests for whether an element is a metal or nonmetal is what kind of oxide it forms. When metals react with oxygen, they produce basic oxides. Dissolve these in water and you get alkaline (basic) solutions. Calcium oxide mixed with water produces calcium hydroxide. Sodium oxide in water produces sodium hydroxide, a strong base.
Nonmetals do the opposite. Their oxides are acidic. Sulfur dioxide dissolves in water to form sulfurous acid. Phosphorus oxide in water makes phosphoric acid. Carbon dioxide in water forms carbonic acid, the mild acid in sparkling water. This pattern holds so consistently that there’s a clear trend across any row of the periodic table: basic oxides on the left (where the metals are), acidic oxides on the right (where the nonmetals are).
Metalloids: The In-Between Elements
The six or seven elements hugging the staircase line don’t commit to either side. Silicon, germanium, arsenic, antimony, tellurium, and boron are metalloids. They look metallic, with a grayish sheen, but they don’t conduct electricity nearly as well as true metals. Their electrons are more tightly bound to their nuclei than in metals but not as tightly as in typical nonmetals. This makes them semiconductors, materials that conduct electricity under some conditions but not others.
Their chemistry splits the difference too. Metalloids form covalent crystals like nonmetals do, with atoms locked in fixed positions by shared electron pairs. But unlike most nonmetals, they don’t form simple negatively charged ions. Their electronegativity values fall right in the middle range, neither strongly attracting electrons nor easily giving them up. This intermediate behavior is precisely why silicon became the foundation of the electronics industry: its conductivity can be tuned by adding tiny amounts of other elements, a trick that pure metals and pure nonmetals can’t pull off.
Exceptions Worth Knowing
The metal/nonmetal divide is one of the most reliable patterns in chemistry, but a few elements break the rules. Mercury is a metal that’s liquid at room temperature, unlike the solid you’d expect. Bromine is a nonmetal that’s also liquid at room temperature, unlike the gases and brittle solids typical of its group. Carbon in the form of graphite conducts electricity despite being a nonmetal, because one of its electrons per atom is free to move through its layered structure.
Gold and platinum are metals that resist reacting with oxygen, which is why they’re found in nature as pure metal nuggets rather than locked in mineral ores. Most other metals corrode over geological time scales through oxidation, rusting away into oxides and other compounds. The handful of “noble” metals that resist this process have been prized by humans for thousands of years precisely because they stay shiny.