Classifying Elements: Metals, Nonmetals, Metalloids

Elements are classified as metals, nonmetals or metalloids using their position in the periodic table and their properties: metals lie to the left and below a stair-step line, nonmetals to the right and above it, and metalloids along the line. In brief: across a period, elements change from metal to nonmetal (sodium to chlorine), and down a group metallic character increases (carbon is a nonmetal, tin is a metal); oxides of metals are usually basic, oxides of nonmetals are usually acidic.

This article uses the periodic table to classify elements and predict their behavior. It looks at how metallic character varies in the table, how the type of oxide helps to tell the classes apart, the properties of period 3 from sodium to argon, how many elements belong to each class, a worked classification exercise and why some elements are hard to classify.

How does position tell the class?

The elements that are generally considered metalloids are boron, silicon, germanium, arsenic, antimony and tellurium. They lie along a diagonal line that runs from boron to astatine and separates the metals on the lower left from the nonmetals on the upper right. Hydrogen is a nonmetal despite its position in group 1. All of the elements in the d-block and f-block are metals. In the main groups, the elements to the left of the line are metals and those to the right are nonmetals.

How does metallic character change in the table?

• Down a group: metallic character increases, because the valence electrons are farther from the nucleus and are lost more easily. In group 14, carbon is a nonmetal, silicon and germanium are metalloids, and tin and lead are metals.
• Across a period (left to right): metallic character decreases, because the nuclear charge increases and holds the electrons more strongly. In period 3, sodium is a metal and chlorine is a nonmetal.
• The most metallic elements are therefore at the bottom left (francium and cesium), and the most nonmetallic at the top right (fluorine, excluding the noble gases).

What happens across period 3?

How do oxides show the class of an element?

Metal oxides are generally ionic and basic: they react with acids to give a salt and water, and the soluble ones form alkalis in water. For example, MgO + 2HCl → MgCl₂ + H₂O and Na₂O + H₂O → 2NaOH. Nonmetal oxides are molecular and acidic: they react with bases, and many form acids in water: SO₃ + H₂O → H₂SO₄ and CO₂ + H₂O ⇌ H₂CO₃. Amphoteric oxides, such as those of aluminum, zinc and many metalloids, react with both acids and bases: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O, and Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄].

How many elements are in each class?

The count depends on the source, as the borderline cases such as astatine, polonium and selenium are classified differently. About three quarters of the elements are metals.

Worked examples

1. Classify by position. Arsenic (Z = 33) lies on the stair-step line in group 15, so it is a metalloid. Tin (Z = 50) is below the line in group 14, so it is a metal. Selenium (Z = 34) is in group 16 and to the right of the line, so it is a nonmetal.
2. Predict the oxide. An element X in group 2 forms XO. As a metal in group 2, XO is basic and reacts with water to form X(OH)₂. A nonmetal Y in group 16 forms YO₂, acidic, like SO₂, which dissolves in water to give an acid.
3. Test results. A solid is shiny, hammers into a sheet, conducts electricity well and its oxide dissolves in water to give a solution with a pH of 13. These properties show that it is a metal, probably from group 1 (a strongly basic oxide).
4. Which is more metallic? Compare germanium and tin (same group): tin is lower, so it is more metallic. Compare magnesium and aluminum (same period): magnesium is to the left, so it is more metallic. Compare Mg and Ge: both are in different groups and periods, but magnesium is a metal and germanium is a metalloid, so magnesium is more metallic.
5. Acid-base behavior. The oxide of an unknown element reacts with both hydrochloric acid and sodium hydroxide solution. It is amphoteric, which suggests an element near the boundary between metals and nonmetals, such as aluminum, or zinc, a metal with an amphoteric oxide.
6. Mass of base. 6.20 g of Na₂O (62.0 g/mol) is 0.100 mol and reacts with water to give 0.200 mol of NaOH. In 1.00 L the concentration is 0.200 mol/L and the pOH is 0.70, so the pH is 13.3.

Which elements are hard to classify?

The change from metal to nonmetal is gradual. Antimony and arsenic have a metallic form and a nonmetallic form, and bismuth, which is below the line, is a metal with a poor conductivity. Polonium and astatine are often called metalloids, but their classification is uncertain, because of the small amounts that exist and their intense radioactivity. Carbon in the form of graphite conducts electricity, while the diamond form does not. For these reasons the lists of metalloids differ, and the class labels should be regarded as a useful guide rather than a strict rule.

How did chemists develop these classes?

Lavoisier distinguished metals from nonmetals in his table of 1789. In the 19th century, Berzelius sorted elements according to whether they acted as electropositive or electronegative in electrolysis. The term metalloid was used early in the 19th century for nonmetals that look like metals and later for elements with properties between those of metals and nonmetals. As semiconductors became technologically important in the 20th century, the category of metalloids gained importance.

Which mistakes are common?

Mistake 1: Treating the line as an exact boundary

It is an approximate guide. Some elements next to it have mixed properties.

Mistake 2: Classifying hydrogen as a metal because it is in group 1

Hydrogen is a nonmetal, except in extreme conditions such as the interior of giant planets.

Mistake 3: Believing that all metals have basic oxides

Some metals form amphoteric or acidic oxides, such as aluminum, zinc and chromium(VI) oxide.

Mistake 4: Saying that metallic character increases across a period

It decreases from left to right and increases down a group.