Metals, Nonmetals, Metalloids

If you draw a diagonal line from boron (B) down to astatine (At) on the periodic table, you have just divided the elements into two fundamentally different camps. Everything to the left of that line is a metal. Everything to the right is a nonmetal. And the handful of elements that sit right along the line? Those are the metalloids - the elements that can’t quite make up their minds.

This three-way classification is one of the first things you should recognize when you see an element on the MCAT. Knowing whether an element is a metal, nonmetal, or metalloid immediately tells you how it bonds, whether it conducts electricity, and whether it is more likely to lose or gain electrons.

Metals

Metals make up the vast majority of the periodic table - roughly 80% of all known elements. They dominate the left side and center of the table, including all of the s-block (except hydrogen), the entire d-block (transition metals), the f-block (lanthanides and actinides), and the lower-left portion of the p-block.

Physical properties of metals:

• Lustrous (shiny) - metallic bonding allows free electrons to reflect light
• Malleable - can be hammered into thin sheets without breaking
• Ductile - can be drawn into thin wires
• Good conductors of heat and electricity (due to the “sea of electrons” in metallic bonding)
• High melting and boiling points (generally, with exceptions like mercury)
• Solid at room temperature (except mercury, which is a liquid)

Chemical properties of metals:

• Low ionization energies - it takes relatively little energy to remove their valence electrons
• Low electronegativities - they do not attract electrons strongly
• Tend to lose electrons and form cations (positive ions)
• Form ionic compounds when they react with nonmetals
• Form basic oxides (metal oxides dissolved in water produce bases)

Nonmetals

Nonmetals occupy the upper-right portion of the periodic table. There are only about 17 nonmetals, but they are disproportionately important in chemistry and biology. Carbon, nitrogen, oxygen, hydrogen, phosphorus, and sulfur make up the backbone of every biological molecule you will ever study.

Physical properties of nonmetals:

• Dull (not lustrous) - no free electron sea to reflect light
• Brittle as solids - they shatter rather than bend
• Poor conductors of heat and electricity (they are insulators)
• Low melting and boiling points (many are gases at room temperature)
• Exist in all three states at room temperature: solids (C, S, P, I2), liquids (Br2), gases (H2, N2, O2, F2, Cl2, noble gases)

Chemical properties of nonmetals:

• High ionization energies - they hold onto their electrons tightly
• High electronegativities - they attract electrons from other atoms
• Tend to gain electrons and form anions (negative ions)
• Form covalent compounds when they bond with other nonmetals
• Form acidic oxides (nonmetal oxides dissolved in water produce acids)

Metals vs. Nonmetals: A Comparison

| Property | Metals | Nonmetals |
|----------|--------|-----------|
| Appearance | Lustrous (shiny) | Dull |
| Malleability | Malleable (can be hammered) | Brittle |
| Conductivity | Good conductors | Poor conductors (insulators) |
| State at room temp | Mostly solids (except Hg) | Gases, liquids, or solids |
| Ionization energy | Low | High |
| Electronegativity | Low | High |
| Electron behavior | Lose electrons (form cations) | Gain electrons (form anions) |
| Oxide character | Basic oxides | Acidic oxides |
| Bonding with each other | Metallic bonding | Covalent bonding |
| When metal + nonmetal react | Form ionic compounds | Form ionic compounds |

Metalloids (Semimetals)

Metalloids are the elements that lie along the “staircase line” (also called the zigzag line or diagonal line) on the periodic table. They have properties intermediate between metals and nonmetals.

The metalloids you should know are: B, Si, Ge, As, Sb, Te, Po, At.

Key properties of metalloids:

• Intermediate conductivity - they are semiconductors, meaning they conduct electricity better than nonmetals but worse than metals
• Intermediate appearance - some have metallic luster, some do not
• Semiconductor behavior - conductivity increases with temperature (opposite of metals, whose conductivity decreases with temperature)
• Can form both covalent and ionic bonds depending on the reaction partner

The semiconductor property of metalloids is their most important practical application. Silicon and germanium are the foundation of the entire electronics industry. Their ability to conduct electricity under controlled conditions makes them essential for computer chips, solar cells, and transistors.

Hydrogen: The Oddball

Hydrogen sits in Group 1 but is not a metal. It has one valence electron like the alkali metals, but it behaves like a nonmetal in most situations. It has high ionization energy, forms covalent bonds (H2, H2O, CH4), and can even gain an electron to form the hydride ion (H-) in compounds like NaH.

The MCAT treats hydrogen as a nonmetal. Do not let its position in Group 1 confuse you.

Predicting Behavior from Position

The real power of this classification is prediction. When you encounter an unfamiliar element on the MCAT:

1. Locate it on the periodic table - left side, right side, or along the staircase?
2. Classify it - metal, nonmetal, or metalloid?
3. Predict its behavior - will it lose or gain electrons? Will it form ionic or covalent bonds? Will it conduct electricity?

For example, if a passage mentions tellurium (Te) and you are unsure of its properties, find it on the table. It sits right on the staircase line - it is a metalloid. You can immediately predict that it has intermediate conductivity, can behave as either an electron donor or acceptor depending on context, and its oxide will have amphoteric (both acidic and basic) character.