Chemical Behaviour of Metals and Non-metals

Introduction

Physical properties tell you how an element looks and handles, but chemistry is about how elements react . Here too, metals and non-metals behave in opposite ways. Metal atoms tend to give away electrons; non-metal atoms tend to take or share them. This single difference explains the kinds of compounds each class forms, including the very different oxides produced when they burn in oxygen.

Core explanation

Metals lose electrons. Metal atoms have one, two or three valence electrons. They reach a stable arrangement most easily by losing these electrons, forming positive ions (cations):

- Na → Na⁺ + e⁻ - Mg → Mg²⁺ + 2e⁻ - Al → Al³⁺ + 3e⁻

The charge on the ion equals the number of electrons lost, which for main-group metals matches the group number (Group 1 gives 1+, Group 2 gives 2+, Group 13 gives 3+).

Non-metals gain or share electrons. Non-metal atoms have five, six or seven valence electrons. Gaining a few electrons is easier than losing many, so when they react with metals they form negative ions (anions):

- Cl + e⁻ → Cl⁻ - O + 2e⁻ → O²⁻

When two non-metals react with each other, neither is willing to give up electrons, so they share pairs of electrons in covalent bonds instead, forming molecules such as H₂O, CO₂ and NH₃.

Types of compound.

Reacting elements What happens to electrons Compound type Example --- --- --- --- Metal + non-metal Transferred from metal to non-metal Ionic Sodium chloride, NaCl Non-metal + non-metal Shared Covalent (molecular) Water, H₂O Metal + metal Pooled in a sea Alloy (metallic) Brass (copper and zinc)

Oxides. Almost every element reacts with oxygen to form an oxide . Metals form ionic oxides, which are usually solids with high melting points, such as magnesium oxide (MgO) and calcium oxide (CaO). Non-metals form covalent oxides, which are often gases or low-melting substances, such as carbon dioxide (CO₂) and sulfur dioxide (SO₂). As the next topic explores, metal oxides behave as bases , while non-metal oxides are generally acidic .

Reactions with dilute acids. Many metals react with dilute acids, releasing hydrogen gas and forming a salt. Non-metals generally do not react with dilute acids in this way. This offers another chemical test to tell the classes apart.

Step-by-step reasoning

To predict the formula of the compound formed between calcium and chlorine:

1. Calcium is a Group 2 metal, so it loses 2 electrons to form Ca²⁺. 2. Chlorine is a Group 17 non-metal, so each atom gains 1 electron to form Cl⁻. 3. Two Cl⁻ ions are needed to balance one Ca²⁺ ion. 4. The ionic compound is CaCl₂, calcium chloride.

Visual explanation

Draw a sodium atom with one outer electron and a chlorine atom with seven. An arrow carries the single electron from sodium across to chlorine. Sodium's circle shrinks and gains a "+" sign; chlorine's circle grows and gains a "−" sign. The two oppositely charged ions then attract each other.

Real-world analogy

A metal atom is like someone with a couple of loose coins burning a hole in their pocket, happy to hand them over. A non-metal atom is like a collector who is just one or two coins short of a complete set and eager to receive them. When the two meet, the transfer happens naturally.

Real-world example

When magnesium ribbon burns in air, it produces a brilliant white flame and a white powder of magnesium oxide, an ionic solid. When carbon in charcoal burns, it produces carbon dioxide, an invisible covalent gas. The very different products reflect the metal and non-metal behaviour of the two elements.

Why?

Why do metals lose electrons rather than gain them? A metal atom with one or two valence electrons would need to gain six or seven electrons to fill its outer shell, which costs far too much energy. Losing the few outer electrons, which are relatively far from the nucleus and weakly held, is much easier.

Common misconception

"Atoms want to form ions because they want a full shell." Atoms do not have desires. Ions form when the overall process releases energy or leads to a more stable arrangement; losing or gaining electrons alone often costs energy, but the strong attraction between the resulting ions more than repays it.

Worked example

Question: Predict the type of compound and its formula when aluminium reacts with oxygen.

Reasoning: Aluminium is a Group 13 metal and forms Al³⁺. Oxygen is a Group 16 non-metal and forms O²⁻. A metal and a non-metal form an ionic compound. To balance charge, 2 × (+3) = +6 and 3 × (−2) = −6, so two aluminium ions combine with three oxide ions.

Answer: An ionic compound, aluminium oxide, Al₂O₃.

Quick check

1. What type of ion does a Group 2 metal form? Answer: A positive ion with a 2+ charge, such as Mg²⁺.

Exam focus

Be able to state that metals form positive ions and non-metals form negative ions, and to predict whether a compound is ionic or covalent from the elements it contains. Examiners reward the phrase "electrons are transferred from the metal to the non-metal".

Advanced insight

The tendency to lose or gain electrons is described more precisely by electronegativity , a measure of how strongly an atom attracts bonding electrons. Fluorine is the most electronegative element and caesium among the least. When two bonded atoms differ greatly in electronegativity the bond is ionic; when they are similar it is covalent; in between, the electrons are shared unequally, giving a polar covalent bond.

Summary

Metals tend to lose electrons to form positive ions; non-metals tend to gain electrons to form negative ions or to share electrons with other non-metals. Metal and non-metal combine to make ionic compounds, while two non-metals make covalent compounds. Metals form ionic, basic oxides such as MgO, and non-metals form covalent, usually acidic oxides such as CO₂.

Practice questions

1. Write an equation showing a potassium atom forming an ion. Answer: K → K⁺ + e⁻ 2. Is hydrogen chloride, HCl, ionic or covalent? Explain. Answer: Covalent, because hydrogen and chlorine are both non-metals and share a pair of electrons. 3. Predict the formula of the compound formed between sodium and oxygen. Answer: Na₂O, because two Na⁺ ions are needed to balance one O²⁻ ion. 4. State one difference between the oxide of magnesium and the oxide of carbon. Answer: Magnesium oxide is an ionic, high-melting solid that is basic; carbon dioxide is a covalent gas that is acidic.