Metals and Non-metals on the Periodic Table

Introduction

The periodic table is not just a list of elements in order; it is a map. One of the first things a chemist reads from that map is whether an element is a metal or a non-metal . A thick zigzag line, often called the staircase line , runs across the right-hand side of the table and splits the elements into these two broad classes. Knowing which side of the line an element sits on lets you predict a great deal about how it looks and how it reacts.

Core explanation

Where the line runs. The staircase line starts between boron (B) and aluminium (Al) in Group 13 and steps down and to the right past silicon, germanium, arsenic, antimony and tellurium, finishing near polonium and astatine (At) at the bottom. Elements to the left of the line are metals; elements to the right are non-metals. Hydrogen is the odd one out: it sits at the top left but is a non-metal.

Most elements are metals. Of the 118 known elements, roughly 90 are metals. All of Groups 1 and 2, the whole transition-metal block in the middle, the lanthanides and actinides, and the lower parts of Groups 13 to 16 are metals. The non-metals are a small cluster in the top right corner, plus hydrogen: about 17 elements in total, including carbon, nitrogen, oxygen, the halogens and the noble gases.

Elements on the line. Elements that touch the staircase line, such as boron, silicon, germanium, arsenic, antimony and tellurium, show a mixture of metallic and non-metallic behaviour. They are called metalloids (or semi-metals). Different textbooks draw the line slightly differently, so the exact list of metalloids varies a little.

Why the line is there. The split follows the number of valence electrons (outer-shell electrons). Metals on the left have few valence electrons — typically one, two or three — and lose them fairly easily to form positive ions. Non-metals on the right have many valence electrons — typically four to seven — and tend to gain or share electrons to complete their outer shell. The noble gases, with full outer shells, are also non-metals. As you move across a period, the number of valence electrons rises, so the character of the elements changes from metallic to non-metallic, and the line marks roughly where that change happens.

A useful rule of thumb. Metallic character increases towards the bottom left of the table and decreases towards the top right. That is why the dividing line slopes: going down a group, the change from metal to non-metal happens further to the right.

Step-by-step reasoning

To decide whether an unfamiliar element is a metal or a non-metal:

1. Find the element on the periodic table using its symbol or atomic number. 2. Locate the staircase line running from boron down to astatine. 3. If the element is to the left of the line (and is not hydrogen), classify it as a metal. 4. If it is to the right of the line, or is hydrogen, classify it as a non-metal. 5. If it touches the line, expect in-between behaviour and call it a metalloid.

Visual explanation

Open the interactive periodic table and colour the elements by class. A large block of one colour fills the left and centre (metals), a narrow diagonal band of a second colour follows the staircase (metalloids), and a small triangle of a third colour sits in the top right corner (non-metals), with hydrogen alone at the top left.

Real-world analogy

Think of a map of a country with a river running diagonally across it. Most of the land and most of the towns lie on one bank (the metals), a smaller region lies on the other bank (the non-metals), and a few villages sit right on the riverbank and share features of both sides (the metalloids).

Real-world example

A copper electrical cable shows both classes working together. The copper wire inside is a metal from the left of the line and carries the current. The plastic sheath around it is made mostly of carbon and hydrogen, non-metals from the right of the line, and it insulates the wire so the current cannot escape.

Why?

Why does the dividing line run diagonally rather than straight down? Going down a group, atoms get larger and the outer electrons are further from the nucleus, so they are lost more easily. This makes elements lower in the table more metallic, so the switch to non-metallic behaviour is pushed further to the right in lower periods.

Common misconception

"Every element in the right-hand part of the table is a non-metal." Tin, lead and bismuth are found on the right-hand side, yet they are clearly metals. What matters is which side of the staircase line an element lies on, not simply whether it is in the right half.

Worked example

Question: Classify magnesium (Mg), sulfur (S) and silicon (Si) as metal, non-metal or metalloid, and explain using their positions.

Reasoning: Magnesium is in Group 2, far to the left of the staircase line, so it is a metal. Sulfur is in Group 16, Period 3, to the right of the line, so it is a non-metal. Silicon is in Group 14, Period 3, and touches the line.

Answer: Magnesium is a metal, sulfur is a non-metal and silicon is a metalloid.

Quick check

1. Is potassium (Group 1) a metal or a non-metal? Answer: A metal, because Group 1 lies well to the left of the staircase line.

Exam focus

Be ready to shade or identify the metal and non-metal regions on an outline periodic table. Remember that hydrogen is a non-metal despite its position, and that about four-fifths of all elements are metals.

Advanced insight

The boundary is not perfectly sharp, and some elements change character with conditions or form. Tin, for example, exists as metallic white tin at room temperature but slowly turns into grey tin, a brittle form with semiconducting properties, when kept below about 13 °C. Under enormous pressure, even hydrogen is predicted to become metallic, which scientists think happens deep inside Jupiter.

Summary

A staircase line from boron to astatine divides the periodic table into metals on the left and non-metals on the right. About 90 elements are metals; the non-metals form a small group in the top right plus hydrogen. Elements touching the line are metalloids with mixed properties. The division reflects valence electrons: metals have few and tend to lose them, while non-metals have many and tend to gain or share them.

Practice questions

1. Name the two elements at the start and end of the staircase line. Answer: It starts at boron (B) and ends near astatine (At). 2. Explain why hydrogen is described as an exception to the position rule for metals. Answer: Hydrogen sits at the top of the left-hand side of the table, where metals are found, but it is a non-metallic gas. 3. Chlorine has seven valence electrons and sodium has one. Use this to explain which is the metal. Answer: Sodium is the metal because atoms with few valence electrons tend to lose them, whereas chlorine, with seven, tends to gain an electron like a non-metal. 4. Roughly what fraction of the known elements are metals? Answer: About four-fifths (roughly 90 out of 118).