What do elements and compounds have in common?

Key Concepts

⚛ A pure substance can be classified as either:

(i) an element

(ii) a compound

⚛ An element is a pure substance that cannot be decomposed into simpler substances.

· Elements are listed in the Periodic Table of the Elements.

· Each element has a 1 or 2 letter symbol.

⚛ A compound is a pure substance that can be decomposed into simpler substances.

· Compounds are made up of two or more elements.

· Compounds are represented by chemical formula using the symbols of the elements found in the compound.

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Elements

An element is defined as a pure substance that cannot be decomposed (broken down) into simpler substances.
For example, if you heat nitrogen gas it will get hotter, but it won't break down (decompose) into anything simpler than nitrogen.

There are 92 naturally occurring elements.
In addition, there are more than 20 man-made elements (synthetic elements or artifical elements).
All the known elements, both naturally occurring and synthetic elements, are listed in the Periodic Table of the Elements.

Each element has been given a unique name and a unique chemical symbol.
This chemical symbol of an element is most often based on the current name of the element, or, in some cases, it is based on an older name for the element.(1)
The chemical symbol of an element can be made up of either 1 letter or 2 letters:

⚛ the first letter of the chemical symbol of an element is always a capital letter (upper case letter)

For example:

hydrogen is an element with the chemical symbol H

carbon is an element with the chemical symbol C

nitrogen is an element with the chemical symbol N

⚛ if there is a second letter in the chemical symbol of an element it is always a lower case letter

For example:

helium is an element with the chemical symbol He

calcium is an element with the chemical symbol Ca

neon has is an element with chemical symbol Ne

Chemistry teachers often ask their students to memorise the names and chemical symbols of the first 20 elements in the periodic table of the elements, so here they are:(2)

You may also be asked to memorise the names and chemical symbols of some common, useful elements such as:

At room temperature and pressure elements can be present as solids, liquids or gases.
Of the 92 naturally occurring elements:

• 2 elements are liquids
• 11 elements are gases
• all the other elements are solids

The names and chemical symbols of elements that occur at room temperature and pressure as solids, liquids, and gases is given in the table below:

The most common elements found in the universe are:

• hydrogen (H)
• helium (He)
• oxygen (O)
• carbon (C)

The Earth's atmosphere is made up of a mixture of different gases, but is mostly made up of two elements:

• nitrogen (which makes up about 78% of the atmosphere)
• oxygen (which makes up about 21% of the atmosphere)

Common elements found in the Earth's crust are:

• oxygen (O)
• silicon (Si)
• aluminium (Al)
• iron (Fe)
• calcium (Ca)
• sodium (Na)
• potassium (K)
• magnesium (Mg)
• hydrogen (H)

The most common elements found in living things are:

• carbon (C)
• hydrogen (H)
• oxygen (O)
• nitrogen (N)
• phosphorus (P)
• sulfur (S)

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Compounds

A compound is a pure substance that can be decomposed into simpler substances because it is made up of 2 or more elements.

For example, water is made up of two different elements; hydrogen (H) and oxygen (O)
An electric current can be passed through water to decompose (break up) water into the elements hydrogen and oxygen.

Each compound is represented by a chemical formula(3) showing which elements are present in the compound.

For example, water is made up of two different elements; hydrogen (H) and oxygen (O)
The chemical formula for water must include the symbols for both these elements, H and O
The chemical formula for water is actually H2O

Examples of some common compounds that you should know are shown in the table below:

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Worked Examples: Elements and Compounds

(Using the StoPGoPS model for problem solving.)

Question 1

Oxygen gas (chemical formula O2) makes up about 21% of the Earth's atmosphere.

Is oxygen gas an element or a compound?

What is the question asking you to do?

Decide whether oxygen gas is an element or a compound.

What information have you been given in the question?

Name: oxygen gas
Chemical formula: O2

What is the relationship between what you know and what you need to find out?

(i) Elements cannot be decomposed into simpler substances because they are made up of only one element.

(ii) Compounds can be decomposed into simpler substances because they are made up of 2 or more elements.

Answer the question.

Oxygen gas (chemical formula O2) is made up ONLY of the element oxygen (chemical symbol O)

Oxygen gas must be an element.

Is your answer plausible?

If you look in the periodic table of the elements you will find oxygen (O) listed as an element.

Therefore oxygen is an element. It doesn't matter whether you refer to it as oxygen gas, liquid oxygen, or solid oxygen, it all refers to the element oxygen.

State your answer.

Oxygen gas is an element.

Question 2

Calcium carbonate (chemical formula CaCO3) is found in limestone.

Name the elements found in the compound calcium carbonate.

What is the question asking you to do?

Name the elements found in calcium carbonate, CaCO3

What information have you been given in the question?

Name of compound: calcium carbonate

Chemical formula of compound: CaCO3

What is the relationship between what you know and what you need to find out?

(i) Compounds are made up of 2 or more elements.

(ii) The chemical formula of the compound includes the symbol of each element found in the compound.

Answer the question.

(i) Break the chemical formula up into the symbols of the elements:

(ii) Use the periodic table of the elements to find the name of the element corresponding to each chemical symbol:

Is your answer plausible?

Work backwards.

Use the periodic table to find the symbol of each element you named:
calcium, Ca
carbon, C
oxygen, O

See if you can arrange these chemical symbols like they are displayed in the chemical formula:
CaCO

Since this corresponds to the same order we find in the chemical formula (CaCO3: CaCO) with no symbols added nor missing, we are reasonably confident that our answer is plausible.

State your answer.

Calcium carbonate is made up of the elements calcium, carbon and oxygen.

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Sample Question: Elements and Compounds

Which of the following groups contains elements only?

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