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Elements and Compounds Introductory Chemistry Tutorial

Key Concepts

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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:

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)

H
hydrogen
  He
helium
Li
lithium
Be
beryllium
B
boron
C
carbon
N
nitrogen
O
oxygen
F
fluorine
Ne
neon
Na
sodium
Mg
magnesium
Al
aluminium
Si
silicon
P
phosphorus
S
sulfur
Cl
chlorine
Ar
argon
K
potassium
Ca
calcium
 

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

Fe
iron
Ni
nickel
Cu
copper
Zn
zinc
Ag
silver
Sn
tin
Pt
platinum
Au
gold
Hg
mercury
Pb
lead

Elements can be present in nature as solids, liquids or gases.
Of the 92 naturally occurring elements:

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

Liquid Elements Gaseous Elements Solid elements
(some examples below)
  1. mercury (Hg)
  2. bromine (Br)
  1. hydrogen (H)
  2. helium (He)
  3. nitrogen (N)
  4. oxygen (O)
  5. fluorine (F)
  6. neon (Ne)
  7. chlorine (Cl)
  8. argon (Ar)
  9. krypton (Kr)
  10. xenon (Xe)
  11. radon (Rn)
  • lithium (Li)
  • carbon (C)
  • sodium (Na)
  • magnesium (Mg)
  • aluminium (Al)
  • silicon (Si)
  • phosphorus (P)
  • sulfur (S)
  • potassium (K)
  • calcium (Ca)
  • zinc (Zn)

The most common elements found in the universe are:

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

Common elements found in the earth's crust are:

The most common elements found in living things are:

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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:

compound name compound formula elements present
water H2O hydrogen (H) and oxygen (O)
ammonia NH3 nitrogen (N) and hydrogen (H)
carbon monoxide CO carbon (C) and oxygen (O)
carbon dioxide CO2 carbon (C) and oxygen (O)
sodium chloride NaCl sodium (Na) and chlorine (Cl)
sodium hydroxide NaOH sodium (Na), oxygen (O) and hydrogen (H)
calcium chloride CaCl2 calcium (Ca) and chlorine (Cl)
calcium carbonate CaCO3 calcium (Ca), carbon (C) and oxygen (O)
calcium nitrate Ca(NO3)2 calcium (Ca), nitrogen (N) and oxygen (O)
calcium phosphate Ca3(PO4)2 calcium (Ca), phosphorus (P) and oxygen (O)
calcium sulfate CaSO4 calcium (Ca), sulfur (S) and oxygen (O)
methane CH4 carbon (C) and hyrogen (H)
ethanol C2H5OH carbon (C), hydrogen (H) and oxygen (O)

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Worked Examples

(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:

Chemical Formula: CaCO3
Symbol of each element Ca C O

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

Chemical Formula: CaCO3
Symbol of each element Ca C O
Name of each element calcium carbon oxygen

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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Footnotes:

(1) You can find out more about the discovery and naming of elements in the History of the Elements tutorial.

(2) IUPAC recognizes both aluminium and aluminum as names for the element with the chemical symbol Al.
However, it should be noted that the spelling on the IUPAC's periodic table is aluminium NOT aluminum.

(3) There are different types of "chemical formula" depending on what sort of comound you are dealing with.
We will just use the very general term "chemical formula" to cover any type of formula.
You can find out more about some of these different formulae by reading the following tutorials:

empirical formula

molecular formula

formula of salts

formula of binary inorganic compounds

formula of ligands and complex ions

2-dimensional structural formula

Condensed structural formula