Atoms in Elements and Compounds

Learning Objective

In this lesson we will learn about the different ways atoms are arranged in pure substance – elements and compounds.

Learning Outcomes

By the end of this lesson you will be able to:

  • Describe elements and compounds.

  • Describe atoms, molecules and lattices.

  • Illustrate the different ways atoms can be arranged in elements, with examples.

  • Illustrate the different ways atoms can be arranged in compounds, with examples.

 

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Introduction

  • Atoms are the fundamental building blocks of matter.
  • Depending on the type of element or compound they form part of, atoms may exist individually, in molecules, or in a lattice structure.
  • The infinite number of possible combinations of different types of atoms accounts for the enormous number of compounds that exist.

 
atoms pure substances elements compounds

Although there are only 118 different types of atoms, there are countless compounds that exist.

(Image: Cygaretka, Wikimedia Commons)

 

Pure Substances

  • Pure substances are those which have fixed physical and chemical properties.
    For example, they have a consistent composition throughout, fixed melting and boiling points, and chemically react in predictable ways.
  • Physical combinations of pure substances are known as mixtures.
    Mixtures have variable physical and chemical properties.

 

Elements

  • The simplest type of pure substance is an element.
    Elements are substances that cannot be chemically broken down into separate substances.
    For example, iron and sulfur are both elements. Once they are purified, they cannot be chemically separated into simpler substances.
  • There are 118 known elements – 91 naturally occurring elements and 27 synthetic (man-made) elements.
    All of these are represented on the periodic table of the elements.

 

iron element pure substance    sulfur element pure substance

Iron (left) and sulfur (right) are both elements. They cannot be chemically broken down into simpler substances.

(Images: Chemicalinterest, Wikimedia Commons; Ben Mills, Wikimedia Commons)

 

Compounds

  • Different elements can chemically combine to form substances known as compounds.
    Compounds are pure substances, but since they are formed from more than one element, they can be chemically broken down back to these elements.
    For example, the elements iron and sulfur can chemically combine to form the compound iron sulfide; iron sulfide can therefore be chemically broken down back to iron and sulfur.

 
iron sulfide compound pure substance

Iron sulfide is a compound. It can be chemically broken down into the elements iron and sulfur.

(Image: Benjah-bmm27, Wikimedia Commons)

 

Chemical Subunits

  • Pure substances have fixed properties because they are made up of only one type of chemical subunit.
    A chemical subunit can be defined as the smallest fraction of a pure substance that still has the properties of that substance.

 

Atoms

  • The simplest type of chemical subunit is an atom.
    Atoms are the fundamental building blocks of all matter.
  • Atoms only rarely exist as individual subunits.
    The only atoms that exist individually are those of the noble gases.
    These are the elements which are located in group 18 of the periodic table. They include helium, neon and argon.

 
helium atoms individual monatomic

Helium is an example of an element with atoms arranged individually.

 

  • More often, atoms are connected to other atoms by strong forces of attraction known as chemical bonds.
    These bonded atoms can form two types of chemical structures: molecules and lattices.
    Molecules and lattices can consist of one type of atom (and therefore form the subunit of an element), or consist of more than one type of atom (and therefore form the subunit of a compound).

 

Molecules

  • Molecules are discrete chemical subunits consisting of fixed arrangements of atoms held together by chemical bonds.
  • Most non-metal elements other than the noble gases have molecules as their subunits.
    These include hydrogen, nitrogen and chlorine.

 
chlorine atoms molecules

Chlorine is an example of an element with atoms arranged in molecules.

 

  • Most compounds have molecules as their subunits.
    These include small molecules, such as water, methane and ethanol, but also large molecules, such as carbohydrates, fats and proteins.

 
water atoms molecules

Water is an example of a compound with atoms arranged in molecules.

 

Lattices

  • Lattices are continuous networks of atoms in a fixed arrangement, held together by chemical bonds.
  • All metal elements consist of atoms arranged as a lattice.
    Carbon and silicon are two examples of non-metal elements whose atoms are arranged as a lattice.

 
carbon atoms lattice

Carbon is an example of an element with atoms arranged in a lattice.

 

  • Silicon dioxide and sodium chloride* are two examples of compounds whose atoms are arranged as a lattice.
    *Most compounds that exist as lattices – such as sodium chloride – are ionic compounds; therefore, the lattices are actually made up of positive and negative ions rather than atoms.

 
silicon dioxide atoms lattice

Silicon dioxide is an example of a compound with atoms arranged in a lattice.

 

Summary

  • Atoms are the fundamental subunits of matter.
    They may exist individually (in some elements) or joined to other atoms by chemical bonds, forming molecules or lattices (in most elements and all compounds).
  • Molecules are discrete structures containing two or more atoms bonded together in a fixed arrangement.
    Molecules can consist of one type of atom, or more commonly, more than one type of atom.
  • Lattices are continuous networks of atoms joined by chemical bonds.
    As with molecules, lattices can consist of one type of atom, or more commonly, more than one type of atom.
  • Elements and compounds are both pure substances as they are made up of one type of chemical subunit.
  • Elements are composed of one type of atom.
    These atoms may exist separately, as molecules or as a lattice.
  • Compounds are composed of more than one type of atom.
    These atoms may exist as molecules or as a lattice.

 
enzyme protein molecule structure

Some molecules, such as proteins, can contain thousands of atoms.

(Image: Splette, Wikimedia Commons)

 

(Header image: Anusorn, Adobe Stock)

 

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