Learning Objective

In this lesson we will revise the classification of matter and atomic structure.

Learning Outcomes

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

  • Distinguish between pure substances and mixtures.

  • Distinguish between elements and compounds.

  • Describe the arrangement of atoms in pure substances.

  • Describe the subatomic structure of atoms, including protons, neutrons and electrons.

  • Define atomic number and mass number, and explain what isotopes are.

  • Describe the configuration of electrons for the first 20 elements.

 

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Pure Substances and Mixtures

  • Chemistry is the study of the physical and chemical properties of matter.
  • Matter can be divided into pure substances and mixtures.
  • Pure substances have fixed physical and chemical properties, such as melting and boiling point, and chemical reactivity.

 
gold pure substance element

Gold is a pure substance: it has fixed physical and chemical properties.

(Image: James St. John, Wikimedia Commons)

 

  • Mixtures have variable physical and chemical properties.
    They are physical combinations of two or more pure substances, which can be physically separated.

 
mud mixture

Mud is a mixture: it has variable physical and chemical properties.

(Image: Hans, Pixabay)

 

Elements and Compounds

  • Pure substances can be divided into elements and compounds.
  • Elements cannot be broken down into simpler substances.
    Examples of elements include chlorine, sulfur and copper.

 

chlorine gas pure element    chlorine gas pure element    copper pure element

Chlorine, sulfur and copper are elements: they have fixed physical and chemical properties, and cannot be broken down into simpler substances.

(Images: W. Oelen, Wikimedia Commons; Ben Mills, Wikimedia Commons; Jonathan Zander, Wikimedia Commons)

 

  • Compounds can be broken down (chemically) into simpler substances.
    Examples of compounds include water, methane and copper sulfate.

 

Copper sulfate is a compound: it has fixed physical and chemical properties, but can be broken down into simpler substances –
the elements copper, sulfur and oxygen.

(Image: Stephanb, Wikimedia Commons)

 

Chemical Subunits: Atoms, Molecules and Lattices

  • A chemical subunit can be defined as the smallest particle of a pure substance that still has the properties of that substance.
  • The simplest type of chemical subunit is an atom.
    Atoms are the building blocks of all matter.
  • Atoms can exist individually, but more often they are connected to other atoms by chemical bonds, forming molecules or lattices.
  • Molecules are discrete arrangements of atoms.

 
molecules structure atom arrangement

Molecules

 

  • Lattices are continuous networks of atoms.

 
lattice structure atom arrangement

Lattice

 

  • Molecules and lattices may be composed of the same type of atom, or they may be composed of two or more different types of atoms, in fixed arrangements.

 

Arrangement of Atoms in Pure Substances

  • Pure substances have fixed properties because they are made up of one type of chemical subunit.
  • Mixtures have variable properties because they are made up of more than one type of chemical subunit.
  • Elements cannot be broken down because their subunits cannot be simplified.
    They are made up of one type of atom.
    This atom may exist individually, or as molecules or lattices.
    Examples
    The atoms in helium exist individually.
    The atoms in oxygen exist as molecules containing pairs of oxygen atoms.
    The atoms in diamond (carbon) exist as a continuous network.

 

helium atoms arrangement    oxygen molecules atom arrangement    carbon lattice atom arrangement

 

  • Compounds can be broken down (chemically) because their subunits can be simplified.
    They are made up of more than one type of atom.
    These atoms may exist as molecules or lattices.
    Examples
    The atoms in carbon dioxide exist as molecules containing one carbon atom and two oxygen atoms.
    The atoms in quartz (silicon dioxide) exist as continuous networks of silicon and oxygen atoms, in a 1:2 ratio.

 

carbon dioxide molecules atom arrangement    silicon dioxide lattice atom arrangement

 

Structure of Atoms

  • Atoms are made up of three main subatomic particles – protons, neutrons and electrons.
  • Protons and neutrons form a cluster at the centre of the atom, called the nucleus.
  • Electrons orbit the nucleus, in electron shells, forming an electron cloud.

 
structure of atom

Structure of an atom

 

Protons

  • Protons are subatomic particles with a positive charge (+1).
  • Protons have a mass of 1 atomic mass unit.
  • The number of protons in an atom determines the type of atom.
    Examples
    If an atom has 2 protons, it is a helium atom.
    If an atom has 6 protons, it is a carbon atom.
    If an atom has 26 protons, it is an iron atom.
  • The number of protons in an atom is known as the atomic number.

 
atomic proton number

 

Neutrons

  • Neutrons are subatomic particles with no charge.
  • Neutrons have a mass of 1 atomic mass unit.
  • The number of neutrons in a particular type of atom can vary.
    For example, 99% of carbon atoms have 6 neutrons, while 1% have 7 neutrons.
  • The total number of protons and neutrons in an atom is known as the mass number.

 
mass number protons neutrons

 

  • Atoms that have the same number of protons but a different number of neutrons are called isotopes.
    Isotopes have the same atomic number, but different mass numbers.

 

carbon-12 isotope mass number            carbon-12 isotope mass number

Carbon-12 and carbon-13 are isotopes of carbon.

 

Electrons

  • Electrons are subatomic particles with a negative charge (-1).
  • Electrons have a mass of 1/1840 of an atomic mass unit.
    Therefore, almost all of the mass of an atom is in the nucleus.
  • The number of electrons in an atom is the same as the number of protons.
    Therefore, atoms have no overall charge.

 

Electron Configuration

  • The arrangement of electrons in shells around the nucleus of an atom is referred to as the electron configuration.
  • Electron shells fill up from the inner shell first.
    The further away from the nucleus a shell is, the larger it is and the more electrons it can hold.
  • The first shell (closest to the nucleus) can hold a maximum of 2 electrons.
    The second shell can hold a maximum of 8 electrons.
    The third shell can hold a maximum of 18 electrons, but once it has 8 electrons, the fourth shell begins to fill.
    For example, even though shell 3 can hold a maximum of 18 electrons, potassium has eight electrons in the third shell and one electron in the fourth shell.
  • The maximum number of electrons that can exist in a shell depends on the shell number, or energy level, n.
    It is given by the formula:

 
maximum electrons shell filling formula

 

Shorthand Electron Configuration

  • The electron configuration of an element can be written in an abbreviated form, showing the numbers of electrons in each shell.
    Example
    A silicon atom has 14 electrons.
    There are two electrons in the first shell, eight electrons in the second shell and four electrons in the third shell.
    The electron configuration of silicon can therefore can be written as 2,8,4.

 
short hand electron configuration silicon

Silicon has the electron configuration 2,8,4.

 

Valence Electrons

  • The outer electron shell is known as the valence shell.
    It can hold a maximum of 8 electrons, regardless of which energy level it is.
  • Electrons in the valence shell are known as valence electrons.
    In the example above, silicon has four valence electrons.
  • The number of valence electrons largely determines the chemical properties of an atom.

 

Summary

  • Pure substances are made up of one type of chemical subunit.
    They have defined physical and chemical properties.
  • Elements are pure substances made up of one type of atom.
    These atoms may exist individually, or as molecules or lattices.
    Elements cannot be broken down into simpler substances.
  • Compounds are pure substances made up of two or more different types of atoms, joined in fixed structures by chemical bonds.
    These structures may be molecules or lattices.
    Compounds can be broken down (chemically) into simpler substances.
  • Atoms are made up of three main subatomic particles – protons, neutrons and electrons.
    Protons and neutrons form a cluster at the centre of the atom, called the nucleus.
    Electrons orbit the nucleus, in electron shells, forming an electron cloud.
  • Protons have a mass of 1 atomic mass unit and a positive charge (+1).
    Neutrons have a mass of 1 atomic mass unit and no charge.
    Electrons have a mass of 1/1840 of an atomic mass unit and a negative charge (-1).
  • Atoms that have the same number of protons but a different number of neutrons are called isotopes.
  • The atomic number of an atom is equal to the number of protons in the atom.
    It determines the type of atom.
  • The mass number of an atom is equal to the total number of protons and neutrons in the atom.
    This is the number of particles in the nucleus, which largely determines the mass of an atom.
  • Electron configuration describes the arrangement of electrons in shells around the nucleus of an atom.
    A maximum of 2 electrons can occupy the first shell.
    A maximum of 8 electrons can occupy the second shell.
    A maximum of 18 electrons can occupy the third shell, but the fourth shell will begin to fill once the third shell contains 8 electrons.
    A maximum of 8 electrons can occupy the valence shell (outermost shell).

 
molecules

(Image: MasterTux, Pixabay)

 

(Header image: geralt, Pixabay)

 

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