Properties of Matter

Inquiry question: How do the properties of substances help us to classify and separate them?

Physical properties of a substance are the characteristics we can observe (qualitatively and quantitatively).  This includes colour, magnetism particle size, melting point, boiling point and density.

A homogeneous mixture is one in which the components that make up the mixture are uniformly distributed throughout the mixture, e.g. salt dissolve in water, copper sulfate solution.

A heterogeneous mixtures is in which the components of the mixture are not uniform, i.e. there are local regions with properties that are different from other local regions. Different samples from the mixture are not identical to each other, e.g. sand and iron filings, dirt and water.

Some separation techniques based on physical properties are:

• chromatography

• filtration

• evaporation

• crystallisation

• simple distillation and fractional distillation

• separating funnel

• sublimation

• magnetic attraction

We can calculate the percentage composition by weight of component elements and/or compounds by:

Percent by mass  = mass of component total ÷ mass x 100%

We name inorganic substances using the International Union of Pure and Applied Chemistry (IUPAC) naming conventions (nomenclature).  Basic rules are followed to create an IUPAC name for an inorganic compound:

1. The cation (metal) is always named first with its name unchanged.

2. The anion (non-metal) is written after the cation, modified to end in –ide.

Elements are pure substances that are composed of only one type of atom, so they cannot be chemically or physically decomposed.  An element can be a metal, a non-metal or a metalloid. We classify the elements based on their properties and position in the periodic table of elements (PTOE) through their:

– physical properties

– chemical properties

Atomic structure and atomic mass

Inquiry question: Why are atoms of elements different from one another?

An atom has a very small nucleus surrounded by an electron cloud.  In the nucleus there is a least one proton (which has a positive charge).  The number of protons tells us the Atomic Number (Z) of the element.  Each element has a different Atomic Number.  All elements, except hydrogen, also have at least one neutron (no charge) in the nucleus.  The electron (with a negative charge) is a fundamental particle and very small (about 1840 times less massive than a proton).

Normally, the atom has no overall charge, so:

the number of protons = the number of electrons

When finding the Mass Number (A), the mass of the electron is ignored and the number of nucleons (protons and neutrons) is totalled:

Mass Number = number of protons + number of neutrons

An isotope of an element has the same number of protons, but a different Mass Number (with either more or less neutrons).

Radioactivity of an isotope occurs if its nucleus is unstable.  It may randomly decay and emit radioactive particles or rays.  There are three types of radiation that may be emitted from an unstable nucleus:

1. alpha particles - a particle made up of 2 protons and 2 neutrons

2. beta particles - a particle that is either negative (an electron) or positive (a positron)

3. gamma rays - high energy electromagnetic waves

When a nucleus loses an alpha particle, A decreases by 4 and Z decreases by 2.  When a nucleus loses a beta (electron) particle, A is unchanged and Z increases by 1.  When a nucleus loses a beta+ particle (positron), A is unchanged and Z decreases by 1.  When a nucleus loses a gamma ray, both A and Z are unchanged.

Electrons are arranged in shells that occur in discrete energy levels, usually given numbers n = 1, 2, 3, ... counting from the inner shell outwards.  Unlike the game Frogger, these energy levels become closer as the energy level increases.

An electron configuration shows how many electrons are in each shell.  Some examples are:

• helium has 2 (only 2 electrons in the outer shell)

• carbon 2,4 (2 electrons in the inner shell and 4 electrons in the outer shell)

• silicon 2, 8, 4 (2 electrons in the inner shell, 8 electrons in the next shell and 4 electrons in the outer shell)

Electrons in the outer shell give the element its properties and are important in chemical reactions, e.g. carbon and silicon have similar properties and are in the same group in the PTOE, since they both have 4 electrons in the outer shell.

An orbital is a region of space around the nucleus of an atom through which 1 or 2 electrons may randomly move.  These are different shapes, including: spherical (s), double-pear shaped (p), different complex shapes (d) and 'funny' (f) Each energy level has a different number of orbitals: 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f.

We can calculate the relative atomic mass from isotopic composition, using the formula:

relative atomic mass = Σ (isotropic mass x relative % abundance) ÷ 100

For example, oxygen-16 has an abundance of 99.76%, oxygen-17 has an abundance of 0.04% and oxygen-18 has an abundance of 0.2%.  So the relative atomic mass = (16 x 99.76 + 17 x 0.04 + 18 x 0.2)/100 = 16.0044.

An electron may absorb energy (e.g. by light) and go up one energy level (excited state).  It may then relax to its ground state and give out energy (as a photon of the electromagnetic spectrum, e.g. visible light).  We can calculate the energy released by the photon by ΔE = hf.  Using the wave equation v = fλ, we can find the wavelength, since v = speed of light.

Light is given out at different frequencies and can be seen as a thin line on an emission spectrum.  The pattern of lines are unique to each element (and compounds),  Scientist can use atomic emission spctroscopy to find out the composition of an unknown substance.

Periodicity

Inquiry question: Are there patterns in the properties of elements?

Periodicity is the regular occurrence of an event over time, e.g. high and low tides at the beach, or the regular characteristic of elements over the the period of the PTOE, e.g. melting points.  Properties that vary periodically with the atomic number are:

• the melting point of an element varies periodically (which relates to its state at room temperature);

• the atomic radius of an element varies periodically;

• the ionisation energy of an element varies periodically;

• the electronegativity of an element varies periodically.

The periods (rows) of the periodic table increase in atomic number from left-right, and each period corresponds to the number of electron shells of the elements in that period.

The groups (columns) of the periodic table have elements that share similar chemical properties, as they have the same number of valence electrons, e.g. Group 1 or 7 elements have only one valence electron, so are highlyreactive; Group 8 elements (noble gases) have a full shell already, so they do not react, as they are already stable.

Metallic reactivity with water becomes more vigorous as you go down Group 1 metals.  In Group 2, beryllium does not react with water and magnesium only does when heated.  Other elements in Group 2 react with water, but less vigorously than those metals in Group 1.  No other elements react with water at room temperature.

Bonding

Inquiry question: What binds atoms together in elements and compounds?

The valency of an element is a number that gives the combining ability of an element when forming a compound.  For example (and check this with the group on the PTOE):

• Na, K and Ag have valencies of 1. Also F, Cl and Br have valencies of 1.

• Mg, Ca and Zn have valencies of 2. Also, O, S and Se have valencies of 2.

• B, Al, Ga have valencies of 3. Also, N, P and Sb have valencies of 3.

• C, Si, Ge and Sn all have valencies of 4.

Allotropy is the occurrence of the same element in different physical forms.  They may have very similar chemical properties, but very different physical properties.  An example is the difference between soot and diamond, which are both made of carbon atoms only.  Crystalline allotropes of carbon include diamond, graphite and fullerines.  Amorphous allotropes of carbon include coal, charcoal and soot.

Ionic compounds are binary compounds.  They are named with the positive ion first (metal) and the negative ion as a new word with the ending "-ide", NaCl is sodium chloride.

We can use electron dot diagrams (Lewis Diagrams) to show how covalent bonds are formed.

When naming covalent binary compounds:

1. The first element name is the one with the lower electronegativity (usually it is further to the left on the PTOE).

2. Use the element name for the first element and add "-ide" to the second element.

3. Use a prefix on the second element to indicate the number of atoms, i.e. "mono-" for 1, "di-" for 2, "tri-" for 3, "tetra-" for 4, "penta-" for 5, "hexa-" for 6, etc.

Properties of ionic substances and covalent molecular substances are very different.

There are 2 steps to identify Covalent Network Solids:

1. Look at the structure and determine what type of interactions or bonds hold the atoms together.

2. If covalent bonds span the entire structure, it is a covalent network solid.

For example, a lump of calcium carbonate is not a covalent network solid even though it has carbon and oxygen atoms that are covalently bonded, since the  calcium is bonded with ionic bonds to the carbonate groups, and there are other intermolecular forces are holding the carbonate groups close together.  Another example is silicon dioxide, which consists of silicon atoms, each of which is covalently bonded to 4 oxygen atoms and , in turn, each of these oxygen atoms is covalently bonded with 2 silicon atoms.  In this solid, there is a continuous network of covalent bonds connecting the atoms across the whole structure forming a continuous, spanning network of covalent bonds, so this structure is a covalent network solid.