The Chemical Earth

Why study chemistry?

The Earth includes the biosphere, lithosphere,

hydrosphere and atmosphere. Each of these is a

mixture of thousands of different substances, many

of which are useful to us if we: have an understanding of the properties of the

elements and compounds that make up the Earth’s materials

develop efficient processes for separating useful materials

Classification of matter

Pure substances have a fixed composition and fixed properties. They cannot be decomposed by simple physical separation techniques.

Mixtures have variable composition and variable properties. They can be separated into their components by various physical separation techniques.

Elements

Pure substances can be further classified into

elements and compounds. Elements are the simplest pure substances

consisting of only one type of atom. They cannot be broken down (or decomposed).

Compounds

Compounds are also pure substances. They are composed of two or more elements that are chemically bonded together. They are composed of a fixed number of atoms of each component element. They can be decomposed into their component elements or into simpler compounds.

Mixtures

The particles of each component in homogeneous mixtures are distributed uniformly (eg: sugar dissolved in water).

The particles in heterogeneous mixtures are not distributed uniformly (eg: concrete)

Chemical analysis

Chemical analysis is the process of finding out what

is present in a particular chemical sample.

Chemical analysis can be: Qualitative – determining what substances are

present Quantitative – determining how much of each

substance is present in a sample.

Gravimetric analysis

Gravimetric analysis involves separating the components of a material and accurately determining their mass. The percentage composition of the material can then be calculated. Gravimetric analysis can be used to determine the:

composition of a mixture using physical separation techniques

% composition of a compound using chemical and physical separation techniques.

Elements

Elements consist of atoms of the same type. Many

elements exist in nature as molecules. Monatomic molecules: consist of only one atom

(eg: noble gases) Diatomic molecules: a molecule in which two

atoms are bonded together. Polyatomic molecules: a molecule of more than

two atoms bonded together

Reactivity of elements

Elements vary in their tendency to react. Elements

that react readily (eg: calcium, sodium) are usually

found combined with other elements as

compounds. On the other hand elements such as

gold which have low reactivity are often found in

their pure form.

Metals

Relatively high densities Good conductors of heat and electricity Malleable and ductile Shiny surface when freshly cut of cleaned Relatively high melting points

Non-metals

State and form is variable (oxygen is a gas, bromine a liquid, sulfur a solid)

Usually not lustrous Poor conductors heat and electricity Not malleable or ductile Variable melting points

Atomic theory

An atom is made up of three fundamental particles: Protons: positively charged particle Neutrons: neutral Electrons: negatively charged particle

In an uncharged atom:

number of protons = number of electrons

The nucleus

Contains protons and neutrons. Has a positive charge equal to the number of

protons Contains about 99.9% of the mass of the atom Extremely dense Electrons move in space outside the nucleus

Atomic number and atomic mass

Atomic number (Z): number of protons in the nucleus and is fixed for any one element. (NB: In an uncharged atom this will also be the number of electrons)

Atomic mass (A): sum of the number of protons and neutrons in the nucleus

Atomic number and atomic mass

Atomic mass = atomic number + number of neutrons

OR

A = Z + number of neutrons

Therefore:

A - Z = number of neutrons

Atomic number and atomic mass

AZ X

X is the element symbol

A is the atomic mass

Z is the atomic number

Electrons

Electrons are believed to exist in energy levels or shells. The maximum number of electrons in each shell is determined by the formula 2n2 (where n = shell number 1, 2, 3, 4, etc). So:

1st shell: can hold a maximum of 2 electrons 2nd shell: can hold a maximum of 8 electrons 3rd shell: can hold a maximum of 18 electrons 4th shell: can hold a maximum of 32 electrons

Electron configuration

The pattern of electrons in each shell is called the

electron configuration. When determining the

electron configuration of an atom the general rule is:

Starting from the innermost shell, each electron

shell or energy level must be filled before moving

to the next energy level or shell.

NB: potassium and calcium are exceptions

Sodium

This means an atom of Sodium has 11 protons,

11electrons and and 12 neutrons.

Therefore electrons are arranged as: First shell = 2 electrons Second Shell = 8 electrons Third shell = 1 electron

1123Na

Valence energy level

The outermost shell of an atom is referred to as the valence energy level. Similarly, the electrons that occupy the outermost shell are called valence electrons.

In the periodic table elements with the same number of valence electrons occur in the same column or group.

Octet rule

In general, atoms are

most stable when they

have 8 electrons in their

outer-most shell. This

accounts for the lack of

reactivity of the noble

gases.

Ions

Elements can achieve stable electron configurations

by losing or gaining electrons. In doing so they form

ions.

Positively charged ions (cations) are formed when one or more electrons are removed from an atom.

Negatively charged ions (anions) are formed when an atom gains one or more electrons

Electron dot diagrams

Electron dot diagrams are a way of showing the

arrangement of valence electrons in atoms. For

example:1 valence electron

2 valence electrons

7 valence electrons

6 valence electrons

Electron dot diagrams and ionic bonds

Electron dot diagrams can also be used to show

the formation of ions.

Question: why will these two ions be attracted to each other and form an ionic bond?

Polyatomic ions

Polyatomic ions are groups of atoms bonded to

one another that have a net positive or negative

charge. The carbonate ion is an example of a

polyatomic ion.

Polyatomic ions often

have the suffix “ate” or

“ite”.

3CO2-

The subscript indicates that there are 3 oxygen atoms

The superscript 2- indicates that there are two more electrons than the total number of protons possessed by the four atoms

Covalent bonds

Covalent bonds are formed when adjacent atoms

share electrons. For example, a chlorine atom has

the electron configuration (2, 8, 7). Two chlorine

atoms can combine to form a chlorine molecule Cl2

by sharing a pair of electrons (each atom

contributes one electron).

http://www.gcsescience.com/Chlorine-Formation.gif

Electron dot diagrams and covalent bonds

Molecules are a group of two or more atoms held together by covalent bonds.

Bonds in which two electrons are shared are called single covalent bonds and can be represented by a line drawn between the atoms H–H.

Electron dot diagrams and covalent bonds

The number of covalent bonds formed by an atom depends on the number of valence electrons.

In forming the molecule O2, each oxygen accepts a share of two electrons from the other atom. Hence four electrons are shared by the two oxygen atoms. This is called a double covalent bond: O=O

Ionic or covalent?

If one member of a pair of atoms wants to gain electrons while the other wants to lose electrons then the pair will form an ionic bond. If both members want to gain electrons then they will form covalent bonds.Question: Will Group I elements tend to form ionic bonds or

covalent bonds? What about Group 6 elements?

ValencyThe valency of an element is a measure of its

“combining power” (the number of bonds it can form). When an element forms ionic compounds the

valency is the charge the atom carries.

Ex: Na+ = +1 valency When an element forms covalent compounds

valency is the number of covalent bonds the atom forms.

Ex: water is H-O-H, the valency of O = 2 and H = 1

Formula ionic compounds

Ionic compounds are electrically neutral – therefore the number of negative charges must equal the number of positive charges.

Ex: NaCl – the numbers of sodium and chloride ions is equal.

If the charges on the ions are not equal then there will be more ions with the smaller charge.

Ex: the compound formed between Ca2+ and Cl-

is CaCl2 (there are 2 Cl ions for each Ca ion)

Naming ionic compounds

The following naming rules apply to ionic compounds:

1. The cation (positive ion) is named first2. The anion (negative ion) is named second3. The suffix ‘ide’ is added to the non-metal in

simple binary compounds (compounds made up of only two elements)

Ex: NaCl = sodium chloride

Formula covalent molecular compounds

In covalent compounds the formula represents the

number of atoms of each element in one molecule of

the compound. This is also called the molecular

formula.

Ex: H2O – two atoms of hydrogen and one atom of oxygen

Naming molecular compounds

1. The name of the element closer to the bottom or left-hand side of the periodic table is written first.

2. The the suffix ‘-ide’ is added to the end of the name of the second element.

3. The number of atoms of each element is indicated by the prefixes ‘mono-’, ‘di-’, ‘tri-’, ‘tetra-’, ‘penta-’ or hexa-’, which stand for 1, 2, 3, 4, 5 and 6 respectively.

NB: prefix ‘mono-’ is not used for the first-named element.

Chemical equations

In a chemical reaction the arrangement of atoms

is changed to produce new substances but atoms

are neither destroyed or created eg: mass is

conserved.

Chemical reactions are represented by chemical

equations.

Writing chemical equations

Reactants are on the left and products are on the right eg:

magnesium + oxygen magnesium oxide

Coefficients written IN FRONT of formulas show the number of particles of that substance eg:

2Mg = 2 atoms of magnesium Physical state of reactants and products is shown

by (g), (l), (s) – gas, liquid, solid

Writing chemical equations

The number of atoms of each element must be the same on the left and right hand sides of the equation eg:

2Mg + O2 2MgO

2 x Mg 2 x Mg

2 x O 2 x O

The sum of the electrical charges on the left must equal that on the right

Balancing equations

Balancing a chemical equation is done by

changing the coefficients in front of the formulas.

1. Write the word equation

2. Write the formula for all elements and compounds present

3. Alter the coefficients to balance the number of each type of atom on both sides of the equation

4. Write in the physical states

Decomposition reactions

Decomposition is a chemical reaction in which a

compound is broken down into their constituent

elements or simpler compounds. This is achieved

by adding energy as: Heat (thermal decomposition) Light Electricity (electrolysis)

Synthesis reactions

Synthesis is the process of forming a compound

from its component elements or other compounds in

a laboratory. It leads to the formation of a more

complex substance. For example, ammonia can be

synthesised directly by combining nitrogen and

hydrogen gases at high temperatures and

pressures.

Bond energy

A chemical change generally involves the

absorption or release of greater quantities of

energy than a physical change.

Reason: A chemical change involves the breaking of chemical bonds

Covalent & ionic bonding

In covalent bonds electrons are shared between atoms. The energy required to separate atoms joined by a covalent bond is referred to as the bond energy.

Ionic bonds are formed by the electrostatic

attraction between oppositely charge ions. The

energy required to break ionic bonds is referred to as the lattice energy.

Bonding and Physical Properties

All substances are made up of atoms, molecules or ions. It is the organisation of these particles that determines the physical properties of a substance. Solids can be classified into four groups on the basis of their physical properties:

Ionic compounds Covalent molecular compounds Covalent network compounds Metallic substances