Elements, Compounds and Mixture

Elements

- What is an element?

ü  An element is a substance made up of entirely from one type of atom.

- Every element is given a name and a chemical symbol

ü  First letter is always capitalised

ü  Any other letter that follows is in letter case

Element	Chemical Symbol
Hydrogen	H
Carbon	C
Calcium	Ca
Cobalt	Co
Iron	Fe
Gold	Au

- Naming Elements

ü  Name and chemical symbols are standardized internationally by IUPAC (International Union of Pure and Applied Chemistry)

ü  Some elements are named after countries and famous people

- Symbols of Elements

ü  From the name of elements or from the names of elements in Latin

- Periodic Table

ü  Horizontal row of elements are called Periods.

ü  Vertical columns of elements are called Groups.

- Classification of Elements

ü  There are 2 major groups of elements – metals and non-metals.

ü  Iron is a metal. Oxygen is a non-metal.

ü  Metals and non-metals are groped separately on the Periodic Table.

ü  There are some elements called metalloids which behave like both metals and non-metals.

ü  2 ways to classify elements

ü  First is by the number of sub atomic particles which determine their position in the Periodic Table

ü  Second is by their properties such as melting and boiling points etc.

ü  The number of sub atomic particles is related to their properties.

Differences between metals and non-metals

Metals	Non-metals
Shiny (lustrous)	Dull
Solid at room temperature (except Hg)	Usually gases or liquids at room temperature
Malleable, sonorous, ductile	Brittle (if solid)
Usually high melting and boiling points	Usually low melting and boiling points
Good conductors of heat	Usually poor conductors of heat
Good conductors of electricity	Usually poor conductors of electricity

Compounds

- What is a compound?

ü  A compound is a pure substance that contains two or more elements chemically combined.

Compound	Elements present
Common salt (sodium chloride)	Sodium, chlorine
Marble (calcium carbonate)	Calcium, carbon, oxygen
Copper (II) sulphate	Copper, sulphur, oxygen
Hydrogen chloride	Chlorine, hydrogen

- Naming Compounds

ü  A compound made up of 2 elements has a name that ends in –ide.

ü  A compound that contains hydroxide ions, OH- (a negatively charged ion made up of oxygen and hydrogen) is named a hydroxide.

ü  A compound that contains a negatively charged polyatomic ion containing oxygen usually has a name ending in –ate.

- Fixed composition of compounds

ü  A compound is made up of different elements chemically combined in a fixed ratio.

ü  For example, water (H₂O) is a compound made only by joining together 2 atoms of hydrogen to one atom of oxygen.

ü  That is, the ratio of hydrogen atoms to oxygen atoms in water is always 2 : 1.

Mixtures

- What are mixtures?

ü  Mixtures are formed when 2 or more substances are added together without chemical bonds being formed.

ü  Examples of mixtures include muddy water and air.

ü  Air is made up of gases such as nitrogen and oxygen mixed together.

- Different types of mixtures

ü  The ratios of the components of mixture are not fixed. They can be present in any ratio.

ü  A mixture can exist in various ways :

- 2 elements

- A mixture of 2 compounds

- A mixture of 1 element and 1 compound

- Comparison of Mixtures with Compounds

ü  Separation

Mixture	Compound
The components of a mixture can be separated by physical methods, eg filtration, distillation or chromatography.	The elements in a compound can only be separated by chemical reactions or by using electricity.

ü  Properties

Mixture	Compound
The chemical properties of a mixture are the same as those of its components.	The physical and chemical properties of a compound are different from those of the elements in the compound.

ü  Energy Changes

Mixture	Compound
No chemical reaction takes place when a mixture is formed – usually there is little or no energy change.	A chemical reaction takes place when a compound is formed – usually there is an energy change, eg, the reactants get hot.

ü  Composition

Mixture	Compound
The components of a mixture can be mixed in any proportion.	The elements in a compound are always combined in a fixed proportion (by mass).

Changes

- Physical Change

ü  No new substance formed

ü  Usually easily reversible

ü  May or may not involve heating

- Chemical Change

ü  New chemical substance(s) is/are formed

ü  Usually irreversible (some chemical changes can be reversed under special conditions)

ü  Heat energy may be given off or absorbed

ü  Light energy may be given off or absorbed

- Signs of a chemical change

ü  A change in colour

ü  Production of a gas

ü  Formation of a precipitate from mixing solutions

ü  Change of temperatures

ü  ALL THE ABOVE MUST FOLLOW THE FORMATION OF A NEW CHEMICAL SUBSTANCE

- Chemical reactions

ü  A process in which new chemical substance is formed

ü  New chemical substances formed due to rearrangement of atoms

ü  No atoms are created or destroyed, according to the Law of Constant Mass

Reactant à Products

Types of chemical reactions

- Combination (Synthesis)

ü  Two or more substances combine to form a new substance (magnesium + oxygen à magnesium oxide)

- Decomposition

ü  A substance breaks down into two or more simpler substances (calcium carbonate → calcium oxide + carbon dioxide)

3. Combustion of burning

ü  Combination of a substance with oxygen.

ü  Heat and light energy are given off (hydrocarbon + oxygen → carbon dioxide + water)

4. Displacement Reaction

ü  A chemical reaction with an atom / a group of atoms get replaced by another atom / group of atoms (hydrogen bromide + chlorine → hydrogen chloride + bromine)

Atoms

History of the model of the atom

- John Dalton

• Suggested that atoms were tiny, hard balls
• Each chemical element has its own atoms that differed from others in mass
• Atoms were the fundamental building blocks of nature and could not be split
• In chemical reaction, the atoms would rearrange themselves and combine with other atoms in new ways

- J.J Thomson

• Discovered electron
• Called negatively charged particles electrons
• Suggested that electrons come from inside atoms
• Proposed that tiny negatively charge electrons must be embedded in a cloud of positive charge

- Ernest Rutherford

• Proposed that Thomson's model was not right : he said that the positive charge must be concentrated in a tiny volume at the centre of the atom, otherwise the heavy alpha particles fired at the foil could never be repelled back towards their source based on his experiment.

- Niels Bohr

• Suggested that the electrons must be orbiting the nucleus of the atom in certain fixed energy levels (explaining why light was given out when atoms were heated always had specific amounts of energy)

Summary: 

John Dalton introduced a new form of the ancient Greek idea of atoms at the beginning of the nineteen century.

In 1897, J.J. Thomson discovered the electron and suggested the 'plum pudding' model of the atom.

In 1911, Rutherford suggested that electrons orbit the atomic nucleus like planets round the Sun.

In 1914, Bohr modified Rutherford's model by introducing the idea of energy levels.

We can think of the atom as a positively charged nucleus with negatively charged electrons orbiting the nucleus in energy levels (or shells).

Structure of the atom as nucleus with electrons surrounding it (Bohr's Model)
- Atoms are made out of 3 different particles

• Protons
• Electrons
• Neutrons

Isotopes

- Isotopes are atoms of the same element with the same number of protons but different number of neutrons.

- Isotopes have the same chemical properties but slightly different physical properties

Eg.

They are similar except that Hydrogen 1 has no neutrons, hydrogen-2 has one neutron and hydrogen-3 has two neutrons. These hydrogens atoms are known as isotopes.

Uses of Isotopes

- Isotopes that emit high-energy radiation are called radioisotopes. They are classified as radioactive substances. Radiation emitted by radioisotopes is dangerous because it can damage living cells and cause cancer. However, radioisotopes can have important applications and can be safely used if they are handled properly.

Electronic Configuration

The electrons in an atom move around the nucleus in regions known as electron shells. Each electron shell can only hold a certain number of electrons.

Above is a magnesium atom. It has 2 electrons in the first shell, eight electrons in the second shell and 2 electrons in the third shell. Thus, its electronic structure or electronic configuration can also be represented as (2,8,2).