Elements, Compounds and Mixtures

Matter can be classified into three groups, elements, compounds and mixtures. 

What is an element?

An element is a pure substance that cannot be split up into two or more simpler substances.

Elements
Shiny appearance Solids at room temperature and pressure (except mercury) *Malleable  *Sonorous *Ductile High melting points and boiling point (except potassium, sodium, mercury) Good conductors of heat Good conductors of electricity	Dull appearance  Either gases, volatile liquids or solids with low melting points at room temperature Solids with low melting points at room temperature (except carbon) Brittle  Low melting points and boiling points ( except  carbon and silicon) Poor conductors of heat (except carbon in the form of diamond and graphite) Poor conductors of electricity (except carbon in the form of graphite)

Atoms and molecules

Elements are made up of tiny particles called atoms. The atoms of one element are not the same as the atoms of another element. Atoms are the smallest particles an element that have the chemical properties of that element.

Monatomic elements

Atoms are not joined together chemically. Helium, neon, argon, krypton, xenon and radon are the only elements that exist as individual atoms.

Molecule

A molecule is formed when two or more atoms chemically combined to make a group. Molecules formed by the combination of two atoms are called diatomic molecules.

Molecules consisting of three atoms are called triatomic molecules. Molecules that are formed by the combination of four or more atoms are called polyatomic molecules.

What is a compound?

 A compound is a pure substance that contains two or more elements chemically combined. It has a very different properties from the elements that from it. A compound is made up of different elements chemically combined.  It is made up of different elements chemically combined in a fixed ration. The smallest particle of a compound that can exists independently is therefore the molecule. A compound can be represented by a chemical formula.

Decomposition of compounds?
Heat can be used to break down compounds into elements or simpler compounds. Such a chemical reaction is called thermal decomposition. Compounds can also be broken down into elements or simpler substances by electricity.

What is a mixture?
Mixtures are formed when two substances are added together without chemical bonds being formed. Mixtures can be made up of elements and compounds. The components of a mixture are not fixed. They can be present in any ratio. 

What does a chemical formula states?

• the types of atoms (i.e elements) present in the compound
• the ratio of the different atoms present in the compound

Difference between Compounds and Elements

compounds	elements
made up of 2 or more types of atom `	made up of only one type of atom
can be broken down into 2 or more simpler substances	cannot be broken down into 2 or more simpler substances

Difference between mixtures and compounds

	Mixtures	Compounds
Separation	The components of a mixture can be separated by physical methods. (e.g. filtration, distillation or chromatography)	The elements in a compound can only be separated  by chemical reactions or by using electricity.
Properties	The chemical properties of a mixture are the same as hose of its components	The physical and chemical properties of a compound are different from those of the elements in the compound.
Energy Change	No chemical reaction takes place when a mixture is formed- usually there is little or no energy charge.	A chemical reaction takes place when a compound is formed- usually there is no energy change (e.g. heat is given out by the reactants.
Composition	The components of a mixture can be mixed in any proportion	The elements in a compound are always combined in fixed proportion.
Boiling points and melting points	Mixtures do not have a fixed boiling point and melting point.	Compound do have a fixed boiling point and melting point.

Periodic table

The elements are arranged in increasing order of proton number or atomic number as you go left from right across the table. The horizontal rows are called periods and the vertical rows are called groups. There is also a progression of metallic atoms to non-metallic atoms across each period from left to right. Group I metals are alkali metals because they react with water to form alkalis. Group II metals are called alkali earth metals because they react with water to form alkalis. They are naturally occurring as stable compounds within the earth's crust. Group VII elements are called halogens. This ter derived from Greeks halos( salt) and suffix-genes(forming), and of course means "salt former".Group VIII elements are noble gases or inert gases . The name is an allusion to the similarity unreactive Noble metals, so called due to their preciousness, resistance to corrosion and long association with the aristocracy. The block of elements (also called d-block elements) between group II and III are called transition elements or metals. Elements 58-71, elements in the Lanthanoid series are known as Lathanides or rare earth elements. These elements are found on earth in only very small amounts.  Elements 90-103 elements in the Actinoid series are known as radioactive elements. They include most of the well-known elements that are found/use in nuclear reactions.

Atomic structure

Atoms are made up of three different particles- protons, neutrons and electrons. These are called subatomic particles. Protons and neutrons are tightly packed together in the centre of an atom, forming the nucleus of the atom. Electrons move rapidly around the nucleus(made of protons and neutrons).

Electron	Proton	Neutron
carries on negative charge (-1)	carries one positive charge (+1)	carries no electric charge
has a relative mass of 1/1840	has a relative mass of 1	has a relative mass of 1
is represented by the letter, e	is represented by the letter, p	is represented by the letter

All atoms are electrically neutral. An atom has the same number of protons(positively charged) and electrons(negatively charged). The number of protons in an atom is called the proton number or atomic number which is represented by the symbol Z. Each element has a unique proton number, The total number of protons and neutrons in an atom is called the nucleon number. It is represented by A.


Isotopes

Isotopes are atoms of the same element with the same number of protons but different number of neutrons. Isotopes of the same element have the same number of electrons. Isotopes have the same chemical properties but slightly different physical properties. The chemical properties of isotopes are similar because chemical reaction involve only the electrons. The physical properties of isotopes are different because of the difference in masses of isotopes. Isotopes that emit high-energy radiation are called radioisotopes. They are classified as radioactive substances. Radiation emitted by radioisotopes danger.

Arrangement of electrons

The way in which electrons are arranged around the nucleus of an atom is very important because the electron arrangement determines the chemical properties of the atom.

First shell	second, third and subsequent shells
is the closest to the nucleus	can hold up to eight electrons each
can hold a maximum of 2 electrons	Are filled in order;usually the second shell is fully filled before the third shell and so on
is always filled first

The way the electrons are arranged in an atom is called the atom’s electronic structure or electronic configuration. This shell that is the furthest from the nucleus is called the outer shell or the valence shell. The electrons in this shell are called valence electrons. A diagram of an atom’s outer electronic structure shows only the valence electrons in the valence shell.

Examples of electronic configuration: Hydrogen(1), Lithium(2,1) and sodium(2,8,1) 

History of Atoms

	450 BC
	Democritus,Greece - stated that all matter is made up of atoms. He also stated that atoms are eternal and invisible and so small that they can’t be divided, and they entirely fill up the space they’re in.			400 BC
								Aristotle, Greece - provided the method of gathering scientific facts, which proved as the basis for all scientific work.
					350 BC
									Model  II     Planetary System
				skip to	1700's
					1750 AD
	Lavoisier (1777),France - provided the formula for the conservation of matter in chemical reactions, and also distinguished between an element and a compound.
									Model III The Plum-Pudding
								Couloumb (1780's), France -  formulated the Coulomb's law, which states that that the force between two electrical charges is proportional to the product of the charges and inversely proportional to the square of the distance between them, one of the main forces involved in atomic reactions.
				1800 AD
								John Dalton (1803), England - formed the atomic theory, which states that all matter is composed of tiny, indestructible particles called atoms that are all alike and have the same atomic weight.
					1850 AD
	Crookes (1870),England - created the Crookes’ tube and demonstrated that cathode rays travel in straight lines and produce phosphorescence and heat when they strike certain materials.
									Model IV            Rutherford's Nucleus Atom
	W.K. Roentgen (1895), Germany -discovered x-rays while experimenting with cathode-ray tubes.
	Becquerel (1896),France - discovered radioactivity when he investigated uranium and other radioactive substances.							The Curies (1898), France -discovered radium and polonium when they started to investigate radioactive substances
								J.J Thomson (1898), England -discovered the electron and developed the plum-pudding model of the atom.
				1900 AD
	Max Planck (1900), Germany -originated the quantum theory							Albert Einstein (1905), Germany -postulated that light was made up of different particles that, in addition to wavelike behavior, demonstrate certain properties unique to particles.  He also brought forth the theory of relativity.
									The Electron Cloud Atom
	Robert Millikan (1908), USA -found out the electric charge of the electron
								Ernest Rutherford (1909), England -used the results of his gold-foil experiment to state that all the mass of an atom were in a small positively-charged ball at the center of the atom.
	Neils Bohr (1913), Denmark -stated that the electrons moved around the nucleus in successively large orbits. He also presented the Bohr atomic model which stated that atoms absorb or emit radiation only when the electrons abruptly jump between allowed, or stationary, states.
	Geiger (1925),Germany -introduced the first detector of alpha particles and other radiations.
								Erwin Shroedinger (1926), Austria -introduced the Shroedinger Equation, a wave equation that describes the  form of the probability waves that govern the motion of small particles and how these waves are altered by external influences.
	Chadwick (1931),England -discovered the neutrally-charged neutron.
									Detail of an Electron Cloud
	Otto Hahn (1938), Germany  - discovered nuclear fission, in which the nucleus of an atom breaks up into two separate nuclei, while experimenting with uranium.							Lise Meitner (1938), Vienna - worked with Otto Hahn to discover uranium fission.
					1950 AD
	Glen T. Seaborg (1951), USA -isolated and identified elements heavier than uranium, and in the process, added elements number 94 - 102, and 106.
								Murray Gell-Mann and George Zweig (1964), USA - brought forth the idea of "quarks", little bits of matter which when used kind of like building blocks, serve to explain some complex chemical substances.