USING ENGLISH TO CLASSIFY : the composition of matter in chemistry

THE ELEMENTS

Before we know about elements, before that we must knowing about atom,molecule and compound.
 

What is anAtom ?

All substances are made up of matter and the fundamental unit of matter is the atom. The atom constitutes the smallest particle of an element. The atom is made of a central nucleus containing protons (positively-charged) and neutrons (with no charge). The electrons (negatively-charged with negligible mass) revolve around the nucleus in different imaginary paths called orbits or shells.

What is a Molecule ?

A molecule is formed when atoms of the same or different elements combine. A molecule is the smallest particle of a substance that can normally exist independently.
Examples:

-Two atoms of oxygen combine to form a molecule of oxygen [O₂].

- One atom of carbon combines with two atoms of oxygen to form a molecule of carbon dioxide [CO₂].

What is a Compound ?

A compound is formed when atoms or molecules of different elements combine. In a compound, elements are chemically combined in a fixed proportion.
Examples:

• Hydrogen and oxygen are combined in a fixed proportion of 2:1 to form the compound water [H₂O].
• Carbon and oxygen are combined in a fixed proportion of 1:2 to form the compound carbon dioxide [CO₂]. 

  so what is the meaning of elements?

ELEMENT DEFINITION

A chemical element is a substance that cannot be broken down by chemical means. Although elements aren't changed by chemical reactions, new elements may be formed by nuclear reactions.

Elements are defined by the number of protons they possess. Atoms of an element all have the same number of protons, but they can have different numbers of electrons and neutrons. Changing the ratio of electrons to protons creates ions, while changing the number of neutrons form isotopes.

There are 115 known elements, although the periodic table has space for 118 of them. Elements 113, 115, and 118 have been claimed, but require verification to earn a place on the periodic table. Research is also underway to make element 120. When element 120 is made and verified, the periodic table will need to be changed to accommodate it!

EXAMPLES OF ELEMENTS

Any of the types of atoms listed on the periodic table is an example of an element, including:

• copper
• cesium
• iron
• neon
• krypton
• proton - technically a lone proton qualifies as an example of the element hydrogen

EXAMPLES OF SUBSTANCES THAT ARE NOT ELEMENTS

If more than one type of atom is present, a substance is not an element. Compounds and alloys are not elements. Similarly, groups of electrons and neutrons are not elements. A particle must contain protons to be an example of an element. Non-elements include:

• water (composed of hydrogen and oxygen atoms)

• steel
• electrons
• brass (composed of multiple types of metal atoms)

General Chemistry/Chemistries of Various Elements/

Alkali Metals

The alkali metals are the elements in Group 1 (1A). They are lithium, sodium, potassium, rubidium, cesium, and francium.

These elements are best marked by their reactivity. Physically they are soft, shiny (when freshly prepared) solids with low melting points; they conduct electricity well. They all have one valence electron that they lose easily to almost any electronegative substance. Alkali metals are never found in their elemental form outside the lab because they are too reactive. Even the smallest amount of oxygen or water would react with the metal. For this reason, alkali metals have no structural use. They must be kept under inert liquids such as kerosene or in inert gases (nitrogen suffices for any of these elements other than lithium).

Alkali metals have many uses in both biological life and industry. Francium, however, is radioactive and decays rapidly to other elements, so it has no commercial use and its chemical properties are vaguely understood.

All oxidize easily to the +1 oxidation state.

Reactions

Alkali metals react violently with water, halogens, and acids. The reactions release surprising amounts of heat and light. In a chemical equation, alkali metals are represented with an M. Here are some example equations:

${\displaystyle 4{\hbox{M}}_{(s)}+{\hbox{O}}_{2(g)}\to 2{\hbox{M}}_{2}{\hbox{O}}_{(s)}}$	Alkali metals react with oxygen to form oxides, which have a duller appearance and lower reactivity. The oxides are much less reactive than the pure metals.
${\displaystyle {\hbox{M}}_{2}{\hbox{O}}_{(s)}+{\hbox{H}}_{2}{\hbox{O}}\to 2{\hbox{MOH}}_{(aq)}}$	However, the oxides do react vigorously with water to form a hydroxide. The resulting hydroxides of these elements dissociate completely in water to form some of the strongest bases known. "Lye" is an industrial-strength base, which is sodium hydroxide (NaOH).
${\displaystyle {\hbox{NaOH}}_{(aq)}+{\hbox{HCl}}_{(aq)}\to {\hbox{NaCl}}_{(aq)}+{\hbox{H}}_{2}{\hbox{O}}_{(l)}}$	These hydroxides react readily with acids to form water and their corresponding alkali salt in a neutralizationreaction.
${\displaystyle 2{\hbox{M}}_{(s)}+2{\hbox{H}}_{2}{\hbox{O}}\to 2{\hbox{MOH}}_{(aq)}+{\hbox{H}}_{2(g)}}$	The pure alkali metal can also react directly with water. In this case, the metal is a basic anhydride. Gaseous hydrogen is released, which is flammable.
${\displaystyle 2{\hbox{M}}_{(s)}+{\hbox{Cl}}_{2(g)}\to 2{\hbox{MCl}}_{(s)}}$	Exposing an alkali metal to a halogen will cause an extremely exothermic reaction that results in an ionic salt.

Almost every salt of an alkali metal is highly soluble in water. They form conducting solutions, proving their ionic nature.

Group 2 Elements: The Alkaline Earth Metals

The Group 2 alkaline earth metals include Beryllium, Magnesium, Calcium, Barium, Strontium and Radium and are soft, silver metals that are less metallic in character than theGroup 1 Alkali Metals. Although many characteristics are common throughout the group, the heavier metals such as Ca, Sr, Ba, and Ra are almost as reactive as the Group 1 Alkali Metals. All the elements in Group 2 have two electrons in their valence shells, giving them an oxidation state of +2.

HALOGEN

The halogens are five non-metallic elements found in group 17 of the periodic table. The term "halogen" means "salt-former" and compounds containing halogens are called "salts". All halogens have 7 electrons in their outer shells, giving them an oxidation number of -1. The halogens exist, at room temperature, in all three states of matter:

• Solid- Iodine, Astatine
• Liquid- Bromine
• Gas- Fluorine, Chlorine

The Halogens are:

• Fluorine
• Chlorine
• Bromine
• Iodine
• Astatine