Chemistry

Chemistry is the science that describes matter, its properties, the changes it undergoes and the energy changes.

Matter

When we burn a metallic bar of magnesium (Mg) in oxygen (O2), the magnesium combines with oxygen to form magnesium oxide (MgO), a white powder.

When we weight the magnesium oxide we find that it is heavier than the original piece of magnesium.

The increased mass of the product is due to the combination of oxygen with magnesium to form magnesium oxide.

Matter

The Law of Conservation of Matter states that “there is no observable change in the quantity of matter during a chemical reaction or during a physical change”

•Matter is anything that has mass and occupies space

Antoine Lavoisier's Law of Conservation of Mass (1774) led to many new findings in the 19th century

Energy

All the energy involved in any chemical reaction (or physical change), appears in some form after the reaction.

e.g. in some reaction chemical energy is usually converted into heat energy. In other reactions light energy, heat energy or electrical energy is converted into chemical energy.

Energy

The Law of Conservation of Energy states that “energy cannot be created or destroyed in a chemical reaction, it can only be converted form one form to another”

•Energy is the capacity to do work

Energy and matter

In nuclear reactions matter is transformed into energy. the relation between matter and energy was given by Albert Einstein’s famous equation

E = mC2

The Law of Conservation of Matter and Energy states that “the combined amount of matter and energy in the universe is constant”

Elements

All the different substances that exist on our Planet are formed from the combination of 118 known elements.

Each elements is unique with its own characteristics and is conveniently represented by a symbol. For example H stands for hydrogen, C stands for carbon, Na stands for sodium.

In addition each elements has its own atomic mass (Ma) and atomic number (z)

An element is a substance that cannot be decomposed into simpler substances by chemical changes or physical processes

Elements

• When the French chemist Antoine Lavoisier published his famous list of elements in 1789, there were only 33 elements.

• By 1930, the diligent labors of thousands of chemists had increased the tally of naturally occurring chemical elements to 90.

• More recently, physicists in high-energy laboratories have been able to create about 20 highly radioactive, unstable elements that do not exist naturally on Earth, although they are probably produced in the hot cores of some stars.

• The number of chemical elements has now reached 118, and the list is growing. Only about 40 are relevant to basic chemistry.

Elements - Atomic masses and amu

Each element has a characteristic mass relative to every other element. As an example C and H have atomic masses of 12 and 1 respectively.

Experiments have resulted in the establishment of a scale of relative atomic weights (Ma) based on the atomic mass unit (amu), which is defined as exactly 1/12 of the mass of an atom of carbon (12C)

Elements - the periodic table

In 1869, the Russian chemist Dmitri Mendeleyev arranged all known chemical elements in order of their atomic masses and found that similar physical and chemical properties recurred every 8 elements. Each period (row) and group (column) contain a group of elements with similar chemical behavior.

Modern periodic tables of elements are based on the atomic number

Elements - the periodic table

How to read the Periodic Table of Elements

Elements - the periodic table

Groups of elements in nature have similar chemical properties.

Helium, neon, argon, krypton, and xenon are all colorless gases, only two of which combine with other elements under very special conditions; their lack of reactivity leads to the name inert gases (or noble gases) for this group of similar elements.

By contrast, fluorine and chlorine are corrosive green gases that form salts when they combine with metals, hence the name halogens (salt formers) for fluorine, chlorine, bromine, and iodine.

Lithium, sodium, and potassium have such low densities that they float on water and are so highly reactive that they spontaneously burn. These light metals form strong alkalis and appropriately are called the alkali metals.

Elements - the periodic table

The elements to the upper right, above a diagonal line from aluminum (13) to polonium (84), are nonmetals, about half of which exist as gases under normal laboratory conditions.

All the elements in the middle and left of the table are metals, except gaseous hydrogen (1). Most of the metals are shiny, deformable solids and have high conductivities for heat and electricity.

Metalloids are element that have properties in between of those of metals and nonmetals

Elements - the periodic table