Atoms

Using indirect information we can infer the identity and the quality of the hidden parts of a model.

Dalton's atomic theory states:

• All elements are composed of atoms.


• atoms of each element are exactly alike but different than atoms of different elements.


• Compounds are formed by joining of atoms of two or more elements.

The negatively charged particles discovered by Thomson are called electrons.

Rutherford has concluded that the nuclues is surrounded by negatively charged electrons, but most of the atom is actually empty space.

Bohr proposed that electrons move around the nucleus in well-defined circular paths(orbit) called energy levels.

Atomic number (Z) is the number of protons in the nucleus of the atom.

Msss number (A) is the number of nucleons of the nuclues of the atom.

Isotops are atoms of the same element that thave different numbers of nuetrons in their nulei.

Atomic mass is expressed in atomic mass units, its symbol is U or Amu.

The mole is the unit (abbreviated mol), used to measure the amount of substance.

One mole atom of an element conatins exactly Na atoms of this element.

Source of the photo: Spectroscopy: A Key Part of the Astronomer's Toolbox (http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/spectroscopy.htm)

Types of bonds

A chemical bond is a force of that holds together two or more atoms or ions. There are two types of bonds, covalent and ionic bond.

Covalent bonding involves a sharing of electrons.

• Single covalent bonding involves sharing of a pair of electrons between two atoms.


• Double covalent bonding involves sharing of 2 pairs of electrons between two atoms.


• triple covalent bonding involves sharing 3 pairs of electrons between two atoms.

Ionic bond is the electrostatic force of attraction between oppositely charged ions.

Source of the picture: biology( http://www.dwm.ks.edu.tw/bio/activelearner/02/ch2summary.html)

Chemical stability

Stability of Inert Gases:

Studies of inert gases revealed that thier atoms contain eight electrons in their outer energy level(except helium, which contains 2 electrons).

Valence Electrons:

Valence electron are the electrons found on the outer energy level . The number of valence electrons of an element is related to its position in the periodic table. This number has an important role in determining the properties of the element.

Lewis Electron-dot Symbol:

A lewis electron-dot symbol consists of the symbol for the element surrounded by dots, one for each valence electron. The number of dots placed around the symbol of the element is equal to the number of valence electrons.

source of the picture: The Periodic Table(http://facstaff.gpc.edu/~pgore/PhysicalScience/Periodic-table.html)

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