Leaving Cert Chemistry

Topics: Chemical element, Periodic table, Atom Pages: 4 (748 words) Published: February 27, 2014
The Arrangement of Electrons in the Atom

An energy level is defined as the fixed energy value that an electron in an atom may have.

Bohr’s Theory

1.Electrons revolve around the nucleus in orbitals
2.The electrons in an orbit have a fixed energy
Note: Orbitals are also known as energy levels
3.As long as an electron is not in any one particular energy level, it neither gains or loses energy 4.When an atom absorbs energy it jumps to a higher energy level
Note: They do not remain there for long
5.Energy is lost when an electron falls from a high energy level to a lower energy level

Energy Sublevels

Heisenberg’s Uncertainty Principle states that it is impossible to measure both the velocity and position of an electron at the same time.

An orbital is a region in which there is a high probability of locating an electron.

px, py and pz orbitals are dumb-bell shaped

The Periodic Table

An element is a substance that cannot be made simpler by chemical means

Johann Dobereiner divided the elements into groups of three known as triads The three elements had similar chemical properties in which the atomic weight of the middle element was the average of the other two

John Newlands classified the elements into groups of eight known as octaves The eight elements were ordered by increasing atomic weight
The first and eighth element had similar properties

Dmitri Mendeleev arranged the elements in order of increasing atomic weight The elements showed a periodic repetition of chemical properties

How does the Newlands’ Table differ from Mendeleev’s table

Mendeleev left gaps to allow for the discovery of new elements Newlands did not leave gaps
In a few cases Mendeleev reversed the order of certain elements allowing them to fall into certain groups Newlands did not reverse the order of the elements

Henry Moseley used X-rays to determine the atomic number of an element

An atomic number is the number of protons in the...
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