The Periodic Table 

The elements 

Greek philosophers had the idea that the universe was made up of a few basic substances. Earth, air, water and fire

Robert Boyle gave the first accurate definition of an element.

An Element is a substance that cannot be split into simplier substances by chemical means.

Humphrey Davy discovers elements by passing electricity through compounds (Potassium, sodium, calcium, bari1um, strontium, magnesium)

 The Discovery that electric current can be used to obtain elements from their compounds was a turning point in the discovery of elements.

Looking for a pattern: History of the Periodic Table

Johann Dobereiner noticed that the properties of bromine, chlorine and iodine were similar.

Triad-is a group of 3 elements with similar chemical properties in which the atomic weight (relative atomic mass) of the middle element is approx. equal to the average of the other two.

John Newlands noticed that the properties of the elements seemed to repeat with every 8th element. He ordered them in order of increasing atomic weight.

Newland's Octave are arrangements of elements in order of increasing atomic weight in which the first and the eighth element, counting from a particular element, have similar properties.

Flaws with Newlands Octave?

Newlands didn't account that some elements had not yet been discovered.

Newlands of Octaves would not work for the modern periodic table as noble gases have been discovered.

Dmitri Mendeleev arranged the elements in order of increasing atomic weights (relative atomic mass) and placed elements with similar properties under each other in groups.                                                                                                       

Mendeleev's Periodic Table when element are arranged in order of increasing atomic weight (relative atomic mass), the properties of the elements recur periodically.

• All elements with Similar properties in the same vertical column..
• Mendeleev left gaps in his table. 
• Mendeleev predicted the properties of unknown elements
• Mendeleev reversed the order of some elements e.g Iodine and Tellurium 

Henry Moseley discovered a method of determining the number of protons in the nucleus of the atom"

The atomic number of an atom is the number of protons in the nucleus of that atom. 

Moseley showed that if the elements are arranged in order of increasing atomic number, it is not necessary to reverse the order of some elements.

Moseley's x-ray studies showed that Mendeleev had been correct in placing tellurium before iodine.

Moseley showed that by arranging the elements in order of increasing atomic number the elements fall naturally into the correct groups.

The modern periodic table is an arrangement of elements in order of increasing atomic number.

Modern Periodic Law when elements are arranged in order of increasing atomic number, the properties of the element recur periodically.

Mendeleevs	Modern
Elements arranged in order of increasing atomic weight (relative atomic mass)	Elements arranged in order of increasing atomic number
Only 60 elements approx. not all discovered	Over 100 elements
Contains gap for undiscovered elements	No gaps present
Transition elements were not put in a seperate block	Transition elements are in a seperate block

The mass number of an element is the sum of the number of neutrons + protons in the nucleus of an atom of that element

Atomic mass unit = u (or) amu

Z is usually used to indicate the atomic number

A is used to indicate the mass number

23=A  11=Z  Na = Sodium

Relative Atomic Mass, Isotopes, Mass Spectrometry

 Isotopes are atoms of the same element which have different mass numbers due to the different number of neutrons in the nucleus.

Work out the average mass of an atom of an element

75% of Chlorine 35+ 25% of Chlorine 37

75 atoms of mass 35 = 75x35-2625 25 atoms of mass 37 = 25x37=925

Total mass of 100 atoms = 3550

Average mass of 1 atom = 35.5

This average mass of an atom is measured relative to the mass of the carbon 12 isotope. For this reason it is called its relative atomic mass (Ar)

Relative atomic mass-the average of the mass numbers of the isotopes of the element, as they occur naturally. Taking their abundances into account and expressed on a scale in which the atoms of the Carbon-12 isotope have a mass of exactly 12 units.

Mass Spectrometer

1.Vaporisation 

A small amount for the gas or liquid is injected into the instrument at the sample inlet 

2.Ionisation

The electron gun fires high energy electrons at the atoms or molecules of the sample gas. Electrons are knocked off these particles of the sample gas and positive ions are produced.  

3.Acceleration

The positive ions produced in the pass between a series of negatively charged plates in the accelerator and accelerates these positive ions to high speeds.  

4.Separation

Occurs in a magnet field. The ions are separated according to their masses. 

5,Detection

The Positive ions fall on the detector where they are detected electronically.

The Principle of Mass Spectrometry is that positively charged ions moving ina magnetic field are deflected  to different extents according to their masses and are thus separated according to these masses. 

Uses of the mass spectrometer

Identity the presence of isotopes

Measure the relative abundances of isotopes

 Measure relative atomic masses. 

Identify unknown compound; (drugs)

Writing Electron Configurations of Atoms 

The electron configuration shows the arrangement of 'electrons in an atom of an elements

Aufbau Principle when building up the electron configuration of an atom in its ground state, the electrons occupy the lowest available energy levels

Calcium=1s22s22px22py22pz23s2 3px23py2 3pz24s2 

Exceptions

Chromium=1s22s22px22py22pz23s23px23py2 3pz2 4s¹ 3d⁵

Copper=1s22s22px22py22pz23s23px23py2 3pz2 4s¹3d¹⁰

Sublevels that are exactly half filled or completely filled have extra stability

Electron Configurations of Ions 

An Atom that has lost or gained electrons is called an ion

Mg2+ = [1s22s22p63s2]2+

Arrangement of Electrons in Orbitals of Equal Energy

Hund's Rule of Maximum Multiplicity states that when two or more orbitals of equal energy. are available, the electrons occupy them singly before filling them in pairs.

Paul Exclusion states that no more than two electrons may occupy an orbital and they must have opposite spin