Chpt. 4: The Periodic Table. In this Chapter. What is an element??? Chemists and order among the elements The Periodic Table Periodic Table & arrangement of electrons in atoms. Robert Boyle from Co. Waterford first chemist to define the meaning of the word element.
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An element is a chemical substance that cannot be split up into simpler substances by chemical means.
1. The Greeks (400 B.C.)
No. of protons = Atomic No.
After 1800’s more and more elements were being discovered and it was becoming increasingly difficult to understand and memorise all the properties of each element.
Four men were responsible for bringing order among the elements in the form of the Periodic Table:
- Johann Dobereiner
- John Newlands
- Dmitri Mendeleev
- Henry Moseley
A triad is a group of three elements with similar chemical properties in which the atomic weight of the middle element is approximately equal to the average of the other two.
An octave is a group of elements arranged in order of increasing atomic weight, in which the first and the eighth element of each group have similar properties
Dmitri Mendeleev listed all known elements (63) in order of increasing atomic weight and again noticed similar properties on every eight element.
Mendeleev left gaps and predicted elements that had not been discovered.
When elements are arranged in order of increasing atomic weight (relative atomic mass), the properties of the elements vary periodically.
The atomic number of a atom is the number of protons in the nucleus of that atom
*Note: Once the atomic number was known it was seen
that Mendeleev’s table was in order of increasing
The modern Periodic Table is an arrangement of elements in order of increasing atomic number
The Periodic Law
When elements are arranged in order of increasing atomic number, the properties vary periodically
WOMANIUM (WO) Physical properties: Generally soft and round in form. Boils at nothing and may freeze any time. Very bitter if not used well. Chemical properties: Very active and highly unstable. Possesses strong affinity with gold, silver, platinum, and precious stones. Violent when left alone. Turns slightly green when placed next to a better specimen. Usage: An extremely good catalyst for dispersion of wealth. Caution: Highly explosive in inexperienced hands!
MANIUM (XY) Physical properties: Solid at room temperature but gets bent out of shape easily. Difficult to find a pure sample. Due to rust, aging samples are unable to conduct electricity as easily as young samples. Chemical properties: Attempts to bond with WO any chance it can get. Also tends to form strong bonds with itself. Becomes explosive when mixed with Childrium for prolonged period of time. Usage: Possibly good methane source. Caution: In the absence of WO, this element rapidly decomposes and begins to smell.
1919 an instrument called mass spectrometer was built to measure the mass of atoms.
Isotopes are atoms of the same element (i.e. they have the same atomic number) that have different mass numbers due to different numbers of neutrons in the nucleus.
Since the neutron has no charge, the quantity of neutrons in an atom can change slightly without having an effect on the atom.
Example: Neon – 2 isotopes, Chlorine – 2 isotopes, Carbon – 3 isotopes and Hydrogen – 3 isotopes
A sample of chlorine is found to consist of 75% 3517Cl and 25% 3717Cl . Calculate the average mass of an atom of chlorine.
Average Mass = (Mass isotope 1 x % abu) + (Mass isotope 2 x % abu)
The relative atomic mass (Ar) of an element:
- is 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.
(average mass of an atom, measured relative to the mass of the carbon -12 isotope)
1/12 mass of atom of carbon-12
Note: ratio therefore no units
Ar= (Mass isotope 1 x % abu) + (Mass isotope 2 x % abu)
An element, X, consists of 92.2% atoms with a mass 28, 4.7% of atoms with a mass 29 and 3.1% of atoms with a mass 30. What is the relative atomic mass? What is the element?
The two isotopes of chlorine have mass numbers of 35 and 37 respectively. Taking the relative atomic mass of chlorine to be 35.46, calculate the % of each isotope present in the element
Write the electronic configuration for potassium showing the number of electrons in each main energy level.
From periodic table potassium has atomic number 19 therefore since neutral atom:
no. of protons = no. of electrons = 19
1st energy level = 2 e-
2nd energy level = 8 e-
3rd energy level = 8 e-
4th energy level = 1 e-
So, electronic configuration of potassium is (2,8,8,1)
Write the electronic configuration for the following showing the number of electrons in each main energy level:
- you must be able to write the electronic configurations of the first 20 elements in terms of the number of electrons in each main energy level.
- number of electrons in outer shell is same as group number e.g. Lithium group I has one outer electron, Boron group III has three outer electrons
Main energy level
An energy sublevel is a group of orbitals, within an atom, all having the same energy.
One orbital in an ‘s’ energy sublevel
Three orbitalsin a ‘p’ energy sublevel - X, Y, Z
Five orbitalsin a ‘d’ energy sublevel
In terms of energy s<p<d<f.
Electrons will fill sublevels in order of increasing energy.
P sublevel can hold 6 electrons, 2 in each px, py and pz orbital
d sublevel can hold 10 electrons, 2 in each of the 5d orbitals
Sublevel – s, p, d, f
Orbitals – 1s, 2s, 2px etc.
Electrons – 2 in each orbital
Aufbau Principle states that when building up the electronic configuration of an atom in its ground state , the electrons occupy the lowest available energy level.
No. of electrons
Main energy level
Type of sublevel
Write the electronic configuration for the elements from 1 to 24.
The ‘4s’ energy sublevel is slightly lower in energy than the ‘3d’ energy sublevel. Therefore, its orbital must be filled with electrons before the orbitals in the 3d sublevel.
If p, d and f shells are exactly half filled or completely filled they will have extra stability:
24) Cr = 1s2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d5
29) Cu = 1s2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d10
You must be able to write the electronic configurations of the first 36 elements in terms of their s, p, d configurations!!!
What is electronic configuration of Al3+ ion? What neutral atom has the same configuration?
Write the electronic configuration (s, p) of the S2- ion. What neutral atom has the same configuration?
Identify the species represented by [ 1s2, 2s2, 2p6]2+
Identify the species represented by:
[1s2, 2s2, 2p6, 3s2, 3p6]2-
The electronic configuration of nitrogen is:
1s2, 2s2, 2p3
The question is how are the three electrons in the p sublevel distributed among the three p orbitals – x, y, z?
Hund’s Rule of Maximum Multiplicity
(Bus Seat Rule)
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
1s2, 2s2, 2p3
1s2, 2s2, 2px1, 2py1, 2pz1
2p sublevel – 1e- will enter px orbital
- 1e- will enter py orbital
- 1e- will enter pz orbital
The Pauli Exclusion Principle states that no more than two electrons may occupy an orbital and they must have opposite spin.
= electron spinning clockwise
= electron spinning anti-clockwise
Complete ‘arrows-in-boxes’ diagrams and the extended form of electronic configurations for the first ten elements
The ‘arrows-in-boxes’ diagrams or extended forms of electronic configurations need only be given if you are specifically asked about the distribution of electrons in the p sublevel.