trend in melting points of elements in period 3

Melting

When a substance melts, some of the attractive forces holding the particles together are broken or loosened so that the particles can move freely around each other but are still close together. The stronger these forces are, the more energy is needed to overcome them and the higher the melting temperature.

Sodium, magnesium and aluminium are all metals. They have metallic bonding, in which positive metal ions are attracted to delocalised electrons. Going from sodium to aluminium:
• the charge on the metal ions increases from +1 to +3 (with magnesium at +2)and the number of delocalised electrons increases so the strength of the metallic bonding increases and the melting points and boiling points increase.

Silicon is an element with some of the properties of metals and some of the properties of non-metals. Silicon has giant covalent bonding. It has a giant lattice structure similar to that of diamond, in which each silicon atom is covalently-bonded to four other silicon atoms in a tetrahedral manner.

Silicon has a very melting point and boiling point because all the silicon atoms are held together by strong covalent bonds which need a very large amount of energy to be broken.

Phosphorus, sulphur, chlorine and argon are all non-metals, and they exist as small, separate molecules. Phosphorus, sulphur and chlorine exist as simple molecules, with strong covalent bonds between their atoms. Argon exists as separate atoms (it is monatomic).
Their melting and boiling points are very low because when these four substances melt or boil, it is the van der Waal’s forces between the molecules which are broken. These bonds are very weak so little energy is needed to overcome them.

Sulphur has a higher melting point and boiling point than the other three because:
• phosphorus exists as P4 molecules
• sulphur exists as S8 molecules
• chlorine exists as Cl2 molecules
• argon exists individual Ar atoms
• the strength of the van der Waal’s forces decreases as the size of the molecule decreases
• so the melting points and boiling points decrease in the order S8 > P4 > Cl2 > Ar

Forensic scientist :)

My future career: Forensic Scientist.

Requirements: > complete the whole education course, must enter either JC/ Poly.
> PH.D in Chemistry
> Complete a forensic science course in the Faculty of Science in
National University of Singapore.
> At least a BSc (2nd upper Hons) in Chemistry, preferably with
physics and engineering sciences.
> A postgraduate degree (MSc, PhD)) in a relevant discipline or
working experience in a forensic laboratory is an advantage.
> Strong theoretical foundations and a good working knowledge of
general chemistry and instrumental techniques in the identification
and characterisation of materials.
> Proficiency in written and oral communication is required as there
is a requirement to routinely write reports on forensic findings and
present expert testimony in Court.

My Responsibility :
I will join a team of forensic scientists in the Forensic Physics and Chemistry Laboratory examining a wide range of physical evidence. i would have to trace evidence (glass, arson, paints, soils, fibres), unknown organic and inorganic substances, fire and explosion debris, firearms components, marks and prints and bloodstain patterns.I am also expected to work independently as well as in a team.
I must have a strong passion for science, good bench skills and highly developed skills in logical reasoning, problem solving and critical thinking. Error-free, quality work and sound conclusions that can withstand Court scrutiny are expected for every case I handle.

What i would be covering during my job :

Forensic Chemistry and Physics Laboratory services are built around HSA's core strengths in identifying microscopic or trace level amounts of material in complex mixtures, and in comparing marks, prints and patterns. Forensic Chemistry and Physics Laboratory is Singapore's national laboratory for the integrated forensic examination of the following types of physical evidence:

> Fire and explosions
> Chemical analysis and trace evidence
> Marks, impressions and physical examinations
> Scene analysis and reconstruction
> Research and commercial cases

As Analysts gazetted under the Health Sciences Authority Act, Forensic Chemistry and Physics Laboratory (FCPL) scientists provide prima facie evidence and expert testimony in Singapore Courts.

FCPL also provides an extensive range of services on the examination of handwriting, signatures, ink, paper and printersencountered in criminal investigation and civil disputes. Our clients include law enforcement agencies such as Singapore Police Force and private individuals and organizations such as law firms and banks.

The laboratory is one of the few worldwide qualified to examine both English and Chinese handwriting and signatures. The officers have rendered services to many countries comprising Hong Kong, United States of America, Australia, Germany, Brunei and Malaysia.

HSA's research works are well-received in international conferences, and some of them are published in peer-reviewed journals.

I would also have to:

> Examination of handwriting and signatures in English, Chinese and romanized scripts;
> Examination of tampered and counterfeit documents;
> Differentiation of inks;
> Chemical analysis of paper and inks;
> Visualisation of indented writing;
> Identification of rubber stamps and rubber stamp impressions;
> Determination of the sequence of two intersecting lines;
> Physical fitting of torn/cut documents;
> Examination of photocopies and facsimile documents;
> Examination of typewritten and computer printed documents;
> Conducting lectures, seminars and training sessions for interested parties

EnDpOiNt AnD eQuIvAlEnCe PoInT.

End point of a titration can be measured in many ways.
Some of which are by using:
- pH indicator: This is a substance that changes colour in response to a chemical change. An acid-base indicator (e.g., phenolphthalein) changes colour depending on the pH.
- A potentiometer can also be used. This is an instrument that measures the electrode potential of the solution.
- pH meter: This is a potentiometer that uses an electrode whose potential depends on the amount of H+ ion present in the solution
- Colour change: In some reactions, the solution changes colour without any added indicator.


Difference between endpoint and equivalence point:

Equivalence Point: Equivalence-point means the moment in your slow, dripping addition,at which the amount of base added is chemically equivalent to the acid in the flask, (or vice-versa) so the two kinds of reagent are exactly cancelling out.

Endpoint: the end-point is the perceptual event at which the indicator actually does change, and you decide you are "done", and you actually read the quantity of measuring reagent you have added. Such as when the solution finally changes colour and does not change back right away.

Moles :)

i have learnt a lot about mole concept during chemistry lesson today. i am also able to see how concentration, mass, volume and Avogadro's Constant are connected to moles.

To calculate moles, i can use many formulas. Some examples are:

1. mole = mass/ Mr
2. mole= concentration/ volume
3. mole= no. of particles/ Avogadro's Constant.


i also learnt how to find Ar by this formula:

I and Y are isotopes of an element.

Ar= ( relative isotopic mass of I x % abundance of I) + ( relative isotopic mass
of substance Y x % abundance of substance Y) / 100

i've learnt quite a number of things in chem class today. i have learnt how to write balanced equations, find out the problem, purpose of the question. i also learnt how to identify if the reaction is a chemical/ physical reaction.

i've learnt many things.
some examples of binary covalent componds are water and ammonia. i've also learnt about prefixes. they are :
Prefixes
1 - mon(o)
2 - di
3 - tri
4 - tetr(a)
5 - pent(a)
6 - hex(a)
7 - hept(a)
8 - oct(a)
9 - non(a)
10 - dec(a)
Roots of the Nonmetals
H - hyd
C - carb
N - nitr
P - phosph
As - arsen
O - ox
S - sulf
Se - selen
F - fluor
Cl - chlor
Br - brom
I - iod

i also learnt that Metallic atoms hold some of their electrons relatively loosely, and as a result, they tend to lose electrons and form cations. i also know that when a metallic element and a nonmetallic element combine, the nonmetallic atoms often pull one or more electrons far enough away from the metallic atoms to form ions. The positive cations and the negative anions then attract each other to form ionic bonds.