Introduction of Intermolecular Forces

Introduction of Intermolecular Forces

I think just by looking at the word “inter” we can know that this is a kind of force that happens between or among molecules. Intermolecular attractions are attractions between one molecule and a neighboring molecule.  They are weak compared to the intermolecular forces, since intermolecular forces create the needed force to keep a molecule together. For example, the covalent bonds which involve the sharing of electron pairs between atoms, which present a much stronger force than the forces present between the two neighboring molecules.

All molecules experience intermolecular attractions, although in some cases those attractions are very weak. Even in a gas like hydrogen (H2) if you slow the molecules down by cooling the gas, the attractions are large enough for the molecules to stick together eventually to form a liquid and then a solid.

Intermolecular forces can be considered by the following forces:

Introduction of Chemical Bonding

Introduction of Chemical Bonding

Chemical bonding can only be put into perspective and be illustrated into a small number of real life example bonding. But the best are the Human Bonding. This is because Human are diverse and vary, however they still need each other. By, definition human bonding can be describe as “ the process of development of a close, interpersonal relationship. It most commonly takes place between family members or friends, but can also develop among groups, such as sporting teams and whenever people spend time together.” Which is very similar to the behaviour of atom, where atoms form chemical bonds in order to make their most outer electron shells (valence shell) more stable.

Chemical bonding between atom happen because they need to “follow” and “obey” the octet rule. Atoms follow the octet rule because they always seek the most stable electron configuration. The octet rule states that elements gain or lose electrons to attain an electron configuration of the nearest noble gas. Noble gases have complete outer electron shells, which make them very stable. Other elements also seek stability, which cause their tendency to have bonding behavior to become an noble gas.

A Brief Past on Chemical Bonding Concepts

• In 1852, E. Frankland proposed the concept of valence. He suggested that each element formed compounds with definite amounts of other elements due to a valence connection.

• Five years later, F.A. Kekule and others proposed a valence of 4 for carbon. Lines were used to represent valence, and this helped the development of organic chemistry

• Following the discoveries of electrons by J.J. Thompson, Gilbert Newton Lewis proposed to use dots to represent valence electrons. His dots made the valence electrons visible to chemists. Example of Lewis dot diagram of NH3:

• With the discovery of X-ray, the X-ray diffractions by crystal allow chemist to calculate details of bond length and bond angles. These data prompted Linus Pauling to look at The Nature of Chemical Bond, a book that introduced many new concepts such as the resonance (resonance structures are two forms of a molecule where the chemical connectivity is the same but the electrons are distributed differently around the structure), electronegativity (Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons), ionic bond (Ionic bonding is a type of chemical bond that involves the electrostatic attraction between oppositely charged ions, and is the primary interaction occurring in ionic compounds. ), and covalent bond ( a chemical bond that involves the sharing of electron pairs between atoms).

• Nevil Vincent Sidgwick and H.E. Powell paid their attention to the lone pairs in a molecule. They developed the valence bond theory, the VSEPR (Valence Shell Electron Pair Repulsion) theory. VSEPR is a valence shell electron pair repulsion, this theory was based on the idea that pairs of electron around the central of atom with the same charge want to be as far as possible from each other. That is why they push away each other and try to be as far as possible.

Boltzmann Distribution of Curve

Boltzmann Distribution of Curve

Boltzmann distribution curve is a graph that shows the distribution of energies of the particle in a sample at a given temperature

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History:

• 1857-Rudolf Clausius wrote a paper which discussed calculating the speed of atoms
• 1860-James Maxwell elaborate on Claudius's findings, showing how to obtain the distribution of atoms’ speed within a gas, determining atoms with greater or lesser speed than average
• 1868-Ludwig Boltzmann published a paper proving Maxwell’s then disputed findings. He did this by applying Maxwell’s mathematical forms into differing pressures, proving that they were correct.

Boltzmann distribution curve with change of  Catalyst:

A catalyst is a substance that increase the rate of a reaction but remain chemically unchanged itself at the end of reaction. With the presence of catalyst the activation energy will be decrease, which will result more chance for successful atom to reaction. Just like what we can see from the picture above.

Boltzmann distribution curve with change of Temperature:

Increase in temperature lead to an increase in Kinetic Energy on the molecules. Therefore the collision or impact energy is stronger which then result the graph to shift the molecular energy to the right. And the peak also decrease because the peak shift to the right, but it still maintain the same amount of molecule. That is because temperature did not affect the activation energy. Only catalyst affect the activation energy. Just like what we can see from the illustration above.

Example of Exercise:

QP12 2015 No.11

The Boltzmann distribution below show the distribution below shows the distribution of molecular energies in a sample of a gas at a given temperature.

Which statement correctly describes the change in such a distribution if the temperature increase?

1. Fewer molecules possess the most probable energy value and this value shifts to the left
2. Fewer molecule possess the most probable energy value and this volume shifts to the right
3. More molecules possess the most probable energy value and this value shift to the left
4. The area under the curve to the distribution increases

Answer: B (since the graph will shift to the right)

Introduction of Atom

Introduction of Atom

We know that Atoms make up everything. From the air we breathe to food we eat and even the sun itself. Because of the study and research that has been done previously throughout history we know that each atom is made up of three parts: proton, neutron, and electron. When atoms combine it make compounds. The most basic atoms are called elements. Right now, scientist knows for sure that there are currently 118 known elements 90 of them occurring naturally and the other 28 are man-made; however in future time, the discovery of a new atom might still be possible.

Now I'm going to break down and give a brief explanation of the three main part that makes up an atom. Protons are positively charged particles found in atomic nuclei. They were discovered by Ernest Rutherford(New Zealand-born British physicist who became known as the father of nuclear physics) in experiments conducted between 1911 and 1919.

The number of protons in an atom defines what element it is. For example, carbon atoms have six protons, hydrogen atoms have one and oxygen atoms have eight. The number of protons in an atom is referred to as the atomic number of that element. The number of protons in an atom also determines the chemical behavior of the element. The Periodic Table of the Elements arranges elements in order of increasing atomic number.

Electrons have a negative charge and are electrically attracted to the positively charged protons. Electrons surround the atomic nucleus in pathways called orbitals. The inner orbitals surrounding the atom are spherical but the outer orbitals are much more complicated.

Neutrons, Neutrons are the particles in an atom that have a neutral charge. They aren't positive like protons. They aren't negative like electrons. Neutrons were discovered by James Chadwick (English physicist) in 1932. are uncharged particles found within atomic nuclei. A neutron mass is slightly larger than proton. Along with protons, neutrons make up the nucleus (the center of an atom, where it will be surounded by orbiting electrons)

Case Study

Case:

History of Atomic Structure  & the Benefit of Knowing Isotope Symbolism

Case presentation:

For years the human mind has been captivated and petrified by a single question, “ what is the world made of? ” I mean we know for example that a wooden table are made up of joining wooden stick that are nailed to one another. But what is the world made up of? Is a very mysterious question for a long time. But now, we all know that Atoms are the foundation of everything. The word atom is derived from the Greek word “atmos” which mean indivisible. Where now we know that atom is the basic “building block” for all matter in the universe. Today I’m going to discuss the historical timeline behind the “finding” of understanding atom, through atomic study that resulted as an atomic theory.

Atomic theory is the scientific theory study of the nature of matter. The theory states that matter is made up of small particles called atoms. Prior to this theory, matter was thought to be able to be divided into any small quantity. 442 BC, Democritus, a Greek philosopher was the pioneer to the study of atom in the first place, where he co-originated the thought with his teacher, Leucippus that all matter is composed of indivisible elements. Long after his “rough” theory was proposed. Thousands of years later, at 1803 John Dalton, a British chemist and physicist, developed a theory that was based and support Democritus perspective on atomic study. John Dalton state “matter is simply composed of atoms of different weights and is combined in ratios by weight” beside that he also proposed that these atoms are spherical, and are in motion.

        1898, JJ Thomson (Sir Joseph John Thomson was an English physicist) discovers the electron, using properties of cathode rays.  Cathode ray tube is a vacuum tube containing one or more electron guns, and a phosphorescent screen used to view images. It is used to accelerate and deflect the electron beam onto the screen to create the images. which enabled scientist to observe and analyze the projectiled images.  In 1911 Ernest Rutherford who performed many experiments to explore radioactivity did an experiment in which he discovered that the atom must have a concentrated positive center charge that contains most of the atom's mass. He suggested that the nucleus contained a particle with a positive charge which we now know as proton.

1913, Niels Bohr (a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory) developed the Bohr atomic model,  in which electrons  orbits around the nucleus, and chemical properties being determined by how many electrons are in the most outer orbits (Valence shell electron). The last “ingredient” of an atom which is the Neutron component was discovered by James Chadwick in 1931. He was later awarded the 1935 Nobel Prize in Physics for his discovery of the neutron.

Now we know that atom of each element is made up of electrons, protons and neutrons. All atoms of the same neutral element have the same number of protons and electrons, but the number of neutrons can differ. Atoms of the same element but different neutrons are called isotopes. Because of these isotopes it becomes necessary to develop a notation to distinguish one isotope from another. By being able to identify the different number between them we can identify the type of atom it is without even looking at the symbol. The atomic symbol has three parts to it:

1. The symbol X: the usual element symbol

2. The atomic number A: equal to the number of protons (placed as a left subscript)

3. The mass number Z: equal to the number of protons and neutrons in the isotope (placed as a left superscript)

Example: Consider two isotopes of gallium, one having the 37 neutrons and the other having 39 neutrons. Write the atomic symbols for each isotope. Solution: