The halogens are the most reactive elements as a family. Fluorine is the most reactive of all the halogens. The reactivity of the halogens decreases down the group. The high reactivity of halogens is due to the following reasons:

(i) Low dissociation energies

All the halogens have very low dissociation energies. As a result, they can readily dissociate into atoms and react with other substances. As shown below, the dissociation energies of halogens are quite low in comparison to common molecules such as H2, O2 and N2.

(ii) High electron affinity

Halogens have very high electron affinity values and therefore, have very strong tendency to gain an electron. Thus halogens are very reactive elements due to their low dissociation energies and high electron affinity values. As clear from the values of bond dissociation energies, fluorine has the lowest bond dissociation energy. This is due to weak F-F bond because of the repulsion between the non-bonding electrons in the small molecule. Therefore, it is most reactive among the halogens.

Some of the important chemical reactions of halogens are discussed ahead.

Thermochemical equations are just like other balanced equations except they also specify the heat flow for the reaction. The heat flow is listed to the right of the equation using the symbol ΔH. The most common units are kilojoules, kJ. Here are two thermochemical equations:

H₂ (g) + ½ O₂ (g) → H₂O (l); ΔH = -285.8 kJ

HgO (s) → Hg (l) + ½ O₂ (g); ΔH = +90.7 kJ

When you write thermochemical equations, be sure to keep the following points in mind:

1. Coefficients refer to the number of moles. Thus, for the first equation, -282.8 kJ is the ΔH when 1 mol of H₂O (l) is formed from 1 mol H₂ (g) and ½ mol O₂.
2. Enthalpy changes for a phase change, so the enthalpy of a substance depends on whether is it is a solid, liquid, or gas. Be sure to specify the phase of the reactants and products using (s), (l), or (g) and be sure to look up the correct ΔH from heat of formation tables. The symbol (aq) is used for species in water (aqueous) solution.
3. The enthalpy of a substance depends upon temperature. Ideally, you should specify the temperature at which a reaction is carried out. When you look at a table of heats of formation, notice that the temperature of the ΔH is given. For homework problems, and unless otherwise specified, temperature is assumed to be 25°C. In the real world, temperature may different and thermochemical calculations can be more difficult.

Certain laws or rules apply when using thermochemical equations:

1. ΔH is directly proportional to the quantity of a substance that reacts or is produced by a reaction.Enthalpy is directly proportional to mass. Therefore, if you double the coefficients in an equation, then the value of ΔH is multiplied by two. For example:

   H₂ (g) + ½ O₂ (g) → H₂O (l); ΔH = -285.8 kJ

   2 H₂ (g) + O₂ (g) → 2 H₂O (l); ΔH = -571.6 kJ

2. ΔH for a reaction is equal in magnitude but opposite in sign to ΔH for the reverse reaction.For example:

   HgO (s) → Hg (l) + ½ O₂ (g); ΔH = +90.7 kJ

   Hg (l) + ½ O₂ (l) → HgO (s); ΔH = -90.7 kJ

   This law is commonly applied to phase changes, although it is true when you reverse any thermochemical reaction.

3. ΔH is independent of the number of steps involved.This rule is called Hess’s Law. It states that ΔH for a reaction is the same whether it occurs in one step or in a series of steps. Another way to look at it is to remember that ΔH is a state property, so it must be independent of the path of a reaction.

   If Reaction (1) + Reaction (2) = Reaction (3), then ΔH₃ = ΔH₁ + ΔH₂

Learn what Le Chatelier’s Principle is and how it can be applied to predict the effect of a change in conditions on chemical equilibrium.

What Is Le Chatelier’s Principle?

Le Chatelier’s Principle is the name given to the principle in which a change in a chemical system prompts an opposing reaction. In chemistry, this principle was discovered independently by Henry Louis Le Chatelier and Karl Ferdinand Braun, so it is sometimes called the Le Chatelier-Braun Principle. The principle can be stated as follows:

If the temperature, concentration, volume, or partial pressure of a chemical system at equilibrium changes, then the equilibrium shifts to compensate for the change.

The more general form of the principle applies to other disciplines. Homeostasis and Lenz’s law are examples. Le Chatelier’s Principle is known by the same name when applied to economic equilibrium.

How Is Le Chatelier’s Principle Used?

Le Chatelier’s Principle is used to predict how a change in pressure, volume, concentration, or temperature will affect chemical equilibrium. Knowing the impact on equilibrium allows chemists to manipulate the chemical reaction. For example, a chemist might apply Le Chatelier’s Principle to maximize yield from a reaction.

My sailboat’s name is the Fiat Lux — “let there be light” in Latin — drawing from both my theological and scientific personae. I sail a Laser, an Olympic class racing dingy, which is an apt boat for a quantum mechanic. The ability to amplify light by stimulating an existing emission process was first predicted by quantum mechanics, then the apparatus to actually do it was built. Laser is really an acronym: Light Amplification by Stimulated Emission of Radiation. The radiation is electromagnetic radiation, not the radioactive radiation.

There’s been a smattering of conversation about light production around my house this weekend between sailing the Laser, setting off fireworks and observing fireflies. One of my teen guests wondered how the fire in fire flies was different from the fire in fireworks. All light is not created in quite the same way….though there are some fundamental similarities.

There are really two fires in fireworks, the thermal explosives that send them skyward, and the “rockets red glare” — the glittering burst of color in the sky. The heat from the thermal explosion (usually blackpowder or a similar substance) is what trigger the colors.

If you ever done a flame test, putting a solid substance or a concentrated solution on a wire loop and placing it in a flame to see what color is produced, you’ve done the same chemistry. The extreme heat excites electrons in an atom or molecule, and as they fall back down to their lowest energy, or ground state, emitting a photon (a bit of light) that just exactly matches the difference in energy between the excited state and the ground state. An orange flame meant you had sodium on the wire, while a violet flame suggested potassium. More properly this technique is called atomic emission spectroscopy.

For atoms the picture you usually see in a high school text of this process is of a ladder, where electrons are shown moving from rung to rung. The larger the distance between the two rungs (or states) the higher the energy of the photon emitted. If the distance corresponds to photons in the visible region, you see a color, otherwise you have to use something fancier to figure out the energy of the photons being released.

Different atoms have different spacings between states and so the colors they emit when heated to high temperatures are likewise different. There are in fact many states, and so many types of photons can be emitted, but few are in the visible region.

If you click here, you can see a simulation of the photons you’d expect to see when an excited sodium atom returns to the ground state. Are you surprised that sodium can be used for yellow-orange in fireworks? Some urban legends suggest that lead (or radioactive barium) are used in fireworks, but if you look at the line spectrum of lead you can see why it can’t be true — there is no rung to rung jump in lead that corresponds to a visible photon. So a lead firework would be invisible! (Lead used to be used to make the fireworks “crackle”…)

(And it’s true that barium salts are used in fireworks, but they are not radioactive. There are no naturally occurring radioactive isotopes of barium.)

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Nuclear Physics is concerned with the fundamental nature of matter. The central focus of this area of study is the relationships between a quantity of energy and its mass, given by E = mc2, and the fact that matter can be converted from one form (energy) to another (particles) in particle accelerators and nuclear reactors.

The results of particle accelerator experiments have led scientist to postulate the existence of three types of forces important in the nucleus: The strong force, the weak force, and the electromagnetic force. These forces are though to account for all types of interaction found in matter. The fourth force found in nature, but not in the nucleus, is the gravitational force. These forces are believed to be generated by the exchange of particles between the interacting pieces of matter. For example, the gravitational force is thought to be carried by particles called gravitons. The electromagnetic force is assumed to be exerted through the exchange of photons. The strong force, not charge related, and only effective at very short distances ( approximately10-13 cm), is postulated to involve the exchange of particles called gluons. The weak force is 100 times weaker than the strong force and seems to be exerted through the exchange of two types of large particles, the W (has a mass 70 times the proton’s mass) and the Z (has a mass 90 times the proton’s mass).

The particles discovered have been classified into several categories. Three of the most important classes are as follows:

1. Hadrons are particles that respond to the strong force and have internal structure. Hadrons consist of Baryons and Mesons. A Baryon is composed of three quarks. A Meson is composed of a quark and an antiquark.
2. Leptons are particles that do not respond to the strong force and have no internal structure.
3. Quarks are particles with no internal structure that are thought to be the fundamental constituents of hadrons. Neutrons and protons are hadrons that are thought to be composed of three quarks each.

Each of these main classes also contains antiparticles. For example the electron, which is a Lepton, has an antiparticle called a positron (“electron” with a positive charge).

The world of particle physics appears mysterious and complicated. For example, particle physicists have discovered new properties of matter they call “color”, “charm”, and “strangeness” and have postulated conservation laws involving these properties. This area of science is extremely important because it should help us to understand the interactions of matter in a more unified way.

Nucleus
Hadrons Leptons
Baryons Mesons
Protons
Neutrons
Other, short-
lived particles Pions
Kaons
Other, short-
lived particles Tau
Muon
Electron Tau neutrino
Muon neutrino
Electron neutrino

Elementary Particles: Leptons and quarks
Quarks
Bottom Top
Strange Charm
Down Up

The Four Forces of Nature

1. Gravatational Force
weakest of the four
acts at long range, varying by 1 / d2
force transmitted by gravitons

2. Electromagnetic Force
originally two forces, electric and magnetic until they were unified into the electromangetic
holds atoms, molecules, liquids, and solids together
acts at long range, varying by 1 / d2
force transmitted by photons
studied in quantum electrodynamics (QED)

3. Weak Nuclear Force
this second weakest force allows matter to disintegrate and transmutate,
beta- decay from neutrons, beta+ decay from protons
acts at very short range
independant of electric charge
acts between leptons and hadrons
acts between hadrons and other hardons

4. Strong Nuclear Force
holds the quarks together to form mesons and baryons
strongest of all the forces
acts at very short range
force transmitted by gluons (coors of quarks)
studied in quantum chromodynamics (QCD)