Atomic Theory and Atomic Structure PDF

# Atomic Theory and Atomic Structure In this chapter, we will study the basic concept of chemistry. An element, compound, or mixture is composed of small particles called atoms. ## Dalton's Atomic Theory The main points of Dalton's atomic theory are: - Everything is composed of atoms, which are...

# Atomic Theory and Atomic Structure In this chapter, we will study the basic concept of chemistry. An element, compound, or mixture is composed of small particles called atoms. ## Dalton's Atomic Theory The main points of Dalton's atomic theory are: - Everything is composed of atoms, which are the indivisible building blocks of matter and cannot be destroyed. - All atoms of an element are identical. - The atoms of different elements vary in size and - Compounds are produced through different whole-number combinations of atoms. - A chemical reaction results in the rearrangement of atoms in the reactant and product compounds. - Atomic theory has been revised over the years to incorporate the existence of atomic isotopes and the interconversion of mass and energy. ## The structure of the Atom - An atom is the smallest unit of matter that retains all of the chemical properties of an element. - Atoms combine to form molecules, which then interact to form solids, gases, or liquids. - For example, water is composed of hydrogen and oxygen atoms that have combined to form water ## Atomic Particles - Atoms consist of three basic particles: protons, electrons, and neutrons. - The nucleus (center) of the atom contains the protons (positively charged) and the neutrons (no charge). - Atoms have different properties based on the arrangement and number of their basic particles. ## Atomic Number - Neutral atoms of an element contain an equal number of protons and electrons. - The number of neutrons can vary to produce isotopes, which are atoms of the same element that have different numbers of neutrons. - For instance, iron, Fe, can exist in its neutral state, ## Mass Number - An element's mass number (A) is the sum of the number of protons and the number of neutrons. - Protons and neutrons both weigh about one atomic mass unit or amu. Isotopes of the same element will have the same atomic number but different mass numbers. ## Isotopes Isotopes are various forms of an element that have the same number of protons but a different number of neutrons. Some elements, such as carbon, potassium, and uranium, have multiple naturally-occurring isotopes. Isotopes are defined first by their element and then by the sum of the protons - Carbon-12 (or ¹²C) contains six protons, six neutrons, and six electrons; therefore, it has a mass number of 12 amu (six protons and six neutrons). - Carbon-14 (or ¹⁴C) contains six protons, eight neutrons, and six electrons; its atomic mass is 14 amu (six protons and eight neutrons). ## Liquids and Solids ### Liquids - The liquid state of matter is an intermediate phase between solid and gas. - The attraction between the particles in a liquid keeps the volume of the liquid constant. - The movement of the particles causes the liquid to be variable in shape ## Evaporation Evaporation is a surface phenomenon in which molecules located near the liquid's edge, escape into the surroundings as vapor. Because the particles of a liquid are in constant motion, they will collide with one another, and with the sides of the container. - Evaporation occurs when surface particles gain enough kinetic energy to escape the system. As the faster particles escape, the remaining particles have lower average kinetic energy, and the temperature of the liquid cools. This phenomenon is known as evaporative cooling. ## Vapour Pressure When a liquid evaporates inside a closed container, the particles cannot escape the system. Some of the evaporated particles will eventually come into contact with the remaining liquid and lose enough of their energy to condense back into the liquid. When the rate of evaporation and the rate of condensation are the same, there will be no net decrease in the - The pressure exerted by the vapor/liquid equilibrium in the closed container is called the vapor pressure. - Increasing the temperature of the closed system will increase the vapor pressure. ## Boiling Point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding environmental pressure - For example, water boils at 100 °C (212 °F) at sea level, but at 93.4 °C (200.1 °F) at 1,905 metres (6,250 ft) altitude. For a given pressure, different liquids will boil at different temperatures. - On the other hand, boiling is a process in which molecules anywhere in the liquid escape, resulting in the formation of vapor bubbles ## Freezing Point The temperature at which a liquid becomes a solid. The temperature at which a liquid, releasing sufficient heat, becomes a solid. For a given substance, the freezing point of its liquid form is the same as the melting point of its solid form. The freezing point of water at a pressure of one ## Surface Tension It is the tendency of liquid surfaces to shrink into the minimum surface area possible. Surface tension is an important factor in the phenomenon of capillarity. Surface tension has the dimension of force per unit length, or of energy per unit area. The two are equivalent, but when referring to energy per it is common to use the term surface energy, which is a more general term in the sense that it applies also to solids. In materials science, surface tension is used for either surface stress or surface energy. ## Measurement of Surface Tension ### Capillary Rise Method The capillary rise method is a test conducted to determine the surface tension of a liquid or the contact angle of liquid with soil or pipe material.  Capillary rise is the rise in a liquid above zero pressure level caused by net upward force that is produced by the attraction of water molecules to a solid surface. Capillary rise takes place due to the combined effect of cohesive and adhesive forces that cause liquids to rise up in tubes of very small diameter. The capillary rise method consists of dipping a thin circular capillary into the liquid to be tested. If the adhesion forces (the interaction between the liquid and the capillary wall) are stronger than the cohesive forces (that between the liquid molecules) the liquid will wet the wall and rise in the capillary tube to a certain level with the - **Downward force** = πr² hd g (1) Where r is the inside radius of the capillary, h is the difference in level of the liquid surfaces inside and outside the capillary, d is the density of liquid and g is the acceleration of gravity. The upward pull equals the surface tension multiplied by the length of the line of contact, i.e. the inside circumference of the capillary, therefore, - **Upward pull** = 2 π r γ (2) At equilibrium, therefor, 2 π r γ = πr² hd g Or γ = πr² hd g / 2 π r = 1/2 r hdg (3) - γ = cm x cm x g/cm³ x cm/sec² - γ = g.cm.sec-²/cm = dyne/cm If the liquid does not wet glass strongly, the angle of contact will not be zero and equation (3) can be modified to give correct values for the surface tension by introduction of cos θ, thus - γ = rhdg/2cos θ (4) ### Example Water at 25°C, rises 15.25cm in a capillary of inside diameter 0.02cm. Calculate the surface tension of water, assuming that the density of water is 1.00 g/cm³. γ = 1/2 r hd g = 1/2 x 0.01 x 15.25 x 1.00 x 980.7 = 74.8 dyne/cm.