chapter3_mhcp



=The Atom: From Philosophical Idea to Scientific Theory=

- The idea of atoms has been around since the ancient Greek era - John Dalton composed a theory of atoms that is still used to explain properties of most modern chemicals. - **Law of Conservation of Mass**: Matter and mass is not created or destroyed in chemical reactions - **Law of definite proportions**: mass ratios of an element in a compound are always the same. - **Law of multiple proportions**: when two or more compounds composed of the same elements, the ratio of the masses of the second element combined with a certain mass of the first element can be seen as a ratio of small whole numbers.

=The Structure of the Atom=

- Cathode-ray tubes make evident the existence of **electrons**; negatively charged subatomic particles with little mass. - Rutherford proved the existence of the atomic **nucleus** by attacking metal foil with a beam of protons. - The nucleus is made up of **protons**, which have an electric charge of +1, and **neutrons** which have no electric charge. - The nucleus have a radius of approximately 0.001 pm (pm = picometers) - Atoms have a radius of approximately 40-270 pm

Mike Halloran, Courtney Pierce Chemistry Period 8 Chapter 3 Atoms: The Building Blocks Of Matter Atom- The word atom is derived from the thoughts of a great scientist in Ancient Greek times Democritus. Atom was based on the Greek meaning-Indivisible. It was thought to be indivisible at the time. However, people doubted the the theory of atoms… One of which was Aristotle. He thought matter was continuous. These ideas inspired many scientists to investigate on the atom. The basic measurement of matter at the time was an element that was thought to be indivisible and that elements combined to make compounds. The thought of Chemical Reactions (a transformation of a substance into a new substance is a chemical reaction) helped the studies of atomic theory This is a portrait of Democritus. in the 1790’s. **The Law of Conservation of Mass** was introduced in the 1790’s. This is the thought that: __Mass is neither created nor destroyed during ordinary chemical reactions or physical changes__. Also, **The** **Law of Definite Proportion** came into effect which stated: __A chemical compound contains the elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound.__ Another law was instated around this time period. That law was **The Law of Multiple Proportions.** This law states that: __If two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers.__  An English science teacher named John Dalton would help explain these laws of science by 5 short statements. 1.) All matter is composed of extremely small particles known as atoms. 2.) Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties. 3.) Atoms cannot be subdivided, created, nor destroyed. 4.) Atoms of different elements combine in simple whole-number ratios A portrait of John Dalton. to form chemical compounds. 5.) In chemical reaction, atoms are combined, separated, or rearranged. Dalton made Democritus’ idea of atoms into a scientific theory that would further be tested by experiments. However, despite Dalton’s great works in the Atomic Theory he was incorrect in some statements such as atoms cannot be subdivided, created, nor destroyed. Also, a given element can have atoms with different masses. The new definition of an Atom is now- The smallest particle of an element that retains the chemical properties of that element. We also now know that the atom has two regions. One being the nucleus and the other is space where electrons are found. The nucleus is made of positively charged Protons and neutrally charged electrons and the extra area is made of some electrons and just extra space. Electrons are negatively charged and they are rather small compared to protons and neutrons. The mass of an electron is 9.109x10-31 kg. The discovery of the electron was in the late 1880’s with the cathode-ray tube experiment. A current was passed through the tube the surface of the tube opposite the cathode glowed. They later thought that the glow was from a stream of particle that were referred to as cathode rays. This lead to two discoveries: Cathode rays were deflected by a magnetic field in the same manner as a wire carrying electric current, which was known to have a negative charge and the rays were deflected away from a negatively charged object. This also proved that the cathode rays are negatively charged. The English Physicist Joseph John Thomson helped back up these statements with a series of experiments throughout 1897. He also named the cathode rays as electrons. Thomson also proved that electrons have a large charge-to-mass ratio. He also proved that electrons are present in atoms of every type of element. Robert A. Millikan further researched the electron and found the measure of the charge of an electron. These discoveries lead to the thoughts that because atoms are electrically neutral they must contain a positive charge to balance the negative electrons and also that because electrons have so much less mass than atoms, atoms must contain other particles that account for most of their mass. Thomson would then propose the idea of the “Plum A portrait of Joseph John Thomson. Pudding Model.” The thought is also like a chocolate chip cookie. The chocolate chips are spread out evenly and yet they don’t make up most of the mass of the cookie. However, this was shortly disproved by the Gold Foil Experiment that was performed by Ernest Ruther ford. This is where alpha particles are shot at a thin piece of gold. They hypothesized that particles would pass through after a slight deflection. However, they were surprised as only 1 in 8000 of the particles had been deflected. This lead The example of the plum pudding model. to the discovery of the positively charged bunch that he called the nucleus. He also said that the volume of the nucleus was very small compared with the total volume of an atom. Rutherford’s student, Niels Bohr, hypothesized that electrons surround the nucleus. The way a nucleus is held together is through Nuclear Forces- These short-ranged proton-neutron, proton-proton, and neutron-neutron forces hold the nuclear particles together.

A Mole: It would be much too hard to count atoms individually so scientists invented a special unit used to express amounts of particles. Also, they started to organize the elements. Scientists decided implement atomic numbers which are simply just the number of protons of each atom of that element. This classification lead to findings of Isotopes. Isotopes are atoms of the same element that have different masses. This occurs because elements are neutral so they can have multiple amounts of neutrons. For example Protium, deuterium, and tritium are all isotopes of hydrogen because they have one proton but they have different neutrons. Also, to find Isotopes you need to know the Mass Number. To find this you simple take the number of protons and neutrons and add them together. Also, scientist started using Relative Atomic Masses and Atomic Mass Unit. Atomic Mass Unit is a measurement that is based around carbon-12. One atomic mass unit (also referred to as AMU) is exactly 1/12 the mass of a carbon-12 atom. However Average Atomic Mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element. These measurements were vital in using the measurement of a mole. A mole is used in counting the number of particles there are in an element or compound. Also, Alvogadro’s Number is vital in understanding the use of moles. Alvogadro’s Number = 6.022x1023. This is used in calculating the number of particles in a mole. Molar mass is the mass of a mole of a substance that is written G/mole.