Atoms and Molecules PDF

An atom is the smallest unit of ordinary matter that forms a chemical elemen ---------------------------------------------------------------------------- History of atomic theory ------------------------ ### Dalton\'s law of multiple proportions In the early 1800s, [John Dalton](https://en.wikipe...

An atom is the smallest unit of ordinary matter that forms a chemical elemen ---------------------------------------------------------------------------- History of atomic theory ------------------------ ### Dalton\'s law of multiple proportions In the early 1800s, [John Dalton](https://en.wikipedia.org/wiki/John_Dalton) compiled experimental data gathered by himself and other scientists and noticed that [chemical elements](https://en.wikipedia.org/wiki/Chemical_element) seemed to combine by weight in ratios of small whole numbers. This pattern is now known as the \"[law of multiple proportions](https://en.wikipedia.org/wiki/Law_of_multiple_proportions)\". The simple ratios suggested that elements combine in multiples of basic units of mass, which Dalton concluded were atoms. ### Kinetic theory of gases Unlike Dalton\'s atomic theory, the kinetic theory of gases describes not how gases react chemically with each other to form compounds, but how they behave physically: diffusion, viscosity, conductivity, pressure, etc. ### Brownian motion In 1827, [botanist](https://en.wikipedia.org/wiki/Botany) [**Robert Brown**](https://en.wikipedia.org/wiki/Robert_Brown_(botanist,_born_1773)) used a microscope to look at dust grains floating in water and discovered that they moved about erratically, a phenomenon that became known as \"[Brownian motion](https://en.wikipedia.org/wiki/Brownian_motion)\". This was thought to be caused by water molecules knocking the grains about. In 1905, **[Albert Einstein](https://en.wikipedia.org/wiki/Albert_Einstein) **proved the reality of these molecules and their motions by producing the first [statistical physics](https://en.wikipedia.org/wiki/Statistical_physics) analysis of [Brownian motion](https://en.wikipedia.org/wiki/Brownian_motion). French physicist [Jean Perrin](https://en.wikipedia.org/wiki/Jean_Perrin) used Einstein\'s work to experimentally determine the mass and dimensions of molecules, thereby providing physical evidence **for the particle nature of matter.** ### Discovery of the electron In 1897, [**J. J. Thomson**](https://en.wikipedia.org/wiki/J._J._Thomson) discovered that [cathode rays](https://en.wikipedia.org/wiki/Cathode_ray) are not electromagnetic waves but made of particles that are 1,800 times lighter than [hydrogen](https://en.wikipedia.org/wiki/Hydrogen) (the lightest atom). Therefore, they were not atoms, but a new particle, the first [*subatomic*](https://en.wikipedia.org/wiki/Subatomic_particle) particle to be discovered. He called these new particles *corpuscles* but they were later renamed [*electrons*](https://en.wikipedia.org/wiki/Electron). ### Discovery of the nucleus Between 1908 and 1913, **Rutherford** and his colleagues performed a series of experiments in which they bombarded thin foils of metal with alpha particles. They spotted alpha particles being deflected by angles greater than 90°. To explain this, Rutherford proposed that the positive charge of the atom is not distributed throughout the atom\'s volume as Thomson believed, but is concentrated in a tiny nucleus at the center. Only such an intense concentration of charge could produce an electric field strong enough to deflect the alpha particles as observed. ### Discovery of isotopes While experimenting with the products of [radioactive decay](https://en.wikipedia.org/wiki/Radioactive_decay), in1**913 [radiochemist](https://en.wikipedia.org/wiki/Radiochemistry) [Frederick Soddy](https://en.wikipedia.org/wiki/Frederick_Soddy) **discovered that there appeared to be more than one type of atom at each position on the [periodic table](https://en.wikipedia.org/wiki/Periodic_table). The term [isotope](https://en.wikipedia.org/wiki/Isotope) was coined by [Margaret Todd](https://en.wikipedia.org/wiki/Margaret_Todd_(doctor)) as a suitable name for different atoms that belong to the same element. J. J. Thomson created a technique for [isotope separation](https://en.wikipedia.org/wiki/Isotope_separation) through his work on [ionized gases](https://en.wikipedia.org/wiki/Ionized_gas), which subsequently led to the discovery of [stable isotopes](https://en.wikipedia.org/wiki/Stable_isotope) ### Bohr model https://upload.wikimedia.org/wikipedia/commons/thumb/1/17/Bohr\_atom\_animation\_2.gif/220px-Bohr\_atom\_animation\_2.gif The Bohr model of the atom, with an electron making instantaneous \"quantum leaps\" from one orbit to another with gain or loss of energy. [***Bohr model***](https://en.wikipedia.org/wiki/Bohr_model) In 1913 the physicist [**Niels Bohr**](https://en.wikipedia.org/wiki/Niels_Bohr) proposed a model in which the electrons of an atom were assumed to orbit the nucleus but could only do so in a finite set of orbits, and could jump between these orbits only in discrete changes of energy corresponding to absorption or radiation of a photon. ### Discovery of the neutron The development of the [mass **spectrometer**](https://en.wikipedia.org/wiki/Mass_spectrometry) allowed the mass of atoms to be measured with increased accuracy. The device uses a magnet to bend the trajectory of a beam of ions, and the amount of deflection is determined by the ratio of an atom\'s mass to its charge. The chemist [**Francis William Aston**](https://en.wikipedia.org/wiki/Francis_William_Aston) used this instrument to show that isotopes had different masses. The [atomic mass](https://en.wikipedia.org/wiki/Atomic_mass) of these isotopes varied by integer amounts, called the [whole number rule](https://en.wikipedia.org/wiki/Whole_number_rule). **The Structure of the Atom** Atoms are made up of particles called protons, neutrons, and electrons, which are responsible for the mass and charge of atoms. - - - - **Key Terms** - - - 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 molecules. Many biological processes are devoted to breaking down molecules into their component atoms so they can be reassembled into a more useful molecule. **Atomic Particles** Atoms consist of three subatomic particles: protons, electrons, and neutrons. The nucleus (center) of the atom contains the protons (positively charged) and the neutrons (no charge). The outermost regions of the atom are called electron shells and contain the electrons (negatively charged). Atoms have different properties based on the arrangement and number of their basic particles. The hydrogen atom (H) contains only one proton, one electron, and no neutrons. This can be determined using the atomic number and the mass number of the element (see the concept on atomic numbers and mass numbers).  **Structure of an atom**: Elements, such as helium, depicted here, are made up of atoms. Atoms are made up of protons and neutrons located within the nucleus, with electrons in orbitals surrounding the nucleus. **Atomic Mass** Protons and neutrons have approximately the same mass, about 1.67 × 10^-24^ grams. Scientists define this amount of mass as one atomic mass unit (amu) or one Dalton. Although similar in mass, protons are positively charged, while neutrons have no charge. Therefore, the number of neutrons in an atom contributes significantly to its mass, but not to its charge. image **Protons, neutrons, and electrons**: Both protons and neutrons have a mass of 1 amu and are found in the nucleus. However, protons have a charge of +1, and neutrons are uncharged. Electrons have a mass of approximately 0 amu, orbit the nucleus, and have a charge of -1. **Exploring Electron Properties**: Compare the behavior of electrons to that of other charged particles to discover properties of electrons such as charge and mass. **Atomic Number and Mass Number** The atomic number is the number of protons in an element, while the mass number is the number of protons plus the number of neutrons.  **Atomic number, chemical symbol, and mass number**: Carbon has an atomic number of six, and two stable isotopes with mass numbers of twelve and thirteen, respectively. Its average atomic mass is 12.11. Scientists determine the atomic mass by calculating the mean of the mass numbers for its naturally-occurring isotopes. Often, the resulting number contains a decimal. For example, the atomic mass of chlorine (Cl) is 35.45 amu because chlorine is composed of several isotopes, some (the majority) with an atomic mass of 35 amu (17 protons and 18 neutrons) and some with an atomic mass of 37 amu (17 protons and 20 neutrons). Given an atomic number (Z) and mass number (A), you can find the number of protons, neutrons, and electrons in a neutral atom. For example, a lithium atom (Z=3, A=7 amu) contains three protons (found from Z), three electrons (as the number of protons is equal to the number of electrons in an atom), and four neutrons (7 -- 3 = 4). **Isotopes** Isotopes are various forms of an element that have the same number of protons, but a different number of neutrons. - - - - **Key Terms** - - - - **What is an Isotope?** 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 and neutrons present. - - Molecule ======== A **molecule** is an [electrically](https://en.wikipedia.org/wiki/Electrically) neutral group of two or more [atoms](https://en.wikipedia.org/wiki/Atom) held together by [chemical bonds](https://en.wikipedia.org/wiki/Chemical_bond). Molecules are distinguished from [ions](https://en.wikipedia.org/wiki/Ion) by their lack of [electrical charge](https://en.wikipedia.org/wiki/Electrical_charge). According to [Merriam-Webster](https://en.wikipedia.org/wiki/Merriam-Webster) and the [Online Etymology Dictionary](https://en.wikipedia.org/wiki/Online_Etymology_Dictionary), the word \"molecule\" derives from the [Latin](https://en.wikipedia.org/wiki/Latin) \"[moles](https://en.wikipedia.org/wiki/Mole_(unit))\" or small unit of mass. **Molecule** (1794) -- \"extremely minute particle\", **Two types** A molecule may be homonuclear, that is, it consists of atoms of one [chemical element](https://en.wikipedia.org/wiki/Chemical_element), as with two atoms in the [oxygen](https://en.wikipedia.org/wiki/Oxygen) molecule (O~2~); or it may be [heteronuclear](https://en.wikipedia.org/wiki/Heteronuclear), a [chemical compound](https://en.wikipedia.org/wiki/Chemical_compound) composed of more than one element, as with [water](https://en.wikipedia.org/wiki/Water_(molecule)) (two hydrogen atoms and one oxygen atom; H~2~O). Atoms and complexes connected by [non-covalent interactions](https://en.wikipedia.org/wiki/Non-covalent_interactions), such as [hydrogen bonds](https://en.wikipedia.org/wiki/Hydrogen_bond) or [ionic bonds](https://en.wikipedia.org/wiki/Ionic_bond), are typically not considered single molecules.[^\[10\]^](https://en.wikipedia.org/wiki/Molecule#cite_note-10) Molecular science ----------------- The science of molecules is called *molecular chemistry* or [*molecular physics*](https://en.wikipedia.org/wiki/Molecular_physics), depending on whether the focus is on chemistry or physics. Molecular chemistry deals with the laws governing the interaction between molecules that results in the formation and breakage of [chemical bonds](https://en.wikipedia.org/wiki/Chemical_bond), while molecular physics deals with the laws governing their structure and properties. ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Bonding ------- Molecules are held together by either [covalent bonding](https://en.wikipedia.org/wiki/Covalent_bonding) or [ionic bonding](https://en.wikipedia.org/wiki/Ionic_bonding). Several types of non-metal elements exist only as molecules in the environment. For example, hydrogen only exists as hydrogen molecule. A molecule of a compound is made out of two or more elements. ### Covalent [***Covalent bonding***](https://en.wikipedia.org/wiki/Covalent_bonding) https://upload.wikimedia.org/wikipedia/commons/thumb/1/19/Covalent\_bond\_hydrogen.svg/220px-Covalent\_bond\_hydrogen.svg.png A covalent bond forming H~2~ (right) where two [hydrogen atoms](https://en.wikipedia.org/wiki/Hydrogen_atom) share the two electrons A covalent bond is a [chemical bond](https://en.wikipedia.org/wiki/Chemical_bond) that involves the sharing of [electron pairs](https://en.wikipedia.org/wiki/Electron_pair) between [atoms](https://en.wikipedia.org/wiki/Atom). These electron pairs are termed *shared pairs* or *bonding pairs*, and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is termed *covalent bonding*. ### Ionic [***Ionic bonding***](https://en.wikipedia.org/wiki/Ionic_bonding)  [Sodium](https://en.wikipedia.org/wiki/Sodium) and [fluorine](https://en.wikipedia.org/wiki/Fluorine) undergoing a redox reaction to form [sodium fluoride](https://en.wikipedia.org/wiki/Sodium_fluoride). Sodium loses its outer [electron](https://en.wikipedia.org/wiki/Electron) to give it a stable [electron configuration](https://en.wikipedia.org/wiki/Electron_configuration), and this electron enters the fluorine atom [exothermically](https://en.wikipedia.org/wiki/Exothermic). Ionic bonding is a type of [chemical bond](https://en.wikipedia.org/wiki/Chemical_bond) that involves the [electrostatic](https://en.wikipedia.org/wiki/Electrostatic) attraction between oppositely charged [ions](https://en.wikipedia.org/wiki/Ion), and is the primary interaction occurring in [ionic compounds](https://en.wikipedia.org/wiki/Ionic_compound). The ions are atoms that have lost one or more [electrons](https://en.wikipedia.org/wiki/Electron) (termed [cations](https://en.wikipedia.org/wiki/Cation)) and atoms that have gained one or more electrons (termed [anions](https://en.wikipedia.org/wiki/Anion)).This transfer of electrons is termed *electrovalence* in contrast to [covalence](https://en.wikipedia.org/wiki/Covalent_bond). In the simplest case, the cation is a [metal](https://en.wikipedia.org/wiki/Metal) atom and the anion is a [nonmetal](https://en.wikipedia.org/wiki/Nonmetal) atom, but these ions can be of a more complicated nature, e.g. molecular ions like NH~4~^+^ or SO~4~^2−^.