Download Ch # 5 Notes

Chem 160
Ch # 5- Atoms, Molecules, Formulas and Subatomic Particles.
Atom: An atom is the smallest particle of an element that can exist and still have
properties of the element.
Atomic Theory of matter:
1) All matter is made up of small particles called atoms.113 types.
2) Atoms of same element are similar to one another.
3) The relative number and arrangement of different types of atoms contained in a
pure substance determined its identity.
4) These atoms rearrange, separate or unite to form a new substance.
5) Atoms can participate in or result from any chemical change, atoms cannot be
destructed.
1 atom= 10-10 m(diameter)
1 atom= 10-23 g(mass)
The Molecule: A group of two or more atoms that is strongly bound to each other.
Diatomic molecule: contains 2 atoms.H,N,O,F,Cl,Br,I,
Triatomic molecule: Contains 3 atoms.
P-4 atoms, S-8 atoms.
Homoatomic molecule: All atoms are of same kind.
Heteroatomic molecule: Two or more different kinds of atoms are present.
Practice Ex:5.1, P.126.
Molecular compounds: Compounds that have heteroatomic molecule as basic structural
unit. Ex: table sugar-sucrose. Molecule is a limit of physical sub-division. Atom is a limit
of chemical sub-division. Two kinds of atoms present, but, one kind of molecule.
Molecules have different properties than that of the atom.
Ionic compounds: Compounds that contain ions. Ex: NaCl.
Natural and Synthetic compounds: The compounds could be naturally or artificially
made.
Chemical Formulas: Made up of symbols. The subscripts indicate the number of atoms.
Read the formula C6H12O6
Practice Ex: 5.2. P.130.
Subatomic Particles : Protons, Neutrons and Electrons.
Subatomic Particle: A very small particle that is a building block of atoms.
Three subatomic particles are: protons, electrons, neutrons.
Protons: +ve charge.-Goldstein. Charge: +1
Electron:-ve charge.- Thomson. Smallest mass.Charge:-1
Neutron: No charge.-Chadwick.
Opposite charges attract, like charges repel.
Arrangement of subatomic particles:
Nucleus: Small, dense, positively charged center of an atom. It contains protons and
neutrons. An atoms entire mass is concentrated to the center.
Nucleon: Any subatomic particle found in the nucleus of an atom. Ex; protons and
neutrons.
Electrons revolve around the nucleus in a very large region. It is mostly empty space.
Volume occupied by electrons is called electron cloud. It is negatively charged.
Charge neutrality of an atom: An atom as a whole is neutral. Number of protons=Number
of electrons in any atom. The +ve and-ve charges cancel each other out.
Size relationships within an atom: Diameter of nucleus= 10-15 m. Entire mass is
concentrated in the center of the atom.
Additional Subatomic Particles: Protons and neutrons are made of Leptons, Mesons,
Baryons.
Atomic Number and Mass Number:
Atomic Numbers
Number of protons present in the nucleus.
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The atomic number of an atom determines which element the atom is.
Every atom with an atomic number of 1 is a hydrogen atom.
Every hydrogen atom contains 1 proton in its nucleus.
Elements in periodic table are arranged according to increasing atomic number.
Element: A pure substance in which all atoms present have the same atomic number, all
atoms have the same number of protons.
Atomic number Z= number of protons=number of electrons.
Atomic mass number: A: atomic mass number=number of protons+ number of neutrons.
Subatomic Particle Makeup of An Atom: Z= # protons= # electrons.
A= # protons + # neutrons.
# neutrons= A-Z.
Practice Ex:5.3, P.136.
Practice, Ex:5.4, P.137.
Isotopes: Atoms of an element that have the same number of protons, but, different
number of neutrons. They have same atomic number, but, different atomic mass numbers.
Isotopes of an element have the same chemical properties.
Isotopes of an element have different physical properties, as they have different neutrons
and thus different mass.
% abundance: Percent of atoms in a natural sample of a pure element that are a particular
isotope of the element.
Isobars: They have same mass numbers, but different atomic numbers.
Practice. Ex. 5.5, P.139.
Atomic Masses: Weighted average mass is called atomic mass.
The listed atomic mass of an element is the average relative mass of the isotopes of that
element compared to the mass of carbon-12 (exactly 12.0000…amu).
Relative Mass: Related to.
Average atom: Common mass value used for atoms is called as the average mass.
Mass of an “average atom” does not vary.
Weighted averages:
1) The number of isotopes that exist.
2) Isotope mass for each isotope, that is, relative mass on C-atom scale.
3) % abundance of each isotope.
Practice, Ex: 5.7, 5.8, P.144.
Evidence supporting the Existence and arrangement of Subatomic Particles:
Discharge Tube Experiments:
1. Gas discharge tubes: Gases at low pressure conduct electricity.
2. Apparatus contains sealed glass tube with electrodes. Electrodes are connected to
a source of electrical power.
3. +ve-anode, -ve-cathode for electrodes.
4. Side arm for vacuum pump. Amount of gas can be varied.Smaller the gas lower is
the pressure.
5. Electrical conductance continues to give a greenish glow called cathode rays.
6. Cathode rays caused certain materials to glow.
7. Cathode rays could be deflected by charged plates or magnetic field.Rays were
attracted to +ve end and thus called electrons. This was discovered by Thomson.
8. Goldstein called cathode rays passing through channels canal rays. Simple canal
rays were called protons.
9. Thomson proposed that atom was a positive sphere containing most mass,
surrounded by negative electrons. Plum pudding.
The Rutherford Experiment/Metal Foil Experiment:
1.Most of the alpha particles passed through the foil with little or no deflection.
2.He found that a few were deflected at large angles and some alpha particles even
bounced back.
3.Rutherford knew that like charges repel.
4.Rutherford concluded that each gold atom contained a positively charged mass that
occupied a tiny volume. He called this mass the nucleus.
5.Most of the alpha particles passed through the gold foil. This led Rutherford to
conclude that a gold atom was mostly empty space.
6.Because alpha particles have relatively high masses, the extent of the deflections led
Rutherford to conclude that the nucleus was very heavy and dense.