ATOMS
THREE POSTULATES OF DALTON’S ATOMIC THEORY
1) An element is composed of identical atoms.
2) Atoms of different elements have different properties.
3) Compounds are atoms of 2 or more elements chemically combined.
THE LAW OF CONSERVATION OF MASS
During a physical change or reaction, or a chemical reaction, matter is neither created nor destroyed.
(Lavoisier – 1777)
THE LAW OF DEFINITE COMPOSITION
A compound always contains the same elements combined in the same proportions by mass. Proposed by Proust in 1779.
Example: H₂O is always 88% Oxygen no matter where it is found.
LAW OF MULTIPLE PROPORTIONS
The same elements may combine to form more than one compound. The ratios of atoms are in small whole numbers. Proposed by Dalton. (For example: H₃O and H₂O₂)
DESCRIBE THE RESEARCH OF J.J. THOMSON
1897- Studied cathode rays in electric and magnetic fields. Identified these rays as negatively charged particles called electrons.
DESCRIBE THE RESEARCH OF MILLIKAN
1909 – Calculated the electric charge and the mass of an electron with his “oil-drop” experiment.
DESCRIBE THE RESEARCH OF RUTHERFORD
1911 – Identified the presence of a small, positively charged atomic nucleus. (Gold foil experiment)
ATOMIC MASS UNIT
The mass of a proton or neutron is equal to 1 atomic mass unit.
symbol- “amu”
1 amu = 1.66 x 10⁻²⁴g
PROTON:
1) LOCATION TN ATOM
2) MASS IN AMU
3)CHARGE
4) SYMBOL
1) atomic nucleus
2) 1 amu
3) + 1
4) p+
ELECTRON:
1) LOCATION IN ATOM
2) MASS IN AMU
3) CHARGE
4) SYMBOL
1) electron cloud surrounding the nucleus
2) 1/1837 amu (sometimes expressed as zero)
3) -1
4) e-
NEUTRON:
1) LOCATION IN ATOM
2) MASS IN AMU
3) CHARGE
4) SYMBOL
1) atomic nucleus
2) 1 amu
3) zero
4) n0
ATOMIC NUMBER
What are the atomic numbers of helium, hydrogen, carbon, oxygen?
The number of protons in the nucleus of an atom of an element.
helium-2
carbon-6
hydrogen-1
oxygen-8
1. MASS NUMBER
2. ISOTOPE
1. The sum of protons plus neutrons in the nucleus of an atom.
2. Atoms which contain the same number of protons but different numbers of neutrons. (Example: hydrogen has 3 isotopes with mass number’s of 1, 2 & 3)
1. MOLECULE
2. ION (LIST EXAMPLES)
1. A group of two or more atoms held together by chemical covalent bonds.
2. Anatom or group of atoms which contains a positive or negative electrical charge. (EXAMPLES: Na+, Cl-, SO₄⁻²)
CATION AND ANION GIVE EXAMPLES
cation – positively charged ion
anion – negatively charged ion
CATIONS: Na+, Fe⁺², NH⁺⁴, Ag⁺
ANIONS· Cl⁻, SO₄⁻² , OH⁻ , P⁻³
1. VALENCE ELECTRONS
2. OXIDATION NUMBER
1. The electrons found in the outermost energy level of an atom.
2. A number (positive or negative) representing the charge on an ion, or the distribution of electrons in an atom involved in a chemical bond.
THREE GENERAL RULES FOR DETERMINING OXIDATION NUMBERS
1) The value is O for atoms of uncombined elements.
2) Hydrogen’s value is +1 (in metallic hydrides: -1 ).
3) Oxygen’s value is -2 (in peroxides: -1 ); (bonded with fluorine: +2).
WHICH ELEMENTS EXHIBIT SEVERAL OXIDATION STATES AND WHY?
The transition elements. Their highest energy sublevels are close in energy (s and d) and easily become involved in bonding.
