Chapter 2 - Atoms, Molecules, and Ions Flashcards
Who’s studies does Atomic theory come from?
Dalton
What are Dalton’s Postulates?
- all atoms of an element are the same
- atoms may move from one substance to another, but they don’t disappear or turn into other elements
- compounds form from specific atoms in specific compositions
What does the law of conservation of mass state?
in a reaction, there is no change in the mass of atoms
ex: you burn something but mass of atoms stays the same
What does the law of constant composition state?
compounds always have the same elements in the same proportion
ex: H2O is always H2O, not HO1, HO3, etc…
What does the law of multiple proportions state?
elements combine to form compounds in whole number ratios
ex: you can’t have HO1/2
Summary of Thompson’s Cathode Tube Experiment
- electrical charge is passed through a gas from + to - plates
- as electricity passes, - plate emits a ray that is drawn to + plate
- this cathode ray could be deflected by a + or - charge
What conclusion was made from Thompson’s Cathode Tube Experiment?
that the rays were negatively charged and consisted of electrons
Summary of Rutherford’s Gold Foil Experiment
- threw alpha particles at gold foil
- hypothesized that if + and - charger were evenly distributed then the collisions would all be the same
- but most went straight through, some scattered, some bounced back
What hypothesis was formed from Thompson’s Cathode Tube Experiment?
atoms made up of positive and negative particles with charges are distributed evenly - led to the “Plum Pudding Model”
What conclusion was made from Rutherford’s Gold Foil Experiment?
that most of an atom is empty space, so there must be an unequal distribution of charge
What is represented by an atom’s Atomic Number “Z”
the number of protons in the atom
Characteristics of an Atom
- (+) charged nucleus (99.9% of the mass)
- (-) charged electrons
- Are described by their Atomic Number “Z”
Can atoms in a element differ in the number of protons “Z”
no
What is represented by an atom’s Mass Number “A”
the number of protons and neutrons in the atom
Isotope
an atom with a different number of neutrons than protons
How can you find the number of neutrons in an atom
By subtracting the Mass Number “A” by the Atomic Number “Z” (A - Z), which is the top number - bottom number in the nuclear symbol
What do Isotopes do if they are unstable
they decompose - transpose into another element (radioactivity)
Radiation
the energy given off in a decay
Why can’t an atom be weighed
because they’re too small and constantly moving
Atomic Mass
refers to 12C (carbon 12) as a reference
- 12C is defined as 12 AMU exactly
- other atoms are measured in relation to this
How to find atomic mass of an isotope?
Atomic Mass = (mass of isotope 1 x percentage) + (mass of isotope 2 x percentage) + (mass of isotope 3 x percentage)
What are the 3 major divisions of the periodic table?
Metals (left), Nonmetals (right), and Metalloids (middle)
Molecules
combination of 2 or more atoms
ex: H2O, CO, CH4
