SEM 1 EXAM Flashcards
how does nucleus stay together
electrostatic repulsion forces protons apart, but strong nuclear force attracts nuclear particles together
if nuclear force is balanced, atoms is stable
what happens if nuclear forces are unbalanced
then the nucleus will be unstable and decay over time
isotopes
atoms of an element with the same number of protons, but different number of neutrons.
chemical properties are similar (atom behaviour in chem reaction is based on arrangement and number of electrons.
different physical properties (mass and nuclear stability)
chemical and physical properties
how elements participate in chemical reaction
feature one can measure and observe
radioactive decay of isotopes
if forces in atom are balanced, atom is stable
if not balanced, atom is unstable and nucleus undergoes radioactive decay to become stable.
in radioactive decay, radiation is emitted, which is useful for radioactive dating and medical diagnosis and treatment
half-life
time taken for half the original sample to decay
relative atomic mass
compares the mass of atom to the mass of another atom
all atoms are compared to carbon 12
isotopes have different masses and abundances on earth
weighted average of the relative masses of the isotopes of an element relative to carbon12
Democritus
disagreed with the idea of infinitely divisible matter, proposed that matter consisted of tiny particles with nothing between them but empty space called “atomos”
dalton
elements composed of atoms
an atom of same element have a same properties, atoms of different elements have different properties
atoms not created nor destroyed or changed into different types during reactions
c reaction is separation, combination , rearrangement of atoms
compounds are combinations of atoms in specific ratio
thomson
found electrons
realised that atoms could be divided further
plum pudding model: electrons embedded in positively changed mass
rutherford
proposed the atom consisted mostly of empty space
electrons orbited around nucleus. most of mass was in the nucleus, all positive charge was located in the nucleus carried by protons
bohr
electrons were in electrons shells around the nucleus, each shell has a specific amount of energy
Chadwick
discovered the existence of neutrons that accounted for around half the mass of the nucleus, had neutral charge
Electrostatic attraction
related to distance the valence electrons are from the nucleus
strength of attraction of protons and electrons
shielding effect
related to number of inner shell electrons, shield the effect of the protons from valence electrons.
reduces ionisation energy
correlates to number of inner electron shells and attraction of valence electrons to nucleus
core charge
how defective the charge of the nucleus is at holding the valence electrons in
increases going towards noble gases
atomic radii
decrease going right
increase going down
electronegativity
ability of atom to attract electron
increases going right
decrease going down as electron is further from nucleus so it is harder to attract
ionisation energy
energy required to remove loosely bound electron
depends on
nuclear charge: more nuclear charge means more protons in nucleus which increases ionisation energy because valence electrons are tightly held on.
distance form nucleus: further distance decreases electrostatic attraction between valence electrons and nucleus. valence electrons are loosely held.
increases towards right
decreases going down because valence shell is further away making it easier to lose electrons
electron affinity
ability to accept electron and from a negative ion
metallic character
set of properties associated with metals
depends on ability to lose valence electrons
decrease going right
increase going down
ground state
shells next to nucleus are taken up by electrons
when electron is at its lowest possible energy levels
how electrons jump
atoms absorb energy, electron around nucleus gain this extra energy and move up to higher energy levels.
electrons can be so excited that they leave (ionisation)
energy levels are discreet. electrons can only exist in specific energy levels
electrons cannot exist between energy levels, therefore
amount of energy absorbed and emitted = difference in one energy level and another. electrons could be excited from one energy level to another by specific amounts of energy that corresponds to the difference in energy levels.
amount of energy absorbed by any sample of one element is the same
excited state
an atom in which electrons occupy higher energy levels than the lowest possible energy levels.
as it moves back to ground state, emits light