CH100 - W1

atoms

matter

constructed from the atom

atom

distinct chemical species, comprising a central positively charged nucleus surrounded by 1 or more negatively charged electron

charge of an atom

always electrically neutral

molecules

collection of atoms with a definite structure held together by covalent bonds

covalent bonds

involve the sharing of electrons between neighbouring atoms

ions

chemical species that have either a positive or negative electric charge

cations

ions with a positive charge

anions

ions with a negative charge

element

collection of one type of atom only

compounds

substances containing two or more elements in definite and unchanging proportion

may be composed of molecules or a covalently-bonded network of atoms

ionic compounds

do not have individual molecules

ionic bonding

transferring of electrons from one element to another and forms cations and anions

what is an ionic compound formed from

formed between a metal and a non-metal

covalent compounds formed from

formed between two non-metals

Law of conservation of mass

no detectable gain or loss of mass occurs in chemical reactions, mass is conserved

law of definitive proportion

in a given chemical compound, the elements are always combined in the same proportions by mass

dalton atomic theory (1)

matter consists of tiny particles - atoms

dalton atomic theory (2)

atoms are indestructible, in chemical reactions the atoms rearrange but do not break apart

dalton atomic theory - DAT (3)

in any sample of a pure element, all atoms are identical in mass and other properties

DAT (4)

the atoms of different elements differ in mass and other properties

DAT (5)

when atoms of different elements combine to form a given compound, the constituent atom in the compound are always present in the same fixed ratio

isotopes

atoms of an element with the same number of protons but different numbers of neutrons

radioactive isotopes/ radioisotopes

have unstable nuclei which undergo spontaneous decay to more stable nuclei

Einstein’s equation

E = mc2

the total energy required to break up a nucleus into its constituent protons and neutrons

binding energy

there is a binding energy holding the nucleolus together, but also tells us that some nuclei are unstable

nuclear chemistry

theoretically all nuclei will try to become larger or smaller to attain a stable number of nucleons

fission

nuclei wanting to become smaller - above mass 56

fusion

nuclei wanting to become larger - below mass 56

range of neutrons is limited by

degree of instability - having too many neutrons, too few neutrons

stable nuclei

does not decay spontaneously

unstable nuclei

certain probability to decay

radioactivity

the spontaneous decomposition of an unstable nucleus into a more stable nucleus by releasing fragments of energy

electromagnetic radiation

the shorter the wavelength the more energy it possesses

gamma rays

very energetic

radio waves

not very energetic

types of radioactive decay

* alpha decay


* beta decay
* gamma decay

alpha decay ratio

increases neutrons to proton ratio

beta decay ratio

decreases neutrons to proton ratio

release of particles

alpha and beta decay

release of energy

gamma decay

alpha particle radiation

parent nucleus loses 2 protons and decreases mass by 4

beta particle radiation

loses an electron

gamma

emission of high energy electromagnetic radiation

does not change the isotope or element

alpha particles

relatively heavy and doubly charged

lose energy quickly in matter

beta particles

much smaller and singly charged

interact more slowly with ,after

hazards of alpha emissions

* easily shielded
* considered hazardous if material is ingested or inhaled

hazards of beta emissions

shielded by thin layers of material

considered hazardous if a ingested or inhaled

half life

time required for half of a sample to decay

radioactivity and half life

the level of radioactivity of an isotope is inversely proportional to its half life

the shorter the half-life the more unstable the nuclei

half life of a radionuclide

constant

Dmitri Mendeleev

elements ordered in order of atomic weights

elements with similar properties put into vertical columns called groups

led to discovery of new elements

group numbers

indicate number of valence electrons

metalloid

mix of non-metal and metal properties

electrons occupy regions of space called

orbitals

electron dot and bond line formula

shows bonds between accurate angles and size but distance is exaggerated

space filling models

accurate versions of molecules does not show bonds

electron density molecule

ball and stick with save filling shape

oxyanions

anions containing a central atom surrounded by oxygen atoms

concentration

amount of solute dissolved in a particular volume of solution

concentration formula

c = n/v

n = no. of moles

v = volume in L

dilution

adding more solvent to the solution causing the concentration to decrease

characteristics of atoms

* can combine with one another
* possesses mass
* contain positive nuclei
* contain electrons
* occupy volumes
* has various properties
* attracts one another

electromagnetic radiation

most useful tool for study the structure of atoms

how is electromagnetic described

in terms of waves - characterised by wavelengths and frequency

wavelength

distance between two adjacent identical points of the wave

frequency

number of wave crests passing a given point per unit time

amplitude

height of crest - eg. intensity or strength

characteristics of light

* carries energy
* Albert einstein postulated that light comes in photons
* proportionality between energy and frequency is known as Planck’s constant

Photon

each photon has an energy that is directly proportional to its frequency

Ephoton = hv photon

ground state

lowered energy state of an atom

excited state

when an atom absorbs a photon

energy level diagram

depicts the changes in energy of an atom

atomic spectra

atoms absorb specific frequencies of light

emmission spectrum

the energy of photons emitted by atoms in excited states

quantisation of energy

* a photo with high energy can cause an atom to lose one of its electrons
* implies that absorption of a photon results in an energy gain for an electron in the atom

properties of electrons

* can identify a probable location
* heisenberg uncertainty principle
* electrons are described as delocalised as waves are spread out
* position of a moving electron cannot be precisely defined
* electrons are always spread out rather than located in one particular place because of their wave properties

Heisenberg uncertainty principle

* possible to determine accurately both the momentum and the position of an electron

orbital

region in space about a nucleus where there is a high probability of finding an electron

principle quantum number (n)

as n increases, the energy of the electron increases, its orbitals get bigger and its electrons are less tightly bound to the atom

azimuthal quantum number (l)

* indexes the angular momentum of the orbital
* identifies the shape of the electron distribution within the orbital
* can be 0 or any smaller positive integer than n

spin quantum number

all electrons have a property called spin

pauli exclusion principle

direct consequence of this principle is that any orbital can contain a max of 2 electrons

Aufbau principle

* ground state of an atom is the most stable arrangement of its elements


* we construct the ground state configuration by placing electron in the orbitals starting with lowest in energy

n and l

energy of an orbital

chemical behaviour

determined by the electrons accessible to an approaching atom or molecule

accessible electrons

valence electrons

inaccessible electrons

core electrons

electron configuration

complete specification of how an atoms electrons are distributed in its orbitals

electron-electron repulsion

the lowest energy situation results when electrons occupy the orbitals that keep them furthest apart

hund’s rule

the lowest energy configuration involving orbitals of equal energies is the one with the maximum number of electrons in the same spin orientation

ionisation energies

large and endothermic

energy required to remove an electron

ionisation energies in a periodic table

* ionisation energy decreases down a group
* decreases going left to right across a period

multi-electron atom

can lose more than one electron but ionisation becomes more difficult as cationic charge increases

electron affinity

exothermic

electron affinity within periodic table

EA becomes more favourable across a period

EA becomes less favourable down a group

3 types of interactions within a molecule

* electrons and nuclei attract one another
* electrons repel each other
* nuclei repel each other

bond length

the separation distance at which the molecule has the maximum energetic advantage over the separated atoms

what is bond energy for

energy required to break the bond

bond formation

considered as the sharing of only two electrons

polar

unequal share of electrons