Module 2: Foundations In Chemistry Flashcards
At the start of the 19th century what did Dalton describe the atom as?
john Dalton described the atoms as solid spheres
what did Jj Thompson do in 1897?
he carried out experiments in which his measurements of charge and mass showed that an atom must contain small negatively charged particles he called ‘corpuscles’ (electrons)
what was the solid sphere idea replaced with?
the plum pudding model
what happened in 1909?
Geiger, marsden and ruthersford conducted the gold foil experiment, they fired alpha particles at thin gold foil,
- from the model they were expecting most of the particles to be deflected very slightly by the ‘positive’ pudding of the atom
what happened instead in the alpha scattering experiment?
most of the particles passed straight through the gold atoms and a small number were deflected backwards which showed the PPM was wrong
what new model did Rutherford come up with?
the nuclear model
what did Henry Moseley discover?
that the charge of the nucleus increased from one element to another, which led Rutherford to investigate further, he discovered that it contained positively charged particles - protons.
- the charges of nuclei were explained that the atoms of elements have different number of protons in their nucleus
what did Rutherford predict after he realised there was a problem that the nuclei of the atoms where heavier than they would’ve been if they just had protons?
that there were other particles in the nucleus, that had mass but no change. and the neutron was discovered by james chadwick
why did neils bohr propose a new model?
after scientists realised that electrons in a cloud around the nucleus of atom would spiral down into the nucleus causing the atom to collapse
what did Bohr’s model entail?
- electrons can only exist in fixed orbits
- each shell has a fixed energy
- when an electron moves around, it emits or absorbs electromagnetic radiations
- because the energy of shells is fixed, the radiation will have a fixed frequency
what did the Bohr model explain?
explained why some elements (the noble gases) are inert, he said that the shells of an atom can only hold fixed numbers of electrons and that an elements reactivity is due to its electrons.
- atoms will react to gain full shells and when an atom has full shells its stable and doesn’t react
what are isotopes?
isotopes of an element are atoms with the same number of protons but different numbers of neutrons
what do isotopes have?
they have the same configuration of electrons, so they’ve got the same chemical properties but slightly different physical properties eg their densities
what is relative atomic mass?
the weighted mean mass of an atom of an element compound to 1/12th the mass of one carbon-12
how do you work out the relative atomic mass?
- multiply each relative isotopic mass by its percentage relative isotopic abundance and add up the results
- divide by 100
what is relative molecular mass (or relative formula mass) Mr?
the average mass of a molecule or formula unit, compared to 1/12th of the mass of an atom of Carbon-12
what is mass spectra?
they are produced by mass spectrometers and is a useful way of measuring the relative atomic mass and can also be used to measure the relative molecular mass, Mr
how can the mass spectra be used to work out the relative atomic masses of different elements?
- multiply each relative isotopic mass by its relative isotopic abundance and add up results
- divide by the sum of the isotopic abundances
what are electrons?
they are shells orbiting the nucleus, each shell corresponds to an energy level in the atom
what is the first shell?
`it has the principal quantum number n=1, the first shell is closest to the nucleus and lowest in energy. As ‘n’ increases so does the energy level
what are shells divided into?
sub shells - different electron shells have different numbers of sub shells, they are called s-, p-, d- and f-.
what do the sub shells have?
different numbers of orbitals which can each hold up to 2 electrons
what is an orbital?
its a bit of space that an electron moves in, they have the same energy as the sub shell they’re within
what happens when two electrons are in an orbital?
they have to spin in opposite directions
what do 4s sub shells have?
- they have a lower energy level than the 3rd sub shell, even though its principal quantum number is bigger therefore it fills up first.
- in general electrons fill orbitals with the same energy singly before they start sharing
What are the charges and masses of the protons neutrons and electrons?
Protons - mass 1 - +1
Neutron - mass 1 - 0
Electron - 1/2000 - -1
How many sub shells and electrons are there in each shell?
1st - sub shell 1s - 2 é
2nd - sub shells 2s 2p - 8 é
3rd - sub shells 3s 3p 3d - 18 é
4th - sub shells 4s 4p 4d 4f - 32 é
when are ions formed?
when electrons are transferred from one atom to another in order to have full outer shells
what is electrostatic attraction?
it holds the positive and negative ions together, its very strong
what’s an ionic bond?
its an electrostatic attraction between two oppositely charged ions
- when oppositely charged ions form an ionic bond you get an ionic compound where the positive charge in the compound balances the negative charge so the total overall charge is zero
what is the structure of a NaCl compound like?
- the Na+ and Cl- ions are packed closely together alternately in a regular structure called a lattice, its giant as its made up of the same basic unit over again
- it forms as each ion is electrostatically attracted in all directions to ions of opposite charge
- its cube shaped and has a high MP due to strong bonds
when do ionic compounds conduct electricity?
- when they’re molten or dissolved not when they’re solid as the ions in a liquid are mobile and carry a charge. in a solid they’re in a fixed position by the strong ionic bonds
do ionic compounds having high melting and boiling points?
- yes as they are held by strong electrostatic forces. it takes lots of energy to overcome these forces
why do ionic compounds tend to dissolve in water?
- water molecules are polar meaning part of it is negative and part of it is positive. the water molecules are attracted to the charged ions which then pull away from the lattice and cause it to dissolve
what is a covalent bond?
the strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms
what is average bond enthalpy?
measure the energy needed to break a covalent bond
- the stronger the bond is, the more energy is required to break it and so the greater the value of the average bond enthalpy
what is dative covalent bonding?
where both electrons come from one atom
eg ammonium ion, NH4+ is formed by dative covalent bonding. it forms when the nitrogen atom in a ammonium molecule donates a pair of electrons to a proton (H+)
what do the shapes of molecules and molecular ions depend on?
the number of pairs of electrons in the outer shell of the central atom
what will electron pairs do?
they will repel each other as much as they can, the types of electron pair affects how much it repels other electron pairs
what do lone pairs do?
they repel more than bonding pairs, this means the greatest angles are between lone pairs of electrons and bond angles between bonding pairs are often reduced as they are pushed together by lone pair repulsion
what’s the order of bond angles?
- lone pair/lone pair angles are the biggest
- lone pair/bonding pair angles are the 2nd biggest
- bonding pair/bonding pair angles are the smallest
what is electron pair repulsion theory?
the way of predicting a molecules shape
what is methane’s shape?
- no lone pairs
- angles are 109.5
- wedge, dashed line, 2 norm
what’s ammonias shape?
- 1 lone pair
- angles are 107
- 2 x’s, dashed line, wedge
what is BeCl2 and CO2’s shape?>
- shape is linear molecule
- angle 180
- 2 electron pairs around central atom
what is BF3’s shape?
- shape is trigonal planar
- angle is 120
- 3 electron pairs around central atom
- no lone pairs
what is NH4+’s shape?
- shape is tetrahedral
- angle 109.5
- 4 electron pairs around central atom
- no lone pairs
- arrow up, dashed line, wedge
what is PF3’s shape?
- shape is trigonal pyramidal
- angle 107
- 4 electron pairs around central atom
- 1 lone pair
- dashed line, wedge
what is H2O’s shape?
- shape is nonlinear/bent
- angle is 104.5
- 4 electron pairs around central atom
- 2 lone pairs
- 4 x’s,
what is PCl5’s shape?
- shape is trigonal bipyramidal
- angle 90 and 120
- 5 electron pairs around central atom
- no lone pairs
- dashed line and wedge - 120
what is SF6’s shape?
- shape is octahedral
- angle 90
- 6 electron pairs around central atom
- no lone pairs
- 2 wedges, 2 dashed lines
what is electronegativity?
an atoms ability to attract the electron pair in a covalent bond
what is the most electronegative element?
- fluorine, oxygen, nitrogen and chlorine are too
- electronegativity increases across period and decreases down a group
- its measures using the Pauling scale, the greater the value the higher its electronegativity
what makes a bond polar?
- in a covalent bond between two atoms with different electronegativities, the bonding pairs are pulled towards the more electronegative atom which makes it polar
what is a dipole?
- in a covalent bond, the difference in electronegativity between two atoms causes a permanent dipole.
- a dipole is a difference in charge between the two atoms caused by a shift in electron density in the bond. the difference in electronegativity, the more polar the bond
what is the covalent bonds in diatomic gases like?
- they are non polar because the atoms have equal electronegativities and so the electrons are equally attracted to both nuclei
- some like carbon and hydrogen have similar electronegativities so bonds between them are non polar
what do polar bonds have?
- permanent dipoles. the arrangement of polar bonds in a molecule determines whether or not the molecule will have an overall dipole.
- if the polar bonds are arranged symmetrically so that the dipoles cancel each other out, then the molecule has no overall dipole and is non polar eg CO2
what happens when polar bonds aren’t arranged so that they cancel each other out?
the charge is arranged unevenly across the whole molecule, and it will have an overall dipole, molecules with an overall dipole are polar eg H20
- to work out whether a molecule has a overall dipole, you draw it out 3D with partial charges and see if they cancel out
where can bonds be purely covalent?
- bonds between atoms of a single element, like diatomic gases as the electronegativity difference between the atom is zero so the bonding electrons are arranged evenly within the bond
what do most compounds come between?
- between the two extremes of covalent and ionic meaning they often have ionic and covalent properties
- you can use electronegativity to predict what type of bonding will occur between two atoms. the higher the difference the more ionic the bonding becomes
what are induced dipole-dipole forced (IDDF)?
- they cause all atoms and molecules to be attracted to each other even noble gases, this is because electrons in charge clouds are always moving fast.
- the electrons in an atom are likely to be more to one side than the other, the atom would have a temporary dipole.
what can a temporary dipole do?
- that dipole can cause another temporary (induced) dipole in the opposite direction on a neighbouring atom. the two dipoles are then attracted to each other
- the dipoles are constantly being created and destroyed, the overall effect Is for the atoms to be attracted to each other
what do large molecules have?
- they have large electron clouds meaning stronger IDDF, molecules with greater surface areas also have stronger IDDF as they have a bigger exposed electron cloud
what happens when you boil a liquid?
- you need to overcome the intermolecular forces so that the particles can escape from the liquid surface, you need more energy to overcome stronger forces so liquids with stronger IDDF will have higher BP
- in Grp 4 hydrides going down the group the IDDF increase because of the number of shells of electrons increases and so atomic size increases
how do IDDF hold iodine molecules in a lattice?
- iodine atoms are held together in pairs by strong covalent bonds to from molecules of I2
- the molecules are held together in a molecule lattice arranged by weak IDDF
what do delta positive and negative charges on polar molecules cause?
- they cause weak electrostatic forces of attraction between molecules. they are permanent dipole-dipole interactions, they occur in addition to induced dipole dipole interactions
what is hydrogen bonding?
- it can only happen when hydrogen is covalently bonded to fluorine, nitrogen or oxygen.
- hydrogen has a high charge density as small, and fluorine, nitrogen and oxygen are very electronegative. the bond is polarised so a weak bond forms between the hydrogen of one molecule and a lone pair of electrons on the F, N, O or another
what effect does hydrogen bonding have on substances?
- they are soluble in water and have higher boiling and freezing points than molecules of a similar size that are unable to form hydrogen bonds.
- water, ammonia and hydrogen fluoride have the highest boiling points when compared with other hydrides in their groups as they need extra energy to break bonds
how is ice held?
- molecules of h20 are held together in a lattice by hydrogen bonds.
- H bonds hold water molecules apart in an open lattice
- the water molecules in ice are further apart than in water
- solid ice is less dense than water and floats
describe the boiling points of group 7 hydrides?
- it increases from HCl to h although the permanent dipole dipole interactions are decreasing the number of electrons in the molecules increases so the strength of the IDDF also increases
- if you have two molecules with similar number of e, the strength of their IDDI will be similar. if 1 substance has molecules that are more polar than the other, it will have stronger permanent dipole dipole interaction and a higher BP
why do simple covalent compounds have low melting and boiling points?
- the intermolecular forces that hold together the molecules in simple covalent compounds are weak so don’t need much energy to break. the melting and boiling are normally low - they’re often liquids or gases at room temp. as IF get stronger, melting and boiling points increase
why are polar molecules soluble in water?
- water is a polar molecule, so only tends to dissolve other polar substances. compounds with H bonds, such as ammonia or ethanoic acid can form H bonds with water molecules, so will be soluble
- molecules that only have IDDF eg CH4 will be insoluble
why don’t covalent compounds conduct electricity?
- some covalent molecules have permanent dipoles, overall covalent molecules are uncharged, so they cant conduct electricity
how do melt or boil a simple covalent compound?
- you have to overcome the IF that hold the molecules together
- you don’t need to break the much stronger covalent bonds that held the atoms together in the molecules, that’s why they have low melting and boiling points
what is the ideal gas equation?
pV = nRT
- p is pressure Pa
- T is temp
- n is amount of moles
- V is volume m3
- R is gas constant 8.31
what the conversions?
- cm2 to m3 is x10-6
- dm3 to m3 is x10-3
- degrees to K is +273
- kPa to Pa is x103
when is a bond polar?
when the bonded atoms are different and have different electronegativity values resulting in a polar covalent bond
when is a bond non polar?
the bonded electron pair is shared equally between the bonded atoms. it occurs when the bonded atoms are the same or if the atoms have the same or similar electronegativities
what does it mean if an atom has more electrons?
- the larger the instantaneous and induced dipoles
- the greater the induced dipole dipole interactions
- the stronger the attractive forces between the molecules
what is a simple molecular substance?
it is made up of simple molecules, small units containing a definite number of atoms with definite number MF eg neon, hydrogen, water and carbon dioxide
