Periodicity Flashcards
what is the trend for elements across the periodic table
increasing atomic size across the row
What cause LDFs
caused by the continual movement of electrons in an atom, which causes a temporary uneven distribution of charge at opposite sides of an atom known as a temporary dipole.
are LDFs strong or weak
weak
what is the trend for no.of electrons in an atom vs LDF magnitude
more electrons the bigger the LDFS
as you go down group 0 what trend do you see in mpts and bpts and why
melting and boiling points increase as more energy is needed to separate the atoms
what happens when a molecular substance melts or boils
the weak LDFs between molecules (intermolecular forces) which are broken, and not the strong covalent bonds (intramolecular forces) holding the atoms together in the molecule
what are the 3 big molecules important to remember
P₄ Phosphorus
S₈ Sulfur
C₆₀ fullerenes
for phosphorus and sulfur what bonds are they held together by
the atoms which make up the molecules are held together by covalent bonds, and the molecules are held together by LDFs
Fullerenes are comprised of
a series of carbons arranged in hexagons and pentagons
why are diamond and graphite unusual
because instead of melting when heated they sublime
what is sublimation
the transition of a substance directly from the solid to the gas phase without passing through an intermediate liquid phase
Explain why sulfur has a higher melting point than phosphorous
sulfur consists of S₈ molecules and phosphorous P₄. The sulfur molecules have more electrons than the phosphorous molecules and so have stronger LDFs. This in turn means sulfur has a higher melting point than phosphorous
Fullerenes are molecular forms of carbon which exist as solids that sublime (solid -> gas) at temps approx. 600°C and above. Describe the intramolecular and intermolecular forces present in fullerenes, and state why their sublimation temps are so high
Fullerenes have molecues with 60 carbons or more arranged in ball shapes . The carbons are held together by covalent bonds (intramolecular forces) and the molecules are held together by LDFs (intermolecular forces). Because the molecules are so big they have a large number of electrons. This results in the LDFs being very strong, so a lot of energy is needed to separate the molecules, so their sublimation point is high.
Explain why:
Diamonds sublimation point is much higher than sulfur’s boiling point
In diamond each carbon atom is covalently bonded to four other carbon in a tetrahedral arrangement. All the outer electrons in the atom of each carbon are used to make single covalent bonds with neighbouring atoms, which results in a giant covalent network. In order to separate the atoms in a covalent network strong covalent bonds have to be broken, which requires a lot of energy so the sublimation point is very high.
Sulfur is molecular and the molecules are held together by weak LDFs. It does not require a lot of energy to separate the molecules so the boiling point is low compared to diamond.
Explain why:
Graphite conducts electricity but diamond doesn’t
in graphite each carbon atom is bonded to only three other carbon atoms. The fourth electron in each carbon is delocalised, so graphite can conduct electricity.
Explain why:
silicon has many properties similar to diamond
Silicon. like carbon, is in group 4 of the periodic table. Silicon forms four covalent bonds with four other silicon atoms and forms a covalent network similar to diamond.
what is the covalent radius
the covalent radius can be used to measure the size of an atom.
the covalent radius is taken as half the distance between the nuclei of two atoms joined by a single covalent bond
Change in covalent radius
What is the trend going down a group and what does this mean
The trend:
Going down the group the no. of occupied electron shells increases so the covalent radius increases.
WHY:
The inner electron shells screen the outer electrons shells from the full pull of the positive nucleus meaning the outer electrons can move further from the nucleus.
Change in covalent radius
What is the trend going across a period and what does this mean
The trend:
Going across the period the no. of electrons decreases by 1 and covalent radius increases
WHY:
There is no increased screening from the inner electron shells as no new shells are added only the no. of electrons on each shell. Thus as atomic no. increases across the period the electrons in the outer shell are more strongly attracted to the nucleus adn so covalent radius decreases
What is the first ionisation energy
the energy required to remove one mole of electrons from one mole of atoms in the gaseous state.
Change in first ionisation energy
What is the trend going across a period and what does this mean
Trend:
First ionisation energy increases
Why:
although electrons are being added, it is to the same energy level. The nuclear charge increases and the electrons are held more tightly, so it becomes harder to remove an outer electron from an atom.
Change in first ionisation energy
What is the trend going down a group and what does this mean
Trend:
First ionisation energy decreases
Why:
The covalent radius of the atoms increases. THis means that the outer electrons are further away from the pull of the nucleus. The number of filled energy levels also increases meaning there is a greater screening effect on the outer energy levels making it easier to remove electrons from the outer electron energy level.
In energy terms, it is not possible for atoms of group 2 elements to form a 3+ ion. WHY?
This is because it would mean removing a third electron form an energy level closer to the nucleus, which require too much energy.
Why are second ionisation energies always higher than first
because although electrons are being removed, the no. of protons in the nucleus remains the same. Therefore, the pull on the remaining electrons is increased.
Electronegativity is …
the attraction of an atom involved in a bond has fr the electrons of the bond.
What is the most electronegative element
fluorine with a value of 4.0
What id the least electronegative element
francium with a value of 8.0
Change in electronegativity values
What is the trend going down a group and what does this mean
Trend:
electronegativity values generally decrease down the group
Why:
covalent radius increases and outer electrons get further from the nucleus. The no. of screening shells also increases.
These factors combine and result in the nucleus having less of an attraction for bonding electrons. Although the nuclear charge increases considerably going down a group, the bonding electrons are far from the nucleus and screened.
Change in electronegativity values
What is the trend going across a period and what does this mean
Trend:
electronegativity values increase
why:
covalent radius decreases and no. of screening shells stay the same, so the attraction the nucleus has for the bonding electrons increases.
