8/10/2019 Gchem

1/5

General Chemistry MCAT:

1. What is the abbreviated electron configuration for sodium?

(a !"e# $s1

(b 1s% %s%%&'$s%$&'

(c !Ar# $s1

(d "a

)*&lanation: The abbreviated electron configuration is written with the next smallest

noble gas followed by the subshells of the specified atom. In this case, the next smallestnoble gas for Na is neon [Ne]. The subshells of sodium following is 3s. Thus the correct

answer is !"#.

%. A gas bubble is located at the bottom of a +ater,filled container. As the bubble is

rising to+ard the surface- +hich of the follo+ing occurs? Assume the gas does not

dissolve into the +ater.

!a# The bubble becomes smaller.

!b# The water level rises.

!c#The bubble becomes larger.

!d# The water level lowers.

)*&lanation: The amount of gas in the bubble remains constant, since none will dissolve

into the water. "s the bubble rises through the water, the external pressure on the bubbledecreases according to

$ressure % &gh where & is the density of the water.

Thus, as the bubble rises, the volume increases as the ratio of the internal pressure toexternal pressure increases. The correct answer is !'#.

$. und/s rule states that:

!a#electrons fill orbitals from the lowest energy to highest energy!b# electrons are paired with each other with the same spin in orbitals

!c# electrons are promoted to higher energy levels only when illuminated by light of

specific fre(uency

!d# electrons do not &air +ith each other in an orbital until all orbitals in thesubshell have one electron of the same s&in

)*&lanation: !"# represents"ufbau)s principle. !*# is false+ electrons fill orbitals with

8/10/2019 Gchem

2/5

opposite spin. !'# while true, does not correspond to und)s rule. Thus, the correct

answer is !-#.

The electrons gather around the nucleus in (uantum orbitals following four basic rulescalled the aufbau &rinci&le.

no two electrons in the atom will share the same four (uantum numbers n, l, m,

and s.

electrons will first occupy orbitals of the lowest energy level.

electrons will fill an orbital with the same spin number until the orbital is filled

before it will begin to fill of the opposite spin number.

electrons will fill orbitals by the sum of the (uantum numbers nand l. rbitals

with e(ual values of !n/l# will fill with the lower nvalues first.

0. Which of the follo+ing has the largest atomic radius?

(a

(b ,

(c Cl

(d Cl,

)*&lanation: "nions are larger than neutral species due to electron repulsion. 0ince 'l is

lower than 1 on the periodic table, it has the larger radius. The ionic form of 'l would be

8/10/2019 Gchem

3/5

larger than neutral 'l. Thus, the correct answer is !-#.

'ations and anions do not actually represent a periodic trend in terms of atomic radius,

but they do affect atomic radius,

A cation is positively charged, meaning that it is an atom that has lost an electron orelectrons. The positive charge of the nucleus is thus distributed over a smaller number of

electrons and electron2electron repulsion is decreased, meaning that the electrons areheld more tightly and the atomic radius is smaller than in the normal neutral atom.

Anions- conversely, are negatively charged ions+ atoms that have gained electrons. In

anions, electron,electron re&ulsion increases and the &ositive charge of the nucleus is

distributed over a large number of electrons. "nions have a greater atomic radiusthan the neutral atom from which they derive.

Atomic 2adius

The atomic radius of an element is half of the distance between the centers of two atomsof that element that are ust touching each other. 4enerally, the atomic radius decreases

across a &eriod from left to right and increases do+n a given grou&. The atoms with

the largest atomic radii are located in 4roup I and at the bottom of groups.

Moving from left to right across a &eriod, electrons are added one at a time to the outerenergy shell. 5lectrons within a shell cannot shield each other from the attraction to

protons. 0ince the number of protons is also increasing, the effective nuclear charge

increases across a period. This causes the atomic radius to decrease.

Moving do+n a grou& in the &eriodic table, the number of electrons and filled electron

shells increases, but the number of valence electrons remains the same. The outermost

electrons in a group are exposed to the same effective nuclear charge, but electrons arefound farther from the nucleus as the number of filled energy shells increases. Therefore,

the atomic radii increase.

8/10/2019 Gchem

4/5

The atomic radius tends to decrease as one progresses across a period because the

effective nuclear chargeincreases, thereby attracting the orbiting electrons and lessening

the radius.

The atomic radius usually increases while going down a group due to the addition of a

new energy level !shell#. The atomic radius increases moving down a group. nce again

protons are added moving down a group, but so are new energy shells of electrons. Thenew energy shells provide shielding, allowing the valence electrons to experience only a

minimal amount of the protons6 positive charge.

owever, diagonally, the number of protons has a larger effect than the si7eable radius.1or example, lithium!89 picometer# has a smaller atomic radius than magnesium!9:

picometer#. "tomic radius decreases from left to right across a period, and also increases

from top to bottom down a group. NT5+ pm%x:;2< meters

http://var/www/apps/conversion/tmp/scratch_6/%2Fwiki%2FEffective_nuclear_chargehttp://var/www/apps/conversion/tmp/scratch_6/%2Fwiki%2FLithiumhttp://var/www/apps/conversion/tmp/scratch_6/%2Fwiki%2FMagnesiumhttp://var/www/apps/conversion/tmp/scratch_6/%2Fwiki%2FLithiumhttp://var/www/apps/conversion/tmp/scratch_6/%2Fwiki%2FMagnesiumhttp://var/www/apps/conversion/tmp/scratch_6/%2Fwiki%2FEffective_nuclear_charge

8/10/2019 Gchem

5/5