Which of the following is not true in regard to atomic radii of main group elements? a. atomic radii generally increase as you move down a group. b. atomic radii generally increase as you move from left to right across a period. c. atomic radii size is related to the number of electrons in the valence level and the number of protons in the nucleus. d. atomic radii decrease in size when the pull of electrons toward the nucleus of an atom is greater.

B. Atomic radii generally increase as you move from left to right across a period.

On moving down a group, the nuclear charge increases but electrons are being added to new shells and the effect of addition of new shells is more pronounced, thus the valence electrons move far from the nucleus, the atomic radii increase.

On moving across a period, the nuclear charge increases but electrons are being added to same shells and the effect of nuclear charge is more pronounced, thus the valence electrons are being tightly held by the nucleus, the atomic radii decrease.

when 25.0 ml of 0.700 mol/l naoh was mixed in a calorimeter with 25.0 ml of 0.700 mol/l hcl, both initially at 20.0 °c, the temperature increased to 22.1 °c. the heat capacity of the calorimeter is 279 j/°c.

the equation for the reaction is:

naoh + hcl → nacl + h₂o

moles of hcl = 0.0250 l hcl ×

0.700

mol hcl

1 l hcl = 0.0175 mol hcl

volume of solution = (25.0 + 25.0) ml = 50.0 ml

mass of solution = 50.0 ml soln ×

1.00

ml soln

= 50.0 g soln

δt=t2-t1

= (22.1 – 20.0) °c = 2.1 °c

the heats involved are

heat from neutralization + heat to warm solution + heat to warm calorimeter = 0

q1+q2+q3=0

nδh+mcδt+cδt=0

0.0175 mol ×

δh

+ 50.0 g × 4.184 j·g⁻¹°c⁻¹ × 2.1 °c + 279 j°c⁻¹ × 2.1 °c = 0

0.0175 mol ×

δh

+ 439.32 j + 585.9 j = 0

0.0175 mol ×

δh

= -1025.22 j

δh=1025.22j0.0175 mol= -58 600 j/mol = -58.6 kj/mol