61. Using the symbol of the previous noble gas to indicate the core electrons, write the electron configuration for each of the fol-
lowing elements.
a. scandium, Z= 21 c. lanthanum, Z= 57
b. yttrium, Z = 39
d. actinium, Z= 89

Scandium, with atomic number of 21 = [Ar]3d¹4s²

Yttrium, with atomic number of 39 = [Kr]4d¹5s²

Lanthanum, with atomic number of 57 = [Xe]5d¹6s²

Actinium, with atomic number of 89 = [Rn]6d¹7s²

Explanation:

The electronic configuration of atoms of elements describes how electrons are arranged in the atomic orbitals of the atom. The electron configurations of atoms in standard notation is usually written sequentially with all electron-containing atomic subshells (s, p, d and f) and the number of electrons they contain in superscript. For example, the electron configuration of Neon is 1s²2s²2p⁶. However, an abbreviated notation can be used especially for elements having large atomic numbers. The symbol of the previous noble gas in square brackets indicating the core electrons is written before the outermost shell electrons of the atom. For example, the abbreviated electron configuration of sodium is [Ne]3s¹.

The electronic configuration of the given elements are shown below:

Scandium, with atomic number of 21 = [Ar]3d¹4s²

Yttrium, with atomic number of 39 = [Kr]4d¹5s²

Lanthanum, with atomic number of 57 = [Xe]5d¹6s²

Actinium, with atomic number of 89 = [Rn]6d¹7s²

first let us lay out the formula for potential

and kinetic energy:

pe = mgh

ke = (mv^2)/2

where: m=mass, v=velocity, g=gravitational

acceleration, h=height

calculating for mass using the known values of

pe:

340j = m (9.81 m/s^2) (50ft) (1m/3.28ft)

m = 2.27 kg

the law of conservation of energy states that

energy is neither created nor destroyed. therefore, the change in kinetic

energy from 50 ft to 35 ft would be equal to the change in potential energy

from 50 ft to 35 ft. however it would be opposite in signs since one is losing

while other is gaining.

ke(35ft) – ke(50ft) = - [ pe(35ft) – pe(50ft]

ke(50ft) = 0 since the brick is initially at rest

ke(35ft) = 340j - 2.27kg (9.81 m/s^2) (35ft)

(1m/3.28 ft)

ke(35ft)

= 102.38 j

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)