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Explanation:

There are five columns here; electronic configuration (electron distribution), number of shells, valence electrons (number of electrons on the outermost shell that can participate in bond formation), period number (on the periodic table) and group number (on the periodic table).

Aluminium-13 (₁₃Al)

This means Aluminium has 13 electrons and thus will have the properties below

Electronic configuration: 1s²2s²2p⁶3s²3s¹

Number of shells: From the electronic configuration written above, Aluminium-13 has 3 shells.

Valence electrons: The number of valence electrons (as seen in the electronic configuration - for the third shell) is 3.

Period number: Aluminium is found in period 3 because it has 3 electron shells

Group number: Aluminium is found in group IIIA because it has 3 electrons in it's outermost shell.

Calcium-20 (₂₀Ca)

This calcium atom has 20 electrons and thus have the following properties.

Electronic configuration: 1s²2s²2p⁶3s²3p⁶4s²

Number of shells: As seen in the electronic configuration above, calcium-20 has 4 electron shells.

Valence electrons: As seen in the electronic configuration also, the total number of electrons/valence electrons in the outermost shell is 2.

Period number: Calcium-20 is found in period 4 of the periodic table because it has four electron shells.

Group number: Calcium-20 is found in Group IIA of the periodic table because it has 2 electrons in it's outermost shell

Tin-50 (₅₀Sn)

This tin atom has 50 electrons and hence have the following properties

Electronic configuration: [Kr] 4d¹⁰5s²5p²

Where [Kr] is the electronic configuration of krypton atom

Number of electron shell: From the electronic configuration above, Tin-50 has 5 electron shells

Number of valence electrons: From the electronic configuration also, it has 4 valence electrons.

Period number: It is found in period 5 because it has 5 electron shells

Group number It is found in Group IVA because it has 4 electrons in it's outermost shell

Barium-56 (₅₆Ba)

Electronic configuration: [Xe] 6s²

Number of shells: 6

Number of valence electrons: 2

Period number: Period 6

Group number: Group IIA

Selenium-34 (₃₄Se)

Electronic configuration: [Ar] 3d¹⁰4s²4p⁴

Number of shells: 4

Number of valence electrons: 6

Period number: Period 4

Group number: Group VIA

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.

the correct answer to this question is #5