An aluminum atom possesses 2 electrons in its inner electron shell, 8 in the next shell, and 3 in its outermost shell. What is the charge on aluminum when it forms an ion?
A. +2
B. -2
C. +3
D. +8

The charge on aluminum when it forms an ion would be +3

The charge that the atom of an element carries depends on the number of valance electrons in its outermost shell and whether it is more likely to gain or lose electrons to achieve a stable state.

Aluminum atom has 3 valence electrons in its outermost shell. It either gains 5 more or donates the 3 valence electrons in order to achieve an octet state. Since it is easier to donate 3 than gain 5, the atom of aluminum will prefer to donate the 3 valence electrons to become stable.

Thus, the aluminum atom will form an ion by carrying +3 charge.

More on ion formation can be found here: link

a mercury barometer is a device that uses the physical properties of mercury in a tube to measure barometric pressure. mercury barometers are used for : -

1. measurement of atmospheric pressure  

2. determining the altitude of a place

3. weather forecasting

4. calibration and checking of aneroid barometers

5. application in fluid mechanics

6. measurement of pressure in aircrafts

7. surface weather analysis

8. preparation of barographs

9. usage in physical sciences like physics, astronomy & chemistry

10. preparation of aircraft altimeters

characteristics of mercury barometer : -

1.used to define and measure atmospheric pressure.

2.on the average at sea level the column of mercury rises to a height of about 760 mm.

3.this quantity is equal to 1 atmosphere.

4.it is also known as standard atmospheric pressure.

5.a barometer in which the weight of a column of mercury in a glass tube with a sealed top is balanced against that of the atmosphere pressing on an exposed cistern of mercury at the base of the mercury column, the height of the column varying with atmospheric pressure.