Identify the missing information for each atom or ion. Note that the atoms and ions are not necessarily neutral.
A Se ion has a mass number of 78 and a charge of −2 . Determine the number of neutrons, protons, and electrons in this ion.

number of neutrons:

number of protons:

number of electrons:

An ion has a mass number of 65, 36 neutrons, and a charge of +1 . Identify the element symbol, and determine the number of protons and electrons in this ion.

element symbol:

number of protons:

number of electrons:

An atom or ion has 42 neutrons , 36 protons, and 36 electrons. Identify the element symbol, and determine the mass number and charge for this atom or ion.

element symbol:

mass number:

charge:

Answer :

(1) Number of neutrons:  44

Number of protons:  34

Number of electrons: 36

(2) Element symbol:  Ni

Number of protons:  29

Number of electrons: 28

(3) Element symbol:  Kr

Mass number:  78

Charge: 0

Explanation :

Atomic number : It is defined as the number of protons or number of electrons.

Atomic number = number of protons

Mass number : It is defined as the sum of number of protons and number of neutrons.

Number of neutrons = Mass number - Atomic number

Part 1:

Element = Se (selenium)

Mass number = 78

Atomic number = 34

Charge on Se = -2

Atomic number = Number of protons = 34

Number of electrons = Number of protons - charge = 34 - (-2) = 36

Number of neutrons = Mass number - Atomic number = 78 - 34 = 44

Part 2:

Mass number = 65

Number of neutrons = 36

Charge on Se = +1

Number of neutrons = Mass number - Atomic number

36 = 65 - Atomic number

Atomic number = 65 - 36 = 29

Atomic number = Number of protons = 29

Number of electrons = Number of protons - charge = 29 - (+1) = 28

The symbol of element is, Nickel (Ni)

Part 3:

Number of neutrons = 42

Number of protons = 36

Number of electrons = 36

Atomic number = Number of protons = 36

Number of neutrons = Mass number - Atomic number

42 = Mass number - 36

Mass number = 42 + 36 = 78

Number of electrons = Number of protons - charge

36 = 36 - charge

Charge = 0

The symbol of element is, Krypton (Kr)

the seeing the electronic configuration we can say that the element is fluorine.

fluorine is the most electronegative element in the periodic table.

but by only seeing the configuration we cannot say about its reactivity because we don't know if its configuration after excitation or by addition of electrons.

but if not then, it is the lightest halogen and exists as a highly toxic pale yellow diatomic gas at standard conditions. as the most electronegative element, it is extremely reactive: almost all other elements, including some noble gases, form compounds with fluorine.