- Using the choices below, which is the complete symbol for the isotope that has the following subatomic particles: 19 protons, 21 neutrons, and 18 electrons.
C.
D.
E.
A.
40 -1
B.
40
21
iR-1
33K+1 4.K+1
izArt1
b. B
A
d. D
e. E
с
c.
a.

The elements in nature have several types of isotopes

Isotopes are atoms whose no-atom has the same number of protons while still having a different number of neutrons.

So Isotopes are elements that have the same Atomic Number (Proton)

The element has : 19 protons, 21 neutrons, and 18 electrons.

atomic mass = protons+neutrons = 19+21=40

atomic number=number of protons=19

charge of elements = protons-electrons = 19-18=+1

The element in the periodic system having the atomic number 19 is K

So symbol of element

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.