This experiment will use a technique known as neutron activation analysis (NAA) to determine the half-lives of two unusual isotopes, 28Al and 20F. The only stable isotopes of these elements are 27Al and 19F. Trace amounts of 26Al and 18F occur naturally, but the isotopes we will generate today are even more unstable and have no significant natural abundance. To produce these very unstable isotopes in measureable concentration, samples containing Al and F are placed next to a source of energetic neutrons. A number of neutron sources can be used; in this lab the samples are placed inside an operating nuclear reactor that emits energetic neutrons. This source has the advantage of a high neutron output that creates the desired isotopes rapidly and at higher concentrations, but it is only feasible for researchers that have access to nuclear reactors. At Oregon State University, the campus TRIGA reactor is set up for NAA experiments. How many neutrons are present in the nucleus of a 20F isotope?

the number of  neutrons are present in the nucleus of a 20F isotope? = 11

Explanation:

solution

recall that flourine has 9protons and 10neutrons

so,

we are going to take the F_20 isotope of flourine

since we know that

number of protons = 9

number of neuron = ?

mass number of flourine = 20

recal the formular

number of neutrons =  mass number - atomic number

number of neutron = 20-9

number of neutron = 11

therefore , the number of neutron that is present in the F_20  isotope nucleus  is 11

assuming that your periodic table has eighteen columns, you should highlight   the four elements in the third column from the left of the table.

neutral atoms of elements after calcium contain d electrons. the energy of d orbitals of a particular main shell lies between the energy of the s and p orbitals of the next main shell. for example, electrons in the have energy higher than those in the orbital while lower than those of the orbital.

by the aufbau principle, electrons fill orbitals one at a time, starting from those of the lowest energy. the orbital is therefore filled right after orbitals and before orbitals.  

potassium has electron configuration and calcium . the s orbital of a main shell holds a maximum of two electrons. the next element would thus have the electron configuration and satisfy the requirements.

a neutral atom of the element contains three electrons on top of the most recent noble gas element. it should thus be placed in the third column of the periodic table. both scandium and yttrium would satisfy the conditions. an aufbau diagram indicates that the 4f orbital is filled before the 5d. however, it appears that exceptions exists for both lanthanum and actinium. neutral atoms of the two d-block elements also demonstrate the and should be highlighted as well.