The animal kingdom is large and diverse. Scientists discover new species all the time, and they need a way to classify them. The science of classification is called taxonomy. In taxonomy, every kingdom of life (plant, animal, fungus, etc) branches into smaller groups called phyla. Each phylum branches into classes, and so on down to species. The presence or absence of a backbone is one of the first traits that scientists use to classify animals. Animals without a backbone are invertebrates. There are many phyla of invertebrates. Earthworms, spiders, jellyfish, starfish and butterflies are all examples of invertebrate animals. Animals with a backbone are vertebrates.

Which of these is an example of an invertebrate?

A
an oak tree, which is a kind of plant
B
a mushroom, which is a kind of fungus
C
a bat, which is an animal with a backbone
D
a snail, which is an animal without a backbone

The answer is D, because "Animals without a backbone are invertebrates."

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.