​Calcium is a metal, in fact it is an alkaline earth metal in Group 2 of the periodic table. Bromine is a halogen in Group 17 and all of the halogens are non-metals.

When metals and non-metals react together, they form ionic compounds. This means that electrons are transferred from one atom to another, which creates positively-charged ions (cations) and negatively-charged ions (anions).

So how is calcium bromide (CaBr2) formed?
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Since calcium has two valence electrons, and bromine has seven (which is one short of a full octet), ONE calcium atom will give away ONE electron to each of two bromine atoms. I use single-headed arrows to show that here:

​This leaves you with a calcium atom that has lost two electrons and therefore has a +2 charge. I know it's weird that losing something causes it to become plus-charged, but electrons are negative so it's like you're subtracting negatives.

The bromine atoms on the other hand gained one electron each, and therefore they now each have a -1 charge.
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These charged particles are shown with their new numbers of valence electrons (zero for calcium, eight for each bromine) and are put in square brackets with the charge written in the top right corner:

But confirm that with your teacher first.
Want to watch me explain it instead? Here you go:

​Boron Trichloride, BCl3, has three chlorine atoms surrounding a single boron atom.

This is because each chlorine brings seven valence electrons with it, and needs just one more electron to complete its octet.

Boron has three valence electrons to start with, but does not need a full eight electrons to be stable. It is a violation of the octet rule, but this is the way things are.
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So, Boron shares ONE electron with each of three chlorine atoms, and each chlorine shares one electron with Boron:

​This is a trigonal planar arrangement and implies that the boron must be sp2 hybridized. The extra unhybridized p orbital is empty, but its presence is what keeps the three sp2 orbitals separated by exactly 120 degrees.
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I have a video where I draw this Lewis Structure, if you're a visual learner:

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​Now, in reality, solid BCl3 is more complicated. The lone pairs on each chlorine atom are attracted to the wide-open slightly-positive charge on the Boron atom, and a Lewis Acid-Base reaction happens: That's fancy chemistry talk for chlorine sharing its lone pair with boron.

This gives the boron atoms in solid BCl3 a tetrahedral geometry; since each of them actually connect with FOUR chlorines each. It makes the entire structure more like a lattice, rather than being individual molecules.

​The Lewis Structure of iron (III) oxide, Fe2O3, consists of five ions: Two iron ions with a +3 charge each, and three oxygen ions with a -2 charge each.

Iron III oxide is an ionic compound, because it consists of a metal and non-metal. These types of atoms have a big enough electronegativity difference that electrons are *transferred* from one atom to another, rather than being shared.

To begin, we note that the iron atoms need a charge of +3 … this is evident in the chemical formula (Fe2O3), since the “3” on the O had to have been criss-crossed down from the iron. It is also obvious in the name: The Roman numeral (III) after iron indicates that its charge in this compound is +3.

So we draw two iron atoms with three valence electrons each.
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Each oxygen atom brings 6 valence electrons (Oxygen is in Group 16 and is two electrons short of a full octet in its outer shell).

​This is an ionic compound, so there is no “hybridization of oxygen in Fe2O3” - it is instead a lattice of alternating positive and negative ions.

​HCN has a hydrogen atom single-bonded to a carbon atom, and that carbon atom is triple-bonded to a nitrogen atom.

These are all non-metals, so the bonds are covalent and HCN is therefore a covalent (aka Molecular) structure.

Carbon brings four valence electrons with it; it needs four more to complete its valence shell. Hydrogen shares one electron with it, and nitrogen shares three. This completes carbon's octet.

Carbon likewise shares one electron back with Hydrogen (this complete's hydrogen's outer shell of two electrons, aka Doublet) and carbon shares three electrons back with Nitrogen. This completes nitrogen's octet.
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You can watch this structure get drawn below, or you can scroll to the bottom of this page for a completed structure.

​What is the hybridization of Carbon in HCN?

Carbon is triple-bonded to nitrogen, and so there are two pi bonds (Remember: The first bond between any two atoms is a sigma bond, and the second/third bonds are pi bonds). This means two p orbitals are required to be left over after hybridization.

2 pi bonds = 2 leftover p orbitals.
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This means only ONE of carbon's p orbitals is available to hybridize, and so the hybridization of C in HCN is "sp".

​What is the hybridization of N in HCN?

Nitrogen is triple-bonded to carbon, and so two pi bonds are required here as well. This means only one of nitrogen's p orbitals is available to be hybridized, and so the hybridization of nitrogen in HCN is "sp".

What is the molecular shape (VSEPR shape) of HCN?
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Because the carbon is connected to two atoms, with no lone pairs on that central carbon, the geometry is AX2, which is "linear". The bond angle is 180 degrees.

​Aluminium oxide is a solid ionic compound, made from atoms of one metal (Aluminum) that have lost three electrons each to become +3 cations, and atoms of a non-metal (oxygen) which have gained two electrons each to become -2 anions.

The numbers 3 and 2 have a lowest common multiple of 6. This means you need TWO aluminum atoms (giving away 2x3=6 electrons total) and THREE oxygen atoms (accepting 3x2=6 electrons total).
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Because the electrons are LOST by the metal, and GAINED by the non-metal, there has been a transfer of electrons and this makes it an ionic compound by definition.

​You can watch this Lewis Structure get drawn below.
I created a video for this post! You're welcome !

Sodium hydroxide is a sodium atom, a metal, having lost one electron to become an Na(+1) ion. It packs itself among hydroxide ions, OH(-1), which are theoretically where sodium's electrons were donated to.

You can watch a video where I explain this structure here:

NH4Cl is an ionic compound, made from a positively-charged ammonium (NH4+) ion, and a negatively-charges chloride (Cl-) ion.

The NH4+ ion is, itself, held together with covalent bonds. that's what makes this Lewis Structure so interesting.

NH4+ contains one nitrogen atom and four hydrogen atoms. Normally this would make 9 valence electrons total, but the positive charge represents the loss of one electron (presumably to the chlorine in this example). This means it has just 8 electrons, enough to complete the octet on nitrogen with a single bond to each of the four hydrogen atoms.

Below, find a video of me drawing this structure step-by-step: