18. Which two substances do not have covalent chemical bonds? 1) MgO 2) CO_(2) 3) Br_(2) 4) CaCl_(2) 5) H_(2) Answer (score: 0.5):

The correct answer is 1) $MgO$ and 4) $CaCl_{2}$.<br /><br />Covalent chemical bonds are formed when atoms share electrons. In the case of $MgO$ and $CaCl_{2}$, the bonds between the atoms are ionic, not covalent. In ionic bonds, one atom donates an electron to another atom, resulting in the formation of positively and negatively charged ions that are held together by electrostatic forces.<br /><br />$MgO$ is composed of magnesium (Mg) and oxygen (O) atoms. Magnesium is a metal that tends to lose electrons, while oxygen is a non-metal that tends to gain electrons. Therefore, magnesium donates electrons to oxygen, forming an ionic bond.<br /><br />$CaCl_{2}$ is composed of calcium (Ca) and chlorine (Cl) atoms. Calcium is a metal that tends to lose electrons, while chlorine is a non-metal that tends to gain electrons. Therefore, calcium donates electrons to chlorine, forming an ionic bond.<br /><br />On the other hand, $CO_{2}$, $Br_{2}$, and $H_{2}$ are all covalent compounds. In $CO_{2}$, each carbon atom shares electrons with two oxygen atoms, forming covalent bonds. In $Br_{2}$, each bromine atom shares electrons with another bromine atom, forming a covalent bond. In $H_{2}$, each hydrogen atom shares electrons with another hydrogen atom, forming a covalent bond.