Section 1: Matter and Substances Preview Bellringer Key Ideas Atoms Chemical Bonds Polarity Summary

Bellringer Look at the different substances and write down which substances you think are composed of elements and which are composed of compounds.

Key Ideas What makes up matter? Why do atoms form bonds? What are some important interactions between substances in living things?

Atoms Every living and nonliving thing is made of matter. Matter is anything that has mass and takes up space. All matter is made of very small particles called atoms. An atom is the smallest unit of matter that cannot be broken down by chemical means. An atom has a positively charged core surrounded by a negatively charged region.

Atoms, continued The nucleus of an atom is made up of positively charged protons and uncharged neutrons. Negatively charged electrons have very little mass and move around the nucleus in a large region called the electron cloud.

Atoms, continued Elements An element is a substance made up of atoms that have the same number of protons. For example, each atom of the element carbon has six protons. Atoms of an element may have different numbers of neutrons. These atoms are called isotopes of elements.

Chemical Bonds The electron cloud of an atom may have levels. Electrons in the outermost level, or shell, are called valence electrons. Atoms tend to combine with each other such that eight electrons will be in the valence shell. When atoms combine, a force called a chemical bond holds them together.

Chemical Bonds, continued Chemical bonds form between groups of atoms because most atoms become stable when they have eight electrons in the valence shell. When atoms of different elements combine, a compound forms. A compound is a substance made of the bonded atoms of two or more elements.

Chemical Bonds, continued Covalent Bonding One way that atoms bond is by sharing valence electrons to form a covalent bond. A molecule is a group of atoms held together by covalent bonds. A water molecule, H2O, forms when an oxygen atom forms covalent bonds with two hydrogen atoms.

Visual Concept: Covalent Bonding Click the button below to watch the Visual Concept.

Chemical Bonds, continued Ionic Bonding Atoms can achieve a stable valence level by losing or gaining electrons, resulting in a positive or negative charge. An ion is an atom or group of atoms that has an electric charge because it has gained or lost electrons. The attractive force between oppositely charged ions is an ionic bond.

Visual Concept: Ion Click the button below to watch the Visual Concept.

Ionic Bonding in Salt Click to animate the image.

Polarity In some covalent bonds, the shared electrons are attracted more strongly to one atom than to the other. As a result, one end of the molecule has a partial negative charge, while the opposite end has a partial positive charge. Molecules with partial charges on opposite ends are said to be polar.

Polarity, continued Solubility The partially charged ends of polar molecules attract opposite charges. Because of this behavior, polar molecules can dissolve other polar molecules and ionic compounds. For example, water can dissolve sugar and salt. Nonpolar substances, such as oil, grease, and wax, do not dissolve well in water.

Ionic Compounds in Water

Polarity, continued Hydrogen Bonds When bonded to an oxygen, nitrogen, or fluorine atom, a hydrogen atom has a partial charge nearly as great as a proton’s charge. It attracts the negative pole of other nearby molecules. This attraction, called a hydrogen bond, is stronger than attractions between other molecules, but not as strong as covalent bonds. Hydrogen bonding plays an important role in many of the molecules that make up living things.

Hydrogen Bonding

Summary All matter is made up of atoms. An atom has a positively charged nucleus surrounded by a negatively charged electron cloud. Chemical bonds form between groups of atoms because most atoms became stable when they have eight electrons in the valence shell. Polar attractions and hydrogen bonds are forces that play an important role in many of the molecules that make up living things.