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Chapter 2.1 The Nature of Matter
Transcript of Chapter 2.1 The Nature of Matter
which means “unable to be cut.”
subatomic particles that make up atoms are protons, neutrons, and electrons subatomic particles in a carbon atom Protons and Neutrons Protons and neutrons have about the same mass. Protons are positively charged particles (+) and neutrons carry no charge at all. Strong forces bind protons and neutrons together to form the nucleus, at the center of the atom. Electrons The electron is a negatively charged particle (–) with only 1/1840 the mass of a proton. Electrons are in constant motion in the space surrounding the nucleus.
They are attracted to the positively charged nucleus but remain outside the nucleus because of the energy of their motion. Because atoms have equal numbers of electrons and protons, their positive and negative charges balance out, and atoms themselves are electrically neutral.
A chemical element is a pure substance that consists entirely of one type of atom More than 100 elements are known, but only about two dozen are commonly found in living organisms Elements are represented by one- or two-letter symbols.
C stands for carbon,
H for hydrogen
Na for sodium
Hg for mercury The number of protons in the nucleus of an element is called its atomic number Carbon’s atomic number is 6, meaning that each atom of carbon has six protons and, consequently, six electrons Isotopes Atoms of an element may have different numbers of neutrons. For example, although all atoms of carbon have six protons, some have six neutrons, some seven, and a few have eight. Atoms of the same element that differ in the number of neutrons they contain are known as isotopes. The total number of protons and neutrons in the nucleus of an atom is called its mass number. Isotopes are identified by their mass numbers; for example, carbon-12, carbon-13, and carbon-14. The weighted average of the masses of an element’s isotopes, in which the abundance of each isotope in nature is considered, is called its atomic mass. Because they have the same number of electrons, all isotopes of an element have the same chemical properties. Radioactive Isotopes Some isotopes are radioactive, meaning that their nuclei are unstable and break down at a constant rate over time. Although radiation can be dangerous, radioactive isotopes have a number of important scientific and practical uses. Geologists can determine the ages of rocks and fossils by analyzing the isotopes found in them. Radiation from certain isotopes can be used to detect and treat cancer and to kill bacteria that cause food to spoil. Radioactive isotopes can also be used as labels or “tracers” to follow the movements of substances within organisms. A chemical compound is a substance formed by the chemical combination of two or more elements in definite proportions. The physical and chemical properties of a compound are usually very different from those of the elements from which it is formed. For example, sodium is a silver-colored metal that is soft enough to cut with a knife. It reacts explosively with cold water. Chlorine is a very reactive, poisonous, greenish gas that was used in battles during World War I.
However, the compound sodium chloride--table salt--is a white solid that dissolves easily in water, is not poisonous, and is essential for the survival of most living things.
Ionic Bonds An ionic bond is formed when one or more electrons are transferred from one atom to another. An atom that loses electrons becomes positively charged An atom that gains electrons has a negative charge These positively and negatively charged atoms are known as ions Ionic bonds form between sodium and chlorine to form NaCl, table salt A sodium atom easily loses its one valence electron and becomes a sodium ion (Na+). A chlorine atom easily gains an electron (from sodium) and becomes a chloride ion (Cl-). These oppositely charged ions have a strong attraction for each other, forming an ionic bond Covalent Bonds Sometimes electrons are shared by atoms instead of being transferred The moving electrons travel about the nuclei of both atoms, forming a covalent bond When the atoms share two electrons, the bond is called a single covalent bond. Sometimes the atoms share four electrons and form a double bond. In a few cases, atoms can share six electrons, forming a triple bond. The structure that results when atoms are joined together by covalent bonds is called a molecule, the smallest unit of most compounds This diagram of a water molecule shows that each hydrogen atom is joined to water’s lone oxygen atom by a single covalent bond. Each hydrogen atom shares two electrons with the oxygen atom. When atoms of the same element join together, they also form a molecule.
Oxygen molecules in the air you breathe consist of two oxygen atoms joined by covalent bonds.