2.1 Atoms are nature's building material. Atoms Any substance in the universe that has ass(see below)and occupies space defined as matter. All matter is com- osed of extremely small particles called atoms. Because of their siz atoms are difficult to study. Not until early in this century did scientists carry out the first experiments sug gesting what an atom is like. The Structure of atoms Objects as small as atoms can be “seen” only indirectly, by using very complex technology such as tunneling microcopy. We now know a great Oxyge deal about the complexities of atomic structure,but the simple view put 8 Neutrons forth in 1913 by the Danish physicist Niels Bohr provides a good starting point. Bohr proposed that every atom assesses an orbiting cloud of subatomic particles called electrons hizzing around a core like the plan- ets of a miniature solar system. At the Proton o Neutron Electron o center of each atom is a small,very dense nucleus formed of two other ve charge (No charge) (Negative charge) kinds of subatomic particles, protons FIGURE 2.2 Within the nucleus, the cluster of Basic structure of atoms. All atoms have a nucleus consisting of protons and neutrons, cept hydrogen, the smallest atom, which has only one proton and no neutrons in its protons and neutrons is held together nucleus. Oxygen, for example, has eight protons and eight neutrons in its nucleus. Electrons by a force that works only over short spin around the nucleus a far distance away from the nucleus subatomic distances. Each proton car- a positive (+) charge, and each electron carries a negative(-)ch Typically an atom has one electron for each proton. The number of protons(the atoms weigbt will be greater on the earth because the earths grav atomic number) determines the chemical character of the itational force is greater than the moons. The atomic atom,because it dictates the number of electrons orbiting mass of an atom is equal to the sum of the masses of its the nucleus which are available for chemical activity. Neu rotons and neutrons. Atoms that occur naturally on earth trons, as their name implies, possess no charge contain from 1 to 92 protons and up to 146 neutrons The mass of atoms and subatomic particles is measured Atomic mass in units called daltons. To give you an idea of just how small these units are. note that it takes 602 million million billion The terms mass and weight are often used interchangeably, (6.02 x 102)daltons to make 1 gram! A proton weighs ap- but they have slightly different meanings. Mass refers to the proximately 1 dalton(actually 1.009 daltons), as does a neu amount of a substance, while weigbt refers to the force tron(1.007 daltons). In contrast, electrons weigh only 18o of gravity exerts on a substance. Hence, an object has the a dalton, so their contribution to the overall mass of an atom same mass whether it is on the earth or the moon, but its is negligible. 20 Part i The Origin of Living thingsweight will be greater on the earth because the earth’s grav￾itational force is greater than the moon’s. The atomic mass of an atom is equal to the sum of the masses of its protons and neutrons. Atoms that occur naturally on earth contain from 1 to 92 protons and up to 146 neutrons. The mass of atoms and subatomic particles is measured in units called daltons. To give you an idea of just how small these units are, note that it takes 602 million million billion (6.02 × 1023) daltons to make 1 gram! A proton weighs ap￾proximately 1 dalton (actually 1.009 daltons), as does a neu￾tron (1.007 daltons). In contrast, electrons weigh only 1 1840 of a dalton, so their contribution to the overall mass of an atom is negligible. 20 Part I The Origin of Living Things Atoms Any substance in the universe that has mass (see below) and occupies space is defined as matter. All matter is com￾posed of extremely small particles called atoms. Because of their size, atoms are difficult to study. Not until early in this century did scientists carry out the first experiments sug￾gesting what an atom is like. The Structure of Atoms Objects as small as atoms can be “seen” only indirectly, by using very complex technology such as tunneling microcopy. We now know a great deal about the complexities of atomic structure, but the simple view put forth in 1913 by the Danish physicist Niels Bohr provides a good starting point. Bohr proposed that every atom possesses an orbiting cloud of tiny subatomic particles called electrons whizzing around a core like the plan￾ets of a miniature solar system. At the center of each atom is a small, very dense nucleus formed of two other kinds of subatomic particles, protons and neutrons (figure 2.2). Within the nucleus, the cluster of protons and neutrons is held together by a force that works only over short subatomic distances. Each proton car￾ries a positive (+) charge, and each electron carries a negative (–) charge. Typically an atom has one electron for each proton. The number of protons (the atom’s atomic number) determines the chemical character of the atom, because it dictates the number of electrons orbiting the nucleus which are available for chemical activity. Neu￾trons, as their name implies, possess no charge. Atomic Mass The terms mass and weight are often used interchangeably, but they have slightly different meanings. Mass refers to the amount of a substance, while weight refers to the force gravity exerts on a substance. Hence, an object has the same mass whether it is on the earth or the moon, but its 2.1 Atoms are nature’s building material. FIGURE 2.2 Basic structure of atoms. All atoms have a nucleus consisting of protons and neutrons, except hydrogen, the smallest atom, which has only one proton and no neutrons in its nucleus. Oxygen, for example, has eight protons and eight neutrons in its nucleus. Electrons spin around the nucleus a far distance away from the nucleus. Proton (Positive charge) (No charge) (Negative charge) Neutron Electron Hydrogen 1 Proton 1 Electron Oxygen 8 Protons 8 Neutrons 8 Electrons