READ 2 1 The Nature of Atoms Any substance in the universe that has mass and occupies space is defined as matter All matter is composed of extremely small particles called atoms Atomic structure includes a central nucleus and orbiting electrons Objects as small as atoms can be seen only indirectly by using complex technology such as tunneling microscopy We not 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 for understanding atomic theory Bohr proposed that every atom possesses an orbiting cloud of tiny subatomic particles called electrons whizzing around a core like the planets 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 Atomic number and the elements Within the nucleus the cluster of protons and neutrons is held together by a force that works only over short subatomic distances Each proton carries a positive charge and each neutron has no charge Each electron carries a negative charge Typically an atom has one electron for each proton and it thus electrically neutral Different atoms are defined by the number of protons a quantity called the atomic number The chemical behavior of an atoms is due to the number and configuration of electrons as we will see later in this chapter Atoms with the same atomic number that is the same number of protons have the same chemical properties and are said to belong to the same element Formally speaking an element is any substance that cannot be broken down to any other substance by ordinary chemical means Atomic mass The terms mass and weight are often used interchangeably but they have slightly different meanings Mass refers to the amount of substance but weight refers to the force gravity exerts on a substance An object has the same mass whether it is on the Earth or the Moon but its weight will be greater on the earth because the Earth s gravitational force is greater on the Earth because the Earth s gravitational force is greater that 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 not that it takes 602 million million billion 6 02 x 10 daltons to make 1 gram g A proton weighs approximately 1 dalton actually 1 007 daltons as does a neutron 1 009 daltons In contrals electrons weight only 1 1840 of a dalton so they contribute almost nothing to the overall mass of an atom Electrons The positive charges in the nucleus of an atom are neutralized or counterbalanced by negatively charged electrons which are located in regions called orbitals that lie at varying distances around the nucleus Atoms with the same number of protons and electrons are electrically neutral that is they have no net charge and are therefore called neutral atoms Electrons are maintained in their orbitals by their attraction to the positively charged nucleus Sometimes other forces overcome this attractions and an atom loses one or more electrons In other cases atoms gain additional electrons Atoms in which the number of electrons does not equal the number of protons are known as ions and they are charged particles An atom having more protons that electrons has a net positive charge and is called a cation For example an atom of sodium Na that has lost on electron becomes a sodium ion Na with a charge of 1 An atom having fewer protons that electrons carries a net negative charge and is called an anion A chlorine atom Cl that has gained one electron becomes a chloride ion Cl with a charge of 1 Isotopes Although all atoms of an element have the same number of protons they may not all have the same number of neutrons Atoms of a single element that possess different numbers of neutrons are called isotopes of that element Most elements in nature exist as mixtures of different isotopes Carbon C for example has three isotopes all containing six protons Over 99 of carbon found in nature exists as an isotopes that also contains six neutrons Because the total mass of this isotope is 12 daltons 6 from protons plus 6 from neutrons it is referred to as carbon 12 Most of the rest of the naturally occuring carbon is carbon 13 and isotope with seven neutrons The rarest carbon isotope is carbon 14 with eight neutrons Unlike the other two isotopes carbon 14 is unstable This means that its nucleus tends to break up into elements with lower atomic numbers This nuclear breakup which emits a significant amount of energy is called radioactive decay and isotopes that decay in this fashion are radioactive isotopes Some radioactive isotopes are more unstable that others and therefore they decay more readily For any given isotope however the rate of decay is constant The decay time is unusually expressed as the half life the time it takes for one half of the atoms in a sample to decay Carbon 14 for example often used in the carbon dating of fossils and other materials has a half life of 5730 years A sample of carbon containing 1 g of carbon 14 today would contain 0 5 g of carbon 14 after 5730 years 0 25 g 11 460 years from now 0 125 g 17 190 years from now and so on By determining the ratios of the different isotopes of carbon and other elements in biological samples and in rocks scientists are able to accurately determine when these materials formed Radioactivity has many useful applications in modern biology Radioactive isotopes are one way to label or tag a specific molecule and then follow its progress either in a chemical reaction or in living cells and tissue The downside however is that the energetic subatomic particles emitted by radioactive substances have the potential to severely damage living cells producing genetic mutations an at high doses cell death Consequently exposure to radiation is carefully controlled and regulated Scientists who work with radioactivity follow strict handling protocols and wear radiation sensitive badges to monitor their exposure over time to help ensure a safe level of exposure Electrons determine the chemical behavior of atoms The key to the chemical behavior of an atom lies in the number and arrangement of its electrons in their orbitals The Bohr model of the atom shows