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Masses of Atoms and the Periodic Table Atomic number • To identify which element an atom is, we look at the number of protons. • The number of protons in an atom is called its atomic number. – every atom with 2 protons is a __________ atom – every atom with 6 protons is a __________ atom – every atom with 8 protons is a __________ atom • Elements are organized on the periodic table by their atomic number Mass number • The sum of the number of protons and neutrons in an atom is the mass number – A fluoride atom with 9 protons and 10 neutrons has a mass number of _________ – A sodium atom with 11 protons and 12 neutrons has a mass number of _________ – A hydrogen atom with 1 proton and 0 neutrons has a mass number of ________ • If we know the mass number and atomic number, we can find the number of neutrons. • Number of neutrons = mass number – atomic number – A carbon atom with a mass number of 14 has how many neutrons? Isotopes • Not all atoms of an element have the same number of neutrons. • Atoms of the same element that have different numbers of neutrons are called isotopes. • Nearly all elements have an isotopes • To identify isotopes, we write the element name, then a dash, then the mass number • Oxygen (O) with 8 protons and 8 neutrons: • Oxygen (O)with 8 protons and 10 neutrons: • Helium (He) with 2 protons and 1 neutron: • Helium (He) with 2 protons and 2 neutrons: • Uranium (U) with 92 protons and 143 neutrons: • Uranium (U) with 92 protons and 146 neutrons: Radioactive isotopes • Many isotopes are unstable. • An unstable atom can emit particles or even change neutrons into protons to become more stable – This is known as radioactivity • Some radioactive particles are very dangerous – For example, radon gas Half-lives • Radioactivity occurs at a constant rate • The time it takes for half of an isotope to decay is called the half-life • After one half-life, 50% of the original isotope remains • After two half-lives, 25% of the original isotope remains • After three half-lives, 12.5% of the original isotope remains Half-life example • Carbon-14 is an unstable isotope of carbon with a half-life of 5700 years. • After 5700 years, half of the C-14 will have decayed into another element. • The ancient Egyptians often mummified cats. Scientists are able to calculate how much C-14 is present in the cat to find out its age. Half-life example • Assume scientists take a reading of the C-14 in a mummified cat and find that only 25% of the expected C-14 remains. • Approximately how long ago was the cat mummified? – – – – Half-life of C-14 is 5700 years. After 1 half-life, 50% of C-14 has decayed After 2 half-lives, 25% of C-14 has decayed 2 half-lives = 5700 years + 5700 years = 11,400 years Atomic Mass • Because most elements have different isotopes, each element has an average atomic mass • The unit we use for calculating atomic mass is the atomic mass unit (amu) • One amu refers to the mass of a proton or neutron • The atomic mass of an element is a weighted average of the masses of its isotopes Review: Mass number vs. Atomic Mass • The mass number is unique for every atom. It is the sum of protons and neutrons. • Different atoms of the same element may have different mass numbers. These are known as isotopes. • Mass number has no unit Review: Mass number vs. Atomic Mass • The atomic mass is the weighted average of the mass of all the different isotopes of an element • Atomic mass is used to describe the mass of an element, not of an individual atom • Atomic mass has the unit amu