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 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