Unit 5 The Atomic Number and Mass Number

Unit 5 The Atomic Number and Mass Number Isotopes

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All Matter is composed of : Atom – Means “unable to be cut” (Greek: atomos) Smallest possible unit of matter that retains the physical and chemical properties of 
its element.

Elements – different types of atom Elements are the simplest substances. There about 100 different elements. Each element is made up of very tiny particles called atoms, and each element is made up of just one particular type of atom, which is different to the atoms in any other element. Gold is an element made up of only gold atoms. Carbon is an element made up of only carbon atoms.

Elements l Substances that can’t be broken down into simpler substances l 92 naturally occurring elements l 25 - 36 chemical elements are essential to life. l 96% of human mass is made up of C, H, O, N

Atoms – the building blocks John Dalton had the first ideas about the existence of atoms over 200 years ago. However, it is only relatively recently that special microscopes (called electron microscopes) been invented that can actually ‘see’ atoms. This image is highly magnified. What could it be showing? The yellow blobs are individual gold atoms, as seen through an electron microscope.

How small is an atom? Atoms are very small – they are about 0. 00000001 cm wide. Think about the thickness of a crisp. The number of atoms you would need to stack up to make thickness of a crisp, is approximately the same number of crisps you would need to stack up to make the height of Mount Everest! That’s roughly 7 million crisps!

Atomic Theory l Atoms are building blocks of elements l Similar atoms in each element l Different from atoms of other elements l Two or more different atoms bond in simple ratios to form compounds (or molecules)

What particles are atoms made of? For some time, people thought that atoms were the smallest particles and could not be broken into anything smaller. Scientists now know that atoms are actually made from even smaller particles. There are three types: proton neutron electron How are these particles arranged inside the atom?

What is the structure of an atom? Protons, neutrons and electrons are not evenly distributed in an atom. The protons and neutrons exist in a dense core at the centre of the atom. This is called the nucleus. The electrons are spread out around the edge of the atom. They orbit the nucleus in layers called shells.

Labelling the atom

Atomic Number (Proton number, Z) Indicates the number of protons in an atom (always the same for an element) 13

How many protons? The atoms of any particular element always contain the same number of protons. For example: l hydrogen atoms always contain 1 proton l carbon atoms always contain 6 protons l magnesium atoms always contain 12 protons. The number of protons in an atom is known as the atomic number or proton number. It is the smaller of the two numbers shown in most periodic tables.

What is the atomic number? What are the atomic numbers of these elements? sodium 11 iron 26 tin 50 fluorine 9

More about atomic number Each element has a definite and fixed number of protons. If the number of protons changes, then the atom becomes a different element. Changes in the number of particles in the nucleus (protons or neutrons) are very rare. They only take place in nuclear processes such as: l radioactive decay l nuclear bombs l nuclear reactors.

How many electrons? Atoms have no overall electrical charge and are neutral. This means atoms must have an equal number of positive protons and negative electrons. The number of electrons is therefore the same as the atomic number. Atoms Protons Neutrons Electrons helium 2 2 2 copper 29 35 29 iodine 53 74 53 Atomic number is the number of protons rather than the number of electrons, because atoms can lose or gain electrons but do not normally lose or gain protons.

Periodic Table • Arranges elements by increasing atomic number • Represents physical and chemical behavior of elements • Repeats similar properties in columns known as chemical families or groups

Learning Check AT 1 State the number of protons for atoms of each of the following: A. Nitrogen 1) 5 protons 2) 7 protons 3) 14 protons B. Sulfur 1) 32 protons 2) 16 protons 3) 6 protons C. Barium 1) 137 protons 2) 81 protons 3) 56 protons Lecture. PLUS Timberlake 19

Solution AT 1 State the number of protons for atoms of each of the following: A. Nitrogen 2) 7 protons B. Sulfur 2) 16 protons C. Barium 3) 56 protons Lecture. PLUS Timberlake 20

Mass Number (Nucleon number, A) Indicates the number of protons and neutrons In the nucleus of an atom

What is mass number? Electrons have a mass of almost zero, which means that the mass of each atom results almost entirely from the number of protons and neutrons in the nucleus. The sum of the protons and neutrons in an atom’s nucleus is the mass number. It is the larger of the two numbers shown in most periodic tables. Atoms Protons Neutrons Mass number hydrogen 1 0 1 lithium 3 4 7 14 27 aluminium 13

What’s the mass number? mass number = number of protons + number of neutrons What is the mass number of these atoms? Atoms Protons Neutrons Mass number helium 2 2 4 copper 29 35 64 cobalt 27 32 59 iodine 53 74 127 germanium 32 41 73

How many neutrons? number of neutrons = mass number - number of protons = mass number - atomic number How many neutrons are there in these atoms? Atoms Mass number Atomic number Neutrons helium 4 2 2 fluorine 19 9 10 strontium 88 38 50 zirconium 91 40 51 uranium 238 92 146

Atoms: true of false?

How are atoms arranged? Where are the electrons found in the atom?

How are electrons arranged? Electrons are not evenly spread but exist in layers called shells. (The shells can also be called energy levels). The arrangement of electrons in these shells is often called the electron configuration. 1 st shell 2 nd shell 3 rd shell Note that this diagram is not drawn to scale – the atom is mostly empty space. If the electrons are the size shown, the nucleus would be too small to see.

How many electrons per shell? Each shell has a maximum number of electrons that it can hold. Electrons will fill the shells nearest the nucleus first. 1 st shell holds a maximum of 2 electrons 2 nd shell holds a maximum of 8 electrons 3 rd shell holds a maximum of 8 electrons This electron arrangement is written as 2, 8, 8.

Calculate electron configurations

Summary: the atom so far The nucleus is: l made up of protons and neutrons l positively charged because of the protons l dense – it contains nearly all the mass of the atom in a tiny space. Electrons are: l very small and light, and negatively charged l able to be lost or gained in chemical reactions l found thinly spread around the outside of the nucleus, orbiting in layers called shells.

What is an isotope? Elements are made up of one type of atom, but there can be slightly different forms of the atoms in an element. Although atoms of the same element always have the same number of protons, they may have different numbers of neutrons. Atoms that differ in this way are called isotopes. For example, two isotopes of carbon: mass number is different atomic number is the same

What are the isotopes of carbon? Most naturally-occurring carbon exists as carbon-12, about 1% is carbon-13 and a much smaller amount is carbon-14. 6 protons 6 neutrons 6 electrons 6 protons 7 neutrons 6 electrons 6 protons 8 neutrons 6 electrons

Properties of isotopes The isotopes of an element are virtually identical in their chemical reactions. This is because they have the same number of protons and the same number of electrons. The uncharged neutrons make little difference to chemical properties but do affect physical properties such as melting point and density. Natural samples of elements are often a mixture of isotopes.

What are the isotopes of hydrogen? Hydrogen-1 makes up the vast majority of the naturallyoccurring element but two other isotopes exist. hydrogen 1 proton 0 neutrons 1 electron deuterium 1 proton 1 neutrons 1 electron tritium 1 proton 2 neutrons 1 electron

What are the isotopes of chlorine? About 75% of naturally-occurring chlorine is chlorine-35 and 25% is chlorine-37. 17 protons 18 neutrons 20 neutrons 17 electrons

What are the isotopes of oxygen? Almost all of naturally-occurring oxygen is oxygen-16, but about 0. 2% is oxygen-18. What are the particle numbers in each isotope below? oxygen-16 8 protons 8 neutrons 8 electrons oxygen-18 8 protons 10 neutrons 8 electrons

Isotopes – true of false?

Stable and Unstable Isotopes • Some isotopes are stable, while others are radioactive and release particles and energy to decay into a more stable form • Small Nuclei - Atoms which contain up to 20 protons (up to Calcium) are usually stable. • Large Nuclei - Larger nuclei tend to be radioactive. All nuclei with more than 84 protons (Polonium and up) are radioactive.

Radioactive isotopes are used for many purposes….

Uses of Isotopes 1 – Smoke detectors – Contain a small amount of americium-241. By utilizing the radioactive properties of this material, smoke from a fire can be detected at a very early stage.

Uses of Isotopes 2 – Food safety The energy from the γray from cobalt-60 passing through food is enough to destroy many disease-causing bacteria as well as those that cause food to spoil, but is not strong enough to change the quality, flavor or texture of the food.

Uses of Isotopes 3 – Carbon dating

Uses of Isotopes 4 – Nuclear medicine Bone Scans - technetium-99 concentrates in areas of high bone growth. Can show abnormalities like stress fractures

Atomic structure – word check

Learning Check AT 3 An atom of zinc has a mass number of 65. A. Number of protons in the zinc atom 1) 30 2) 35 3) 65 B. Number of neutrons in the zinc atom 1) 30 2) 35 3) 65 C. What is the mass number of a zinc isotope with 37 neutrons? 1) 37 2) 65 3) 67 Lecture. PLUS Timberlake 45

Solution AT 3 An atom of zinc has a mass number of 65. A. Number of protons in the zinc atom 1) 30 B. Number of neutrons in the zinc atom 2) 35 C. What is the mass number of a zinc isotope with 37 neutrons? 3) 67 Lecture. PLUS Timberlake 46

Learning Check AT 4 Write the atomic symbols for atoms with the following: A. 8 p+, 8 n, 8 e. B. 17 p+, 20 n, 17 e- ___________ C. 47 p+, 60 n, 47 e- ______ Lecture. PLUS Timberlake 47

Learning Check AT 5 An atom has 14 protons and 20 neutrons. A. Its atomic number is 1) 14 2) 16 3) 34 B. Its mass number is 1) 14 2) 16 C. The element is 1) Si 2) Ca 3) 34 3) Se D. Another isotope of this element is 1) 34 X 16 2) 34 X 14 3) 36 X 14 Lecture. PLUS Timberlake 48