Understanding Atoms Year 9 Science

Contents Atomic structure Bohr shell theory Periodic table

Video: Atomic Structure and the periodic table

Recall from last year’s science that Elements are… Pure substances that cannot be broken down into other chemicals. These substances are made up of only one type of atom (A substance of an element will consist of millions of atoms but they will all be the same atom). Carbon Gold Mercury

What is an ATOM? All matter in the universe is made up of atoms. It is the simplest part of an element.

Atomic structure All atoms have the same basic structure. A CENTRAL region called the NUCLEUS. An OUTER region known as the ELECTRON SHELLS

Inside the nucleus The NUCLEUS contains TWO TYPES of subatomic particles: + - PROTONS (which carry a positive charge) NEUTRONS (which carry no charge ) Electron shells Nucleus -

Atomic Mass Units (AMU) Protons and neutrons weigh about the same x g or ( g) Because this number is so small, Scientist call this number one atomic mass unit (amu).

Electron shells The ELECTRON SHELLS contain only ONE TYPE of subatomic particle: ELECTRONS (which carry a negative charge). They are so small that they can be considered to have a negligible mass (so small it can be ignored). - Electron shells Nucleus - Electrons

What are the three subatomic particles?

Summary of subatomic particles Source: Chapter 2 Understanding Atoms. In Heinemann Science Links 3 (VELS Edition) 2006, pp.27

Atomic Number  The number of protons in an atom’s nucleus is known as its Atomic Number.  This number is different for each different type of atom.  Each element (in the periodic table) is assigned an atomic number. Atomic Number

Atomic Number Question: How many protons do these elements have?

Number of Electrons The number of electrons found in an atom will be the same as the number of protons (atomic number) that the atom’s nucleus contains. Protons carry a positive (+) charge & electrons carry a negative (-) charge. Therefore the numbers of each particle in an atom have to be the same to keep the charges (+/-) balanced. + -

Number of electrons Question: How many electrons do these elements have?

Mass Number The total number of protons AND neutrons found in an atom’s nucleus is known as its Mass number. Mass number is expressed in atomic mass units. (amu) This number is not shown on all Periodic Tables.

Mass Number Question: Helium has a mass number of 4. If the atomic number of Helium is 2; How many protons and neutrons does Helium have? Mass Number = No. of protons + neutron 4 = Therefore Helium has 2 protons and 2 neutrons Boron has a mass number of 11: Boron atoms have an atomic number of 5; How many neutrons does Boron have? Mass Number = No. of protons + neutron 11 = Therefore Boron has 5 protons and 6 neutrons

Mass Number Superscript and subscript numbers are attached to the symbol of an element to indicate its atomic and mass numbers. Example, Mass No X C He Na Atomic No Carbon (C) has an atomic number of 6 & a mass number of 12. Helium (He) has an atomic number of 2 & a mass number of 4 Sodium (Na) has an atomic number of 11 & a mass number of 23. The mass number generally appears above the atomic number, and is usually the larger of the two.

Mass Number Question: Chlorine (Cl) 35 Cl 17 What is the atomic number and mass number of chlorine?

Isotopes Isotopes are atoms of the same elements with different numbers of neutrons. Example: Carbon C C 6 6 Normal Carbon Atom Carbon Atom Isotope N=6 P=6 N=7 P=6

Atomic Mass Versus Mass Number Atomic mass is the weighted average of the mass numbers of isotopes of the element. The Atomic mass of an atom (NOT the mass number) is usually expressed as a number of grams of that element that contain 6 x atoms of that element. ( ) This number is usually on all Periodic tables. The atomic mass is usually very close (if not identical) to the atom’s mass number.

Summary Protons- carry a positive (+) charge, found in nucleus Neutrons- carry no charge, found in nucleus Electrons- carry a negative (-) charge, found in electron shells Atomic number = number of protons Mass number = number of protons + neutrons No. of electrons in neutral atom = atomic number Isotopes - are atoms of the same elements with different numbers of neutrons

Questions Heinemann Science Links 32 pp. 32 Questions 2- 4

Bohr Shell Model Bohr proposed that electrons are found within specific regions of an atom called shells. Electrons in an atom are arranged in shells The first shell - found closest to the nucleus can hold only two electrons. The second shell can hold eight electrons The third shell can hold eighteen electrons. The further out shells can hold progressively more electrons

Bohr also noticed that electrons tend to be naturally found in shells of the lowest energy. The lowest energy shell is the first shell. Therefore the first shell always fills up first before the second shell.

References Chapter 2: Understanding Atoms. In Heinemann Science Links 3 (VELS Edition). ic_table/atomic_mass.htmlhttp:// ic_table/atomic_mass.html

Components of an atom Nucleus Electron Shells

Then one oxygen atom weighs eight times as much as two hydrogens.

Source: s/Propulsion/1-what-is-an-atom.html