The Atom Chapter 25

The Atom The Atom is the smallest part of an element that still has the properties of that element. An element is a substance that cannot be broken down into simpler substances by chemical means.

What are Atoms made of? Atoms are made up of smaller particles called sub atomic particles. There are 3 types of sub atomic particle: Protons Neutrons Electrons

Structure of the Atom Protons and Neutrons are found in the centre of the atom called the nucleus. Electrons are found orbiting the nucleus in shells/orbits.

The Bohr Model

Structure of the Atom Part Mass Charge Location Proton 1 a.m.u. +1 In the nucleus Neutron Electron 1/1840 a.m.u. -1 Orbiting the nucleus

Chemical Symbols Atomic Number Symbol Mass Number

Atomic Number The atomic number of an atom is the number of protons balanced by an equal number of electrons. (The small number) Atomic Number

Mass Number The mass number tells the number of protons and neutrons in the nucleus. (The big number) Mass Number = No of Protons + No of Neutrons Mass Number

Arrangement of Electrons n=1 1st shell holds 2e- n=2 2nd shell holds 8e- n=3 3rd shell holds 8e-

Periodic Table Horizontal lines are called periods. The period tells us the number of shells/orbits in an atom. Vertical lines are called groups. Group number tells us how many electrons are in an atoms outermost shell.

Isotopes Isotopes are atoms of the same element, which have different numbers of neutrons. Same atomic number different mass number. i.e. Carbon 12, carbon 13 and carbon 14.