1.1.2 Atomic Structure

• Elements are made of tiny particles of matter calledatoms
• Each atom is made of subatomic particles calledprotons,neutronsandelectrons
• Their size is so tiny that we can't really compare their masses in conventional units such as kilograms or grams, so a unit called therelative atomic massis used
• The mass of an atom isconcentratedin the nucleus, because the nucleus contains the heaviest subatomic particles (the neutrons and protons)
• The nucleus is alsopositivelychargeddue to the protons
• Electrons orbit the nucleus of the atom, contributing very little to its overall mass, but creating a‘cloud’of negative charge
• Theelectrostaticattractionbetween thepositive nucleusandnegatively charged electronsorbiting around it is what holds an atom together

The mass of the atom is concentrated in the positively charged nucleus which is attracted to the negatively charged electrons orbiting around it

• Theprotons,neutronsandelectronsthat an atom is made up of are called亚原子水平ticles
• These subatomic particles are so small that it is not practical to measure their masses and charges usingconventionalunits(such as grams or coulombs)
• Instead, their masses and charges are compared to each other, and so are called‘relativeatomicmasses’and ‘relative atomic charges’
• These are not actual charges and masses, but rather charges and masses of particles relative to each other
  • Protons and neutrons have a very similar mass, so each is assigned a relative mass of1
  • Electrons are 1840 times smaller than a proton and neutron, and so their mass is often described as being negligible

• The relative mass and charge of the subatomic particles are:

The Mass & Charge of Subatomic Particles Table

• Atoms are electricallyneutral
• This is achieved by having thesame numberof electrons as protons
• Thenegative chargeof an electron exactly cancels out thepositive chargeof a proton