ATOM

Chapter 14

I CAN IDENTIFY THE PROPERTIES OF THE THREE SUBATOMIC PARTICLES OF ATOMS.

I CAN USE A MODEL TO REPRESENT THE STRUCTURE OF AN ATOM AND IDENTIFY THE FORCES THAT HOLD THE SUBATOMIC PARTICLES TOGETHER.

I CAN IDENTIFY THE NUMBER OF SUBATOMIC PARTICLES BASED ON THE PERIODIC TABLE OF ELEMENTS.

Competency G – Properties of Matter

Electric Charge

Electric Charge is a property of matter 2 types of Electric Charge: positive or

negative Electric Charge holds atoms together Opposite charges ATTRACT Same charges REPEL

Electric Charge

Matter is electrically neutral when positive and negative charges are EQUAL and total electric charge is ZERO

History of the Atom

Important Discoveries: 1. J.J. Thomson (1897) discovered electrons He passed electricity through a gas and

noticed smaller negatively charges particles were given off

He proposed negative electrons were sprinkled throughout positively charged atom

History of Atom

2. Rutherford, Geiger and Marsden (1911) discovered atom was mostly empty space with dense core called nucleus Gold Foil experiment –

most atoms passed through gold foil

Atomic Structure

Subatomic

Particles

Location

Charge Mass Other

Proton Nucleus Positive Large Same mass as neutron

Neutron

Nucleus No charge

(neutral)

Large Same mass as proton

Electron

Electron Cloud

Negative Very small

Move very fast

Atomic Structure

Protons and Neutrons are massive compared to electrons

Mass of nucleus determines mass of atom

99% of atom’s mass in nucleus where protons and neutrons are located

Electron cloud is empty space around the nucleus

Electron cloud is 10,000 larger than nucleus – reason why atom is mostly empty space

Forces inside Atoms

Electromagnetic Force: Attractive force between electrons (-) and protons (+) binding electrons to nucleus

Forces inside Atoms

Strong nuclear force: force that holds nucleus together Very strong Attracts neutrons and protons to each other Works at only extremely small distances

Forces inside Atoms

Weak Force: force that will turn a single neutron outside the nucleus into a proton and electron Weaker than electromagnetic force and

strong nuclear force Important when atoms break apart

Forces inside Atoms

Gravity: force of gravity inside atoms is very weak because an atom does not have a large mass

How to tell Different elements apart?

Different elements contain DIFFERENT numbers of protons

Atomic Number: number of protons

All atoms of same element have same number of protons in nucleus

Each element has unique atomic number

Stable Atoms

Stable atoms are neutral atoms In STABLE atom number of protons

EQUALS number of electrons

Different Atoms

PROBLEM SOLVING

What is the atomic number of Helium? How many protons are in Helium? How many electrons are in Helium

What is the atomic number of Carbon? How many protons are in Carbon? How many electrons are in Carbon?

Ions

Ions: atoms that have different numbers of protons than electrons

Ions have an electric charge Positive charged ions: contain more

protons Negative charged ions: contain more

electrons

Isotopes

Isotopes: atoms of the same element that have different numbers of neutrons

Mass number: number of protons + number of neutrons Represents everything in nucleus

Number of neutrons = mass number – atomic number

Referring to Atoms

When referring to atoms, most scientists write the name of the atom and then the mass number of the atom

Example: Nitrogen - 14 or Nitrogen – 15 Both have the SAME number or protons Both have the SAME number of electrons They have DIFFERENT numbers of neutrons

Isotope Notation

For Isotope Notation, scientists write the symbol of the element

Superscripted next to the symbol is the mass number

Subscripted next to the symbol is the atomic number

Isotopes of Hydrogen

PROBLEM SOLVING

How many neutrons are present in an aluminum atom that has an atomic number of 13 and a mass number of 27?

Radioactivity

Most elements have stable isotopes Stable means nucleus stays

together Radioactive means nucleus is

unstable and breaks apart because too many or too few neutrons

Radioactive isotope eventually changes into stable isotope

Radioactivity is a process in which the nucleus spontaneously emits particles as it changes into a more stable isotope

Radioactivity

Radioactivity can change one element into a completely different element

Alpha Decay: when nucleus ejects 2 protons and 2 neutrons Atomic number decreases by 2 Mass Number decreases by 4

Beta Decay: when a neutron splits into a proton and electron Atomic number increases by 1 Mass Number stays the same

Gamma Decay: How a nucleus gets rid of excess energy Nucleus gives off gamma rays Atomic Number and Mass Number stay the same

Electrons – 14.2

Atoms interact with each other through electrons

Example: Chemical bonds involve only electrons

Light Spectrum

Almost all light you see comes from atoms

Spectrum: is a specific pattern of colors given off by an element

Spectrum includes very specific colors or characteristics

Spectral line: individual color in a spectrum

Spectroscope: instrument that separates light into a spectrum

Light

Energy level: allowed energies for electrons

Electrons must be in one energy level or another – not between levels

Electrons change energy levels by absorbing or emitting light

Light is given off when an electron moves from a higher energy level to a lower energy level

Bohr Model

Neils Bohr: Danish physicist who proposed concept of energy levels

When an electron moves from a higher energy level to a lower one, the atom gives up the energy difference between the two levels

The energy comes out as different colors of light

Electrons and Energy Levels

Electrons located in the Electron Cloud

Electron Cloud divided into energy levels

Electrons farther from the nucleus have more energy

Energy Levels

Rules for energy levels: 1. energy of electron has to match 1

of the energy levels 2. each energy level can hold a

certain number of electrons 3. when electrons are added to an

atom, fill lowest empty energy levels first

Energy Levels

1st energy level: 2 electrons 2nd energy level: 8

electrons 3rd energy level: 8

electrons 4th energy level: 18

electrons 5th energy level: 18

electrons