Chapter 4: Atomic Structure

4.2 – The Structure of an Atom

Subatomic Particles

• Protons (Rutherford)- a positively charged subatomic particle in the nucleus– Each nucleus must contain at least one particle

with a positive charge.

Subatomic Particles

• Electrons (Thomson)- a negatively charged particle found in the space OUTSIDE the nucleus– Each electron has a charge of –1.

Subatomic Particles

• Neutrons (Chadwick)- a neutral subatomic particle found in the nucleus– Neutrons have a mass almost exactly equal to that

of a proton.– No charge

A neutron walked into a bar and asked for the price of a drink. "For you," the

bartender replied "no charge."

Rutherford’s Nuclear Atom• the protons and neutrons are located in the nucleus • The electrons are distributed around the nucleus and

occupy almost all the volume of the atom. • the nucleus is tiny compared with the atom as a whole. • If an atom were the size of a football stadium, the nucleus

would be about the size of a marble.

Comparing Subatomic Particles• Protons, electrons, and neutrons can be

distinguished by mass, charge, and location in an atom.

– Everything scientists know about the nucleus and subatomic particles is based on how the particles behave.

Atomic Number• Atomic Number- equals the number of

protons in an atom– The atoms of any given element always have the

same number of protons.• There is one proton in the nucleus of each and every

hydrogen (H) atom…always…no exceptions!

– Each element has a unique (different) atomic number

– Atoms of different elements have different numbers of protons.

Atomic Number

• Each positive charge in an atom is balanced by a negative charge because atoms are neutral. – The atomic number of an element also equals the

number of electrons in an atom.

Mass Number• Mass Number- the sum of the protons and

neutrons in the nucleus of an atom– Using the mass number is the only way to find the

number of neutrons

Atoms of the First Ten Elements

Isotopes

• Isotopes- atoms of the same element that have different numbers of neutrons and different masses.– Every atom of a given element does have the

same number of protons and electrons.– But…every atom of a given element does not have

the same number of neutrons.

Isotopes

• Isotopes of an element have the same atomic number but different mass numbers because they have different numbers of neutrons.– Example: Heavy Water Figure 12, p. 112.

Complete Chemical Symbol• The composition of any atom can be

represented in shorthand notation using atomic number and mass number

• The chemical symbol Au appears with two numbers written to its left.

• The atomic number is the subscript. • The mass number is the superscript.• You can also refer to atoms by using the

mass number and the name of the element.• For example: gold - 197

Calculating Atomic Number

• Uranium-238 has a mass number of 238 with 146 neutrons in the nucleus. Uranium-235 (mass of 235) has 143 neutrons in the nucleus.

• What is the atomic # of uranium?

4.3 MODERN ATOMIC THEORY• Key Concepts:– Electrons move from energy levels when they

gains/loses energy – The electron cloud model describes the possible

locations of electrons around the nucleus – An electron cloud is a good approximation of how

electrons behave in orbitals. – Stable electron configurations

Vocabulary: • Energy levels• Electron Cloud• Orbital• Electron Configuration • Ground State

BOHR’S MODEL• 1885-1962• Danish physicist• His model focused

on the electrons• Electrons move in

orbits around the nucleus

• Like planets around the sun

ENERGY LEVELS • Each electron in orbit has a specific amount

of energy. • If the atom gains/loses energy, the energy of

the electron changes • Energy levels- the possible energies that

electrons in an atom can have• Electrons cannot exist between energy

levels • They must be in one energy level • Each step away from the nucleus represents a

gain in energy

• ***An electron can move from one energy level to another when the atom gains or loses energy

Question:• What determines the amount of energy

gained/lost when an electron moves between energy levels?

Answer:

• The size of the jump between energy levels

EVIDENCE FOR ENERGY LEVELS• Fireworks!!! – Heat produced by the explosion causes e-

to move to higher energy levels –When those e- move back to a lower

energy level, they emit energy (lose energy).

– Some of that energy is released as light – Different elements emit different colors of

light because no 2 elements have the same set of energy levels.

ELECTRON CLOUD MODEL • Out with the old…– Bohr was correct in assigning energy

levels to electrons.– But incorrect in assuming e- move like

planets in a solar system.

• In with the new…– Today, we know that e- move in less

predictable ways.

ELECTRON CLOUD MODEL • Electron Cloud- is a visual model of the

most likely locations for electrons in an atom– The cloud is denser at locations where

electrons most likely are found.

• Scientists use the electron cloud model to describe the possible locations of electrons around the nucleus.

• Example: Propeller blades, page 116, Figure 14.

Atomic Orbitals • The electron cloud represents all the

orbitals in an atom.• Orbital- a 3-D region of space around

the nucleus where an electron is likely to be found

• Example: if you were to put a dot on a map of the school where you were located once every 10 minutes for a week we could see the places you visit the most would have a LOT of dots.

Atomic Orbitals • The dots on the map would be a model

of your “orbital” • They describe your most likely

locations. • An electron cloud is a good

approximation of how electrons behave in their orbitals.

Energy Levels, Orbitals, and Electrons

Energy Level Number of Orbitals Maximum # of Electrons

1 1 2

2 4 8

3 9 18

4 16 32

Electrons & Energy Levels

• Question: How many electrons can be in each orbital?

• Answer: 2

ELECTRON CONFIGURATIONS • Configuration- an arrangement of

objects in a given space• Electron configuration- the

arrangement of electrons in the orbitals of an atom

• The most stable electron configuration is the one in which the electrons are in orbitals with the lowest possible energies

ELECTRON CONFIGURATIONS • Ground state- when all the electrons in an

atom have the lowest possible energies• Example: Lithium has an atomic number of

3, so it has three electrons – Two electrons are in the orbital of the first

energy level, one in the second at its second energy level

– If energy is absorbed, one of the e- can move to an orbital with a higher energy level• This is known as an excited state • It is less stable than the ground state

Gain Energy…Move to Excited State

Energy

Lose Energy…Back to Ground State

Energy

ELECTRON CONFIGURATIONS • Eventually, this e- will lose energy and

return to the ground state. • When helium, neon, argon, krypton,

and xenon return from an excited state they emit light, neon light!