Introduction to Atoms

Chapter 4

Democritus

• Greek Philosopher• “uncutable particle”• Named the atom from the Greek word Atomos • Atoms are small, hard, different

shapes and sizes, always moving and join together

Aristotle

• Greek philosopher• Disagreed with Democritus• Strong influence

Dalton• British Chemist 1766• Elements combine in specific proportions

because they are made of individual atoms• He observed that no matter how large or

small the sample, the ratio of the masses of the elements in the compound is always the same.

• Compounds have a fixed position

Dalton’s Theory

• Dalton proposed the theory that all matter is made up of individual particles called atoms, which cannot be divided.

Main Points of Dalton’s Theory

• All elements are composed of atoms• All atoms of the same element have the

same mass, and atoms of different elements have a different masses.

• Compounds contain atoms of more than one element.

• In a particular compound, atoms of different elements always combine in the same way

What did Dalton notice that all compounds have in common?

• Dalton noticed that the ratio of masses of elements in a

compound is always the same!

Objects with like charges repel, or push apart.

• Objects with opposite charges attract, or pull together.

Thomson’s Experiments• He used a sealed tube of gas

in his experiments. – When the current was on,

the disks became charged and glowing beam appeared in the tube.

– The beam bent toward a positively charged plate placed outside the tube.

• He concluded that the particles in the beam had a negative charge because they were attracted to the positive plate.

• Thomson’s experiments provided the first evidence that atoms are made of even smaller particles

Thomson’s Model

• Thomson found a negative charge, but knew that atoms were neutral.

• Atoms must contain a positive charge as well.

• The negative charges were evenly scattered throughout an atom filled with a positively charged mass of matter.

• Plum Pudding Model

Rutherford’s Atomic Theory(1871-1937)

• He hypothesized that the mass and charge at any location in the gold would be too small to change the path of an alpha particle (positive charge).

• When the gold foil experiment was done more particles were deflected and bounced straight back, as though they had struck an object.

• The alpha particles must have come close to another charged particle

• Rutherford concluded that the positive charge of an atom is not evenly spread through the atom.

• It is concentrated in a very small, central area called the nucleus.

• According to Rutherford’s model, all of an atom’s positive charge is concentrated in its nucleus.

Bohr

• Danish scientist• Discovered electrons travel

around nucleus in definite paths• Located in levels around nucleus• Electrons can jump from one level

to another

Schrodinger and Heisenberg

• Modern theory• Electron clouds surround

nucleus• Electrons do not travel in

definite paths• Movement cannot be predicted

Chapter 4 Section 2

Atom Structure

Nucleus• A dense, positively

charged mass located at the center of the atom

• New Model: All of an atom’s positive charge is concentrated in its nucleus.

Properties of subatomic particles

• 1. Protons

• 2. Electrons

• 3. Neutrons

electron

neutron

proton

Protons

• Positively charged subatomic particle that is found in the nucleus of an atom

• Charge of 1+

Electron

• A negatively charged subatomic particle that is found in the space outside the nucleus

• Charge of 1-

Neutrons

• Neutral subatomic particle that is found in the nucleus of an atom

• Mass almost exactly equal to that of a proton

Subatomic particles

• Distinguished by their mass, charge and location in the atom

Atomic Number

• Atoms of any element have the same number of protons

• Atomic number = number of protons in an atom of that element

• Ex: Hydrogen has 1 proton, atomic number is 1

Mass Number

• Sum of protons and neutrons in the nucleus of an atom

• Number of neutrons = Mass number – Atomic Number

Isotopes

• Every atom of a given element has the same number of protons and electrons, but every element does not have the same number of neutrons.

• Isotopes – have same atomic number, but different mass numbers because they have different number of neutrons

Isotopes

• Atoms that have the same number of protons but have a different number of neutrons

• Atoms that are isotopes of each other are always the same element because the number of protons in each atom is the same

Bohr’s Model of the Atom

• Focused on Electrons that move with constant speed in fixed orbits around the nucleus

• Electrons can change energy levels when the atom gains or loses energy

Electron Cloud Model

• Visual model used to find the locations for electrons

Forces that Work in Atoms

• * Gravity• * Electromagnetic Force• * Strong Force• * Weak Force