Atoms

Atoms are spherically shaped with a tiny, dense, positively charged nucleus, contain protons and neutrons. Most of an atom consists of fast-moving negatively charged electrons traveling through empty space surrounding the nucleus. The electrons occupy different energy levels, similar to the rungs on a ladder.

• The number of protons in an atom identifies it as an atom of a particular element

• The number of protons is called the atomic number

• Elements are arranged on the Periodic Table by increasing atomic number

• On the Periodic Table, each different element is shown in a box

• Atomic number = protons = electrons

• Mass number = protons + neutrons

Isotopes

All atoms of a particular element have the same number of protons and electrons but the number of neutrons may vary. Atoms with the same number of protons but different number of neutrons are called isotopes.

Isotopes differ in mass but have the same chemical behavior. To identify the various isotopes of an element, the mass number is added after the element’s name. It represents the sum of the protons and neutrons in the nucleus.

• Number of Neutrons = Mass number - atomic number

For example, a carbon atom with 6 protons and 6 neutrons is called carbon-12

Scientists

Democritus (460-370 BC)

The first person to propose the idea that matter was not infinitely divisible. He believed matter was made up of tiny individual particles called atomos. He believed that atoms could not be created, destroyed, or further divided.

• Matter is composed of empty space through which atoms move

• Atoms are solid, homogeneous, indestructible, and indivisible

• Different kinds of atoms have different sixes and shapes

• The differing properties of matter are due to the size, shape, and movement of atoms

• Apparent changes in matter result from changes in the groups of atoms and not from changes in the atoms themselves

His ideas were met with criticism from other philosophers.

John Dalton (1766-1844)

Dalton’s Atomic Theory:

• All matter is made of indivisible and indestructible atoms

• Atoms of a given element are identical in their physical and chemical properties

• Atoms of different elements have different physical and chemical properties

• Atoms of different elements can combine in simple whole-number ratios to form chemical compounds

• Atoms cannot be subdivided, created, or destroyed when they are combines separated, or rearranged in chemical reactions

Dalton’s theory explains the law of conservation of mass: atoms of different elements combine to form chemical compounds

Not entirely accurate → Weakness was his belief that the atom was indivisible

J.J. Thomson (1856-1940)

Thomson’s experiments concluded that there were tiny, negatively charged particles called electrons. Thomson proposed that tiny negatively charged electrons must be embedded in a cloud of positive charge, which he known as the “Plum Pudding Model.”

Ernest Rutherford (1871-1937)

A student of J.J. Thomson, Rutherford performed the “gold-foil experiment” which ultimately disproved J.J. Thomson’s theory. With a new model, called the Rutherford model, the electrons orbit around the very small and dense nucleus.

Ions

An atom is an electrically neutral particle composed of electrons, neutrons, and protons. Usually, the number of protons equal the number of electrons. If an atom gains or loses electrons, it is called an Ion.

Cation: A positively charged ion that has lost an electron

• There are more protons than electrons, which makes it positively charged

• ex. Cl²⁺, Ca⁺

Anion: A negatively charged ion that gained an electron

• There are more electrons than protons, which makes it negatively charged

• ex. S^2-, O^-

Nuclear Chemistry

Mass Defect: Difference between the mass of an atom and the mass of its individual particles

Nuclear Binding Energy: Energy released when a nucleus is formed from nucleons. High binding energy means a more stable nucleus

E=mc²

• E: Energy (J)

• m: mass defect (kg)

• c: speed of light (3.00 × 10⁸ m/s)

Types of Radiation:

Transmutation: the changing of one element into another by radioactive decay

Alpha

• Lowest penetration power: stopped by paper, clothes, skin

Beta

• Relative penetration power: stopped by aluminium foil

Positron

• Relative penetration power: stopped by aluminium foil

Gamma

• Greatest penetration power: stopped by concrete, lead

Electron Capture

• Relative penetration power: stopped by aluminium foil

Neutron

• Relative penetration power: stopped by water

Half-Life

Half life is the time required for the number of radioactive nuclides to decay to half of the original value

• Parent isotope → Daughter isotope

• Shorter half life means it is more unstable

• Expressed as t_1/2

Formula: m_f = m_i(1/2)ⁿ

• m_f = final mass

• m_i = initial mass

• n = number of half lives

• n = total time / time in one half life