Atoms, Elements, and the Periodic Table: Foundations of GOB Chemistry

Elements and Chemical Symbols

Definition and Properties of Elements

Elements are pure substances from which all other substances are built. They cannot be broken down into simpler substances by chemical means. Each element is represented by a unique chemical symbol, typically consisting of one or two letters, with the first letter always capitalized.

• One-letter symbols: Examples include C (carbon), N (nitrogen), F (fluorine), O (oxygen).

• Two-letter symbols: Examples include Co (cobalt), Ca (calcium), Al (aluminum), Mg (magnesium).

• Latin-derived symbols: Some elements use symbols from their Latin names, such as Ag (silver, argentum) and Au (gold, aurum).

The Periodic Table

Organization and Structure

The periodic table organizes the known elements (currently 118) into groups with similar properties and arranges them in order of increasing atomic number. The table is divided into vertical columns called groups and horizontal rows called periods.

• Groups: Numbered 1A–8A (representative elements) and 3B–12B (transition elements), or alternatively 1–18 from left to right.

• Periods: Horizontal rows numbered from 1 to 7.

Group Names and Classifications

Groups have specific names based on their properties:

• Group 1A: Alkali metals

• Group 2A: Alkaline earth metals

• Group 7A (17): Halogens

• Group 8A (18): Noble gases

Metals, Nonmetals, and Metalloids

The periodic table is divided by a heavy zigzag line that separates metals (left), nonmetals (right), and metalloids (along the line).

• Metals: Shiny, ductile, good conductors of heat and electricity, mostly solid at room temperature (except mercury).

• Nonmetals: Dull, brittle, poor conductors, often good insulators, low densities and melting points.

• Metalloids: Exhibit properties of both metals and nonmetals, used as semiconductors and insulators.

Elements Essential to Health

Certain elements are essential for human health and are classified as major elements, macrominerals, and microminerals (trace elements):

• Major elements: H, C, N, O

• Macrominerals: Na, Mg, K, Ca, P, S, Cl

• Microminerals: Si, V, Cr, Mn, Fe, Co, Cu, Zn, As, Se, Mo, I

The Atom

Structure of the Atom

An atom is the smallest particle of an element that retains its chemical properties. Atoms consist of a dense nucleus containing protons and neutrons, surrounded by electrons in a large, mostly empty space.

• Protons: Positively charged particles in the nucleus.

• Neutrons: Neutral particles in the nucleus.

• Electrons: Negatively charged particles outside the nucleus.

Dalton’s Atomic Theory

Dalton proposed that atoms are tiny particles of matter, identical for each element, and combine to form compounds. Atoms are rearranged in chemical reactions but are never created or destroyed.

Electrical Charges in Atoms

Atoms contain charged subatomic particles. Like charges repel, and unlike charges attract.

• Protons: +1 charge

• Electrons: -1 charge

• Neutrons: 0 charge

Atomic Models

J. J. Thomson’s "plum pudding" model depicted electrons scattered within a positively charged sphere. Rutherford’s gold foil experiment demonstrated that atoms have a small, dense nucleus.

Mass of the Atom

The mass of an atom is concentrated in the nucleus (protons and neutrons). The atomic mass unit (amu) is used to express atomic and subatomic masses, where 1 amu is defined as 1/12 the mass of a carbon-12 atom.

• Proton mass: ~1.007 amu

• Neutron mass: ~1.008 amu

• Electron mass: ~0.00055 amu (negligible)

Particle	Symbol	Charge	Mass (amu)	Location
Proton	p or p+	+1	1.007	Nucleus
Neutron	n or n0	0	1.008	Nucleus
Electron	e-	-1	0.00055	Outside nucleus

Atomic Number and Mass Number

Atomic Number

The atomic number is the number of protons in an atom and defines the element. It is a whole number unique to each element and appears above the element symbol in the periodic table.

• Atomic number of H = 1 (1 proton)

• Atomic number of C = 6 (6 protons)

• Atomic number of Cu = 29 (29 protons)

Neutral Atoms

In a neutral atom, the number of protons equals the number of electrons, resulting in no overall charge.

• Example: Aluminum has 13 protons and 13 electrons; net charge = 0.

Mass Number

The mass number is the total number of protons and neutrons in the nucleus. It is always a whole number and is specific to each isotope of an element.

• Mass number = number of protons + number of neutrons

• Number of neutrons = mass number − atomic number

Element	Symbol	Atomic Number	Mass Number	Protons	Neutrons	Electrons
Hydrogen	H	1	1	1	0	1
Nitrogen	N	7	14	7	7	7
Oxygen	O	8	16	8	8	8
Chlorine	Cl	17	37	17	20	17
Iron	Fe	26	58	26	32	26
Gold	Au	79	197	79	118	79

Isotopes and Atomic Mass

Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, resulting in different mass numbers. Isotopes are represented by atomic symbols that show both the atomic number and mass number.

• Example: $^{16}_{8}\mathrm{O}$, $^{31}_{15}\mathrm{P}$, $^{65}_{30}\mathrm{Zn}$

Isotope	Atomic Number	Mass Number	Protons	Neutrons
$^{16}_{8}\mathrm{O}$	8	16	8	8
$^{31}_{15}\mathrm{P}$	15	31	15	16
$^{65}_{30}\mathrm{Zn}$	30	65	30	35

Calculating Subatomic Particles from Atomic Symbols

• Number of protons: Equal to atomic number

• Number of neutrons: Mass number − atomic number

• Number of electrons: Equal to number of protons (for neutral atoms)

Example Problems

• Write the atomic symbol for an atom with 8 protons, 8 neutrons, and 8 electrons: $^{16}_{8}\mathrm{O}$

• Write the atomic symbol for an atom with 17 protons, 20 neutrons, and 17 electrons: $^{37}_{17}\mathrm{Cl}$

• Write the atomic symbol for an atom with 47 protons, 60 neutrons, and 47 electrons: $^{107}_{47}\mathrm{Ag}$

Identifying Isotopes

• Atoms with the same number of protons but different numbers of neutrons are isotopes of the same element.

• Atoms with the same number of neutrons can be identified by subtracting the atomic number from the mass number.

Key Equations

• Number of neutrons: $\text{Number of neutrons} = \text{Mass number} - \text{Atomic number}$

• Atomic symbol: $^{A}_{Z}\mathrm{X}$, where $A$ is the mass number, $Z$ is the atomic number, and $\mathrm{X}$ is the element symbol.