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Isotope

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

Atomic mass

The weighted average mass of all naturally occurring isotopes of an element.

Atomic number

The number of protons in an atom’s nucleus.

Bohr model

A model showing electrons moving around the nucleus in specific energy levels.

Valence electrons

The electrons in the outermost energy level of an atom.

Period

A horizontal row on the periodic table.

Group

A vertical column on the periodic table.

Proton

A positively charged particle in the nucleus with a mass of about 1 amu.

Neutron

A neutral particle in the nucleus with a mass of about 1 amu.

Electron

A negatively charged particle outside the nucleus with a mass of about 0 amu.

Carbon-13 protons, neutrons, electrons

6 protons, 7 neutrons, and 6 electrons.

Strontium-88 protons, neutrons, electrons

38 protons, 50 neutrons, and 38 electrons.

Radon-222 protons, neutrons, electrons

86 protons, 136 neutrons, and 86 electrons.

Tellurium-127 protons, neutrons, electrons

52 protons, 75 neutrons, and 52 electrons.

Calcium Bohr model

2, 8, 8, 2 electrons; 20 protons.

Sodium Bohr model

2, 8, 1 electrons; 11 protons.

Boron Bohr model

2, 3 electrons; 5 protons.

Argon Bohr model

2, 8, 8 electrons; 18 protons.

Modern periodic table organization

Elements are arranged by increasing atomic number and repeating chemical properties.

Atomic radius across a period

Atomic radius decreases from left to right.

Atomic radius down a group

Atomic radius increases from top to bottom.

Metallic character across a period

Metallic character decreases from left to right.

Metallic character down a group

Metallic character increases from top to bottom.

Most metallic element

Francium.