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55 Cs Cesium 132.905451966

Cesium

ALKALI METAL
Bohr Model: Electrons arranged in energy shells around the nucleus.

Caesium or cesium is a chemical element with symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C (82 °F), which makes it one of only five elemental metals that are liquid at or near room temperature. Caesium is an alkali metal and has physical and chemical properties similar to those of rubidium and potassium.

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Quick Facts

Atomic Number 55
Period 6
Group 1
Phase Solid
Appearance silvery gold
About Alkali Metals

Alkali metals are highly reactive, soft, silver, and shiny metals found in Group 1. They readily lose their outermost electron to form positive ions (cations) with a charge of +1.

History

Discovered By Robert Bunsen
Named By Unknown

Physical Properties

Atomic Mass 132.905451966
Density 1.93 g/L
Molar Heat 32.21 J/(mol·K)
Phase Transition (Melting & Boiling Points)

Cesium melts at 301.7 K and boils at 944 K.

Atomic Properties

Electron Config [Xe] 6s1
Electronegativity 0.79
Electron Affinity 45.505 kJ/mol
Ionization Energy 375.7 kJ/mol
Orbital Filling Diagram
This diagram visualizes electron configuration according to the Aufbau principle and Hund's rule. Boxes represent orbitals (s, p, d, f), while arrows indicate electrons with spin up or down. Electrons fill lower energy levels first and occupy orbitals singly before pairing.

Photograph

Image of Cesium Source: Wikipedia

External Resources

Wikipedia PubChem Royal Society of Chemistry Periodic Videos (YouTube)

Study Guide

Atomic Number

The number of protons in the nucleus, which defines the element.

Atomic Mass

The average mass of an atom, including protons and neutrons.

Electronegativity

A measure of how strongly an atom attracts electrons in a bond.

Ionization Energy

The energy required to remove an electron from an atom.

Electron Config

The arrangement of electrons in the atom's energy shells.