Choose the correct answer:

A substance that cannot be broken down chemically into simpler substances is a(n) ___________.

• atom

• electron

• element

• nucleus

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Today we know that a(n) ___________ is the smallest unit of an element that has that element's properties.

• atom

• electron

• element

• nucleus

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Negatively charged particles called ___________move around the nucleus.

• atom

• electrons

• elements

• neutron

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Because an atom has the same number of ___________ and electrons, the atom has no overall charge.

• protons

• electrons

• elements

• neutron

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On Earth, elements such as ___________ and any of these: oxygen, silicon, aluminum, nitrogen, iron and calcium are among the most common.

• oxygen

• hydrogen

• elements

• helium

Fill in the blank:

Dmitri Mendeleev created the _______ table of elements.

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In space, the most common elements are __________ and __________.

Fill in the blank:

The center of an atom is its ________.

Fill in the blank:

Two or more atoms joined into a single particle form a(n) _________.

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The table's columns group elements according to ____________.

• molecule

• properties

• elements

• atom